After reading some of joeb's answers and explanation of Greenhills Formula I am totally baffled. It seems to me(at least) that the stability of a bullet has to due with how fast it is rotating, not the twist that got it rotating. If a 30 caliber bullet leaves a 12 inch twist barrel at 3000 f/s, it should be rotating 3000 times a second. Let's say the bullet is stable at that rotational speed. Now lets send the same bullet through the same bullet, or a lead bullet of similar shape and weight, through the same barrel at 1500 f/s like we do with our cast bullets. It is now only turning 1/2 as fast but we know from experience that it will shoot well at that velocity. Do bullets have that much tolerance in rotational velocity or am I missing something? I make no claim to mathematics or engineering backround but would like someone to explain errors in my thing.
Twist rates for dummies
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I need to edit my original post. Ignore " through the same bullet" in the 5th line down. Also, the last word should be "thinking".
Just too many interruptions when I was typing it out. I apologize.
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Hi. I think the answer lies somewhere in between where you would to take into account GH's stabilization formula (and it's derivative offspring,) as well as the rotational threshold of cast bullets. Larry Gibson wrote about this on other forums (here's an example.) In my own observations developing cast loads for my newly rebored 260 REM, 1:8" twist, shooting a 140 gr GC bullet of Lyman #2. I started getting unexplained fliers and sporadic/inconsistent groups as I pushed the velocity over ~1600fps (about 142,000 RPM.) I think your comment about 1500 fps being much more accurate bears this out. I know there are certain bullet types and methods where you can exceed this (e.g. NOE 30XCB 165gr FNGC cast with precision.) In addition to stabilization based on bullet length, weight, and barrel twist, as well as the RPM limit, I also think that pushing a cast bullet too fast through a barrel with a quicker twist than optimal creates a shear stress on the cast lead bearing surface that it might not have on a jacketed bullet. Ed
Growing old is mandatory, growing up, however, is totally optional!
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After reading some of joeb's answers and explanation of Greenhills Formula I am totally baffled. It seems to me(at least) that the stability of a bullet has to due with how fast it is rotating, not the twist that got it rotating. If a 30 caliber bullet leaves a 12 inch twist barrel at 3000 f/s, it should be rotating 3000 times a second. Let's say the bullet is stable at that rotational speed. Now lets send the same bullet through the same bullet, or a lead bullet of similar shape and weight, through the same barrel at 1500 f/s like we do with our cast bullets. It is now only turning 1/2 as fast but we know from experience that it will shoot well at that velocity. Do bullets have that much tolerance in rotational velocity or am I missing something?
Yeah, you're missing something, you're ass end to. Greenhill wrote in the black powder, 1500 fps world. In that world, a .3 dia bullet of a certain maximum length in a 12" twist barrel is stable. It's also stable at 3000 fps, and 8000 fps and a zillion fps.
But, stability is a teeny bit affected by velocity, not affected much at all. EXCEPT in the i'm-almosty-unstable region, where the bullet goes from stable to unstable in a small v delta. Because, stability isn't affected my v very much.
As for rpms, good luck. I've been trying to understand the LG rpm story for 15 years, with no luck.
I make no claim to mathematics or engineering backround but would like someone to explain errors in my thing.
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the stabilization of spinning objects is pretty spooky ... although children play with spinning tops that refuse to fall over ... if you spin them faster and faster they get harder to move .. they like to stay where they are ... bullets are like tops ...
dr. mann was fascinated by tops ..... and got interested in why bullets don't all fly into the same hole ... even when they are highly stabilized. ( he finally decided that bullets really fly reliably to where they are directed as they leave the muzzle ... the problem is they are aimed at different places on the target just as they leave the muzzle ) ....
we used to mount our toy car wheels on a ball bearing axle and whizz them up to about 40,000 rpm with an air gun .... it was very hard to change the axial direction of such a spinning mass. spooky ...
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anyway, greenhill gives the least amount of spin to stabilize a bullet .... you can spin it more, it just becomes more stable .... if a bullet is stable at 1000 rpm, you can drop it from 5000 rpm to 3000 rpm and it is still stable .
top a the mornin to y'all
ken
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I'm going to stab a sacred cow here; if Mann was correct then how is it that given the exact same load a 150 gr 30 caliber Speer, Sierra, Hornady or Nosler SP is much more accurate (shoots smaller groups) out of the same 30 caliber rifle as any M2 150 gr bullet? The reason they shoot smaller groups and are more accurate is because they are more balanced and are less affected in flight by the centrifugal force created by the RPM on the imbalances. Given the same load the barrel "nodes" would be relatively the same for all of the various bullets. Mann's assumption, as stated by Ken, is only partially correct. His assumption fails to consider what other forces act upon the bullet while it is in flight. Considering the title of his book I always thought that assumption of his was odd.
Also once a bullet is stable it is stable. There isn't any "more stable". Over spinning a bullet beyond stabilization only increases the adverse affect the centrifugal force has on any imbalances in the bullet during flight. There is an abundance of evidence demonstrating the best accuracy is achieved by a bullet having just enough spin (RPM) for stabilization. Read any modern work on ballistics, study what bench rest shooters use (both cast and jacketed bullets) and look at what the winners are using. A simple explanation, with easy to understand pictures, can be found in the last few Hornady reloading manuals.
Yes, some do have a problem understanding because it can be "spooky" as Ken mentions. However, it is still science, not a "story".
LMG
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WOW! That is a lot of good information. Thanks to all for taking the time to answer my question.
beagle6
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Every bullet has a minimal rotational velocity required for stabilization. It is the twist rate of the rifling, the velocity and the length of the bullet that determines the minimal rotational velocity required. Yes, there is a "constant" in the formula also that is based on the density of the material of which the bullet is made.
Simply put for a given bullet; the faster the rifling twist the less velocity is required for stability.......or the slower the twist the higher the velocity. A 311284 will become stable at a lower velocity in a 10" twist than in a 12" twist.
LMG
Concealment is not cover.........
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The thing that has always perplexed me, Larry, is that the twist rate and the velocity are not proportional. If I double the velocity, I can not cut the twist rate in half and still get the same result. Intuitively it seems as though for any bullet, there should simply be a magic rpm above which the bullet is stable. However it is clear from my own experience that as the angular velocity decreases (although at a much slower rate than the linear velocity) after firing, a bullet that was stable at the muzzle can become unstable in flight. Also while it is true that a bullet can wobble its axis a fair amount while still maintaining accuracy at the target, I believe the bullets wobble has to increase it drag, accelerate velocity loss and ultimately lead to instability. Underlying all of these confusing facts, it is also seemingly evident that two bullets of exactly the same length and same alloy don't necessarily require the same twist rate. The RCBS 95 gr .245 RN and the NOE 105 gr .245 FN are a case in point. They are the same length but the RCBS bullet is measurably easier to stabilize.
None of this is meant to be argumentative. I am simply taking this opportunity to air some of my own rambling thoughts about twist rates and such.
Jim
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I was a bit apprehensive about starting this discussion but now I'm glad I did . There was a lot of good information submitted and and any time there is an exchange of knowledge by such experienced people, it is beneficial to all.
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... a man has got to know his limitations ....
i can see why i can shoot 2 moa with a good cast bullet rig ... heck, hitting a baseball at 100 steps is pretty good stuff ... smacking a target i can barely see without a 10 power scope is very satisfying ... not into the same hole, but into the same baseball ...
but i never could see why my 788 remmy in 44 mag shot 16 moa with 10 different loads and molds and visibly perfect castings .... it made me sad to have to sell this otherwise really fun gun to a collector ... kinda like putting a beloved dog down for a terminal disease ... ( fwiw it shot 3 moa with sierra mj ) ...
how could it be that bad ... then we read about joeb's mysterious occasional large cast groups for unfathomable reasons from respectable rifles ...
maybe we should be looking at horrible groups to uncover secrets of cast ( un ) predictability ... why are they 12 moa .... not 120 moa ? ...
we could form an elite research team of very large group shooters ... i remember the guy that bot my remmy 44 ...
ken
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Well when a gun will shoot jacketed accurately but not cast, my first thought is that the cast bullets in question are too small in diameter. It could be that the 788 in 44 Mag had a throat angle that was not at all conducive to cast bullet use, it's *possible* that adjusting the leade would have helped - but, Ken, you are a damn good gunsmith and I would think you would have tried this sort of stuff.
My own experience, and what I have read on here and in FS, lead me to believe that really bad accuracy is almost always a bullet fit problem, and that in these cases going to a different powder is a waste of time. And bullet fit problems are almost always that the bullet is too small in diameter - in my limited experience anyway.
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Here's a table showing how stability varies as V and Twist.
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SF < 1.0 Bullet is not stabilized
SF > 1.0 Bullet is marginally stabilized
SF > 1.3 = Bullet is fully stabilized
SF > 1.5 = Bullet is maximally stabilized
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Charlie Dell's formula:
(3.5 x MV1/2 x Caliber2) / Bullet Length = Required Twist Rate
Note the MV squared, twist rqd and stability are affected little by V.100 fps ^1/2 = 10, 900 fps ^1/2 = 30; 1600 fps^1/2 = 40, etc.
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Homer Powley:
(20.62/((BL/DIA)^2.25)SQRT(WT/((1-(V/5705))SF)))
= twist;
SF= ((WT(20.62(DIA^2.25))^2))/((TW*(BL^2.25))^2))/1-(V/5705))
= stability factor.
Now let the stability factor wizards begin.
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joeb
In Charlie Dell's Formula, is bullet length in inches or calibers?
beagle6
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Inches. I'd be happy to send the EXCEL workbooks for all of these.
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The thing that has always perplexed me, Larry, is that the twist rate and the velocity are not proportional. If I double the velocity, I can not cut the twist rate in half and still get the same result.
Stability varies and as the square of twist.. Double the twist and Sg quadruples.
Stability varies (almost) linear. Increasing velocity increases Sg by a small, (almost) constant amount.
(Sg vs. V is a sorta log function, but quite flat >1500 fps.)
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Joe
I would like to have EXCEL workbooks for these. I'll send a PM with my email address.
If memory serves the SF target for optimal accuracy is thought to be well below 1.5. There must be opinions regarding the best range of SF.
Thank you Joe. You make me dust off mathematics I haven't used in many years. Being a numbers geek, I find it to be fun.
Jim
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Searcy
The thing that has always perplexed me, Larry, is that the twist rate and the velocity are not proportional. If I double the velocity, I can not cut the twist rate in half and still get the same result. Intuitively it seems as though for any bullet, there should simply be a magic rpm above which the bullet is stable.
No, they are not proportional. Sometimes what we think there "should be" just isn't. Even in the relative simple formula for determining stability of bullets there are variables. Those variables are accounted for in the constant. If we accept the reality of what really is then we begin to see many erroneous assumptions, myths and old wife's tales for what they are or what they aren't. Unfortunately many prefer to believe those erroneous assumptions, myths and old wife's tales instead of learning as we advance our knowledge of what is really occurring.
However it is clear from my own experience that as the angular velocity decreases (although at a much slower rate than the linear velocity) after firing, a bullet that was stable at the muzzle can become unstable in flight. Also while it is true that a bullet can wobble its axis a fair amount while still maintaining accuracy at the target, I believe the bullets wobble has to increase it drag, accelerate velocity loss and ultimately lead to instability. Underlying all of these confusing facts, it is also seemingly evident that two bullets of exactly the same length and same alloy don't necessarily require the same twist rate. The RCBS 95 gr .245 RN and the NOE 105 gr .245 FN are a case in point. They are the same length but the RCBS bullet is measurably easier to stabilize.
Loss of accuracy at longer ranges is most often assumed to be from a loss of rotational velocity (angular velocity?). However most bullets loss accuracy at longer ranges is not because of a loss of stability due to degradation of rotational velocity but because other forces act upon the bullet causing it to lose stability. What happens to 38 SPL WCs out of 18 3/8" twist barrels near or beyond 50 yards is a good example. What causes the loss of stability is the uneven air resistance on the flat face of the bullet overcomes the dynamic stability of the bullet. Increasing the twist (increases the RPMs and thus the dynamic stability of the bullet) with a 16" twist barrel will keep the bullets flying "stable for another 25 + yards but the air resistance will overcome the stability of the bullet and cause the nose to yaw and then the bullet to tumble.
We also must understand that stability, even though necessary for accuracy, is not the only thing that underlies the causes of inaccuracy. There are many other influences on accuracy that have nothing to do with stability of the bullet. Let's take my previous example of the 150 gr 30 caliber jacketed bullets for instance. The typical 150 gr M2 bullet from the military 30-06 gives typically 2 - 4 moa accuracy (10 shot groups) at 300 yards. The bullet holes are all nice and round thus the bullets are stable. Yet a 150 gr Speer, Hornady or Sierra SP over the exact same load at the same velocity out of the same rifle will give 1 moa +/- groups at the same 300 yards. The bullet holes are nice and round, the same as the M2 bullet holes, thus they are also stable. There must be other things/forces at play that make the M2 bullets less accurate than the commercial bullets which tells us that stability is not the only criteria for accuracy. Factually we can shoot some very poor groups with bullets that are stable.
This is certainly an interesting conversation.
LMG
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... just to keep up the sales of aspirin .... maybe the slower twists also are more gentle to our delicate squishy bullets as they " take the rifling " .... gentle leade angles, gentle twist ..
even the mj hunter benchrest shooters use the slowest twist they can get by with ... 17 twist and 115 gr. 30 cal .. 22lr match shooters use a lot of 17 twist barrels ...
which brings up gain twist barrels .... some shoot well, but doesn't the bullet undergo constant re-squishing as the twist rate increases ?? a friend tried a $$$ custom pacnor gain twist on his very good 22lr match rifle ... didn't shoot better ... but just one data dot ...
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my mental model of a bullet entering a short-leade rifling is similar to one slowly sticking one's finger into a fan .... the tip in both cases is trying to twist everything behind it in an argumentative fashion ...
that is why long tapered leads ( ~0.5 degree included ) and matching tapered bullets tend to work well with mushy bullets ... the bullet only has to move forward 0.00002 inches before it is engraved about 80 per cent as much as it is going to engrave.
fwiw, mental models of how things work can be skewed by wishful thinking ... ed harris has it right ... again ...
ken
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Ken
In reality the reason the slower twists have proven to give better accuracy is simply because they do not spin the bullet as fast given an equal velocity. That means there is less centrifugal force created with the lessor RPMs to act adversely on any imbalances in the bullet during flight.
Having recovered 30 XCB and 311466 cast bullets from 10, 12, 14 and 16" twist there was no additional discernible additional damage/deformation from the bullets shot out of the faster twists given the same load/velocity. However, there was a definite discernible difference in accuracy (10 shot groups at 100, 200 and 300 yards), especially at high RPM/velocities. The slower twists always produced the better accuracy.
LMG
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For those interested who have access to Robert A. Rinker's Understanding Firearm Ballistics on pages 141 - 143 there is an excellent and simplified explanation of the Greenhill Formula.
LMG
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JUST DID THIS, INTERESTING-AT LEAST TO ME
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thanks joe ... now all that remains is for " someone " with a 68 gr. 224 bullet to add a data line from real life range results .... i suppose it would have to be based on " wildly unstable " , not just tipping bullet holes .
we need a research grant ... i am thinking a tunnel of ?600? yards of very heavy water mist would stop a bullet undamaged ...
ken
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Joe,
What .98 long, 68 gr bullet are you using for that graph.
Frank
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The main reason that there is such a large accuracy difference, between the M2 and commercial bullets (especially match grade), is the quality control (especially jacket QC) and construction differences, between the two. There will also be a difference in CG, that may make a difference in accuracy.
Frank
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Joe,
What .98 long, 68 gr bullet are you using for that graph.
Frank
Hornady 68 gr hpbtm; this is the heaviest/longest bullet I've used in my Jacketed Bullet Test. In a 9" twist 22-250 bbl, 16.5" long, it goes sideways/very tipped just below 1250 fps. My plan is to slowly reduce v and see how tipping progresses.
The graph is the same SHAPE for any bullet length/weight, not specific to any bullet.
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The main reason that there is such a large accuracy difference, between the M2 and commercial bullets (especially match grade), is the quality control (especially jacket QC) and construction differences, between the two. There will also be a difference in CG, that may make a difference in accuracy.
Frank
You are essentially correct. The fact is the end result of that lessor quality control is the M2 bullets means they have more imbalance in them than do the much better made commercial bullets. Thus the greater imbalances in the M2 bullets give the centrifugal force more to act on. It's as simple as excellent quality (more consistent in jacket concentric thickness in particular) bullets are more accurate all other things being equal. It applies to cast bullets also.
The CG (basically the center of mass) is well behind the CP (center of pressure) on all bullets I mentioned. The CG in an M2 bullet is a tudge bit farther behind its CP than in the commercial bullets. That does mean the yaws and nutation's caused by the centrifugal force acting on the imbalances would have a slightly greater effect. But then we have seen some lots of M2 bullets that shoot very accurately so how is it that the CG is the cause of inaccuracy? Why is it Hornady M80 bullets (150 gr FMJBTs) shoot more accurately than most milsurp M80 bullets? The reason is simple, as noted, the bullets, particularly the jackets, are much better made with less imbalances. A purview of the section on bullet accuracy in Hornady manuals explains it.
The goal is to get the bullet out of the muzzle as balanced as we can. Otherwise the more unbalanced, the less accurate.
LMG
Concealment is not cover.........
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As a hole, the shape of a bullet, dictates the CG. Tapered jackets and cannalures as well as lead density have a minor effect on jacketed. Lube grooves and their position on cast. Imperfections on both.
As Joe points out, most stabilization programs don't allow for CG in their programs.
While I can't say how much the position of the CG can effect accuracy, I can say the the position of the CG can and will effect the stability of any given bullet, in any given twist rate.
The stability program that I use calculates the appoximent GC, by bullet shape and also calculates the amount of increased twist that it takes to keep a bullet point on, in the transonic range, where the building and uneven sonic waves play hell with the bullet.
I've posted the Geoffery Kolbe program in the past, more than once, on this forum and got no questions or feed back. I can only assume that most think it's BS or don't know how to input the variables but, it's the most accurate program that I've found and will answer questions that many scratch their heads about.
Before I could afford a computer I knew the GH formula and used it but, there cam a day that I found that the famous 150 constant didn't dictate stability. A good friend that I got into Schuetzen shooting built a rifle with my info. I was new to the sport (1985) and I thought all 32/40 where 16 twist so, I told him to get a Douglas 16 twist barrel and the same mold that I used a 1.165 long Ron Long.
Well when we start shooting together, his bullet tipped noticeably, mine didn't and never had. So, I measured my twist, it was 15! What to do, I had cost him a lot of money in my ignorance! I then took his mold and cut .050 off of the base, his bullet straighten up and was competitive with my rifle.........SAVED.
I then used that length bullet in the GH formula changing the constant to 175 and used that number until I got one on a computer.
My second learning experience about 1990:
In my 15 twist rifle, I was able to stabilize the 323471, well enough to shoot a .43 group at 2100 fps. The 323471 is 1.215 long. A friend that I shot with, Stan D. (can't spell his long name) but, he was well known, here in the PNW, that was making bullet molds, he offered to make a custom mold for me. It is 1.15 long and a ogive of .38, weighing 209 gr. The 323471 is 1,22 long with a ogive of .57, weighing 220 gr.
You would think that the 1.15 long bullet, would easily stabilize in the 15 twist, if the 1.22 long Lyman does. It was NOT to be! The 1.15 long bullet had noticeable tipping and I couldn't use it. The CG is the difference.
Regarding yaw, I don't think it effects the BC very much at super sonic speeds, so long as it stays fairly point on. In SS speeds the shock wave is more important that the air waves. It's the point of the bullet that pushes the shock wave, the flatter the point, the harder it is to push the shock wave and the more velocity you loose. That is why spitzers excel at SS speeds, at least regarding BC.
If I could get a drawing of Joe's 68 gr bullet, I can run some figures for him to compare with his velocity experiments.
Frank
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These are the graphs for my shortened and original length Bullets. The original length is 1.16, the shortened (by cutting off .050 from the base band only) 1.11. The spitzer nose is .53 long, with a .075 meplat.
First is the 1.16 in 16 twist, Second, 1.11 in 16 twist.
The last graph is for the 1.16, in a 15 twist.
Frank
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I just want to say this thread has gone well beyond twist rates for dummies - good stuff!
Jim
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This is how Joe's, Miller/GH program deals with the same bullet/twist combo. Note that the Sg figures, for both, say the the bullet won't be stable, especially for the 1.11 long/16 twist.
And the last picture is the match results that I shot with the 1.11 long/16 twist combo.
Frank
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Empirical results outweigh theoretical musings.
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Here are the solutions:
.321"Dia, 190GR, 1.11" long, 1450FPS, 16"twist
GREENHILL 13.9"
DELL 12.4"
POWLEY WITH SF = 1.3, 17.7"
MILLER WITH Sg = 1.243, 16"
Greenhill and Dell suggest that a faster twist is required, Powley and Miller suggest that a 16" twist is at least marginal.
Same story for the 190 grain bullet.
However, I'm missing Frank's point. Please explain.
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My point is a comparison of the different stability programs and their accuracy, when it comes to bullet shape and there for CG.
The one I use, indicates the the 1.11 bullet is fully stable and the accuracy, shows that it's fully stable.
A Sg of 1.243, is not fully stabilized and the bullet should not be able to create the accuracy that it demonstrates, in the match results.
I don't believe anyone would choose a twist/bullet combo in the 1.24 range, either. Miller doesn't suggest anything under 1.4 Sg.
If you change the GH constant to 173, it gives a twist of 16. I found when I shorten that mold, that I could use 175 as a safe constant for my twist calculations, for Spitzer bullets. Spitzer bullets have their CG closer to the base that other common designs.
The program that I use calculates the CG of a bullet, when producing a Sg factor.
Frank
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I'm a little mentally challenger here - can anyone explain the graphs from the Kolbe calculator?
All my crunching for a .338-06 and 1850ish accurate 247L 1.13" long shows a 14" will work.
Having a tube rebored and don't fancy making a mistake.
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750k2
Don't fret it, the14" twist will work just fine with that bullet.
LMG
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Perhaps I am over simplifying this, but, just like the front tires on your car, the more the imbalance of the rotating object the more unstable it becomes as the linear velocity increases. Is that close to what you are saying about bullet quality?
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In a neighboring thread (Trigger work on Ruger #1) OU812 demonstrates that a 12" twist will stabilize a 22 bullet nearly .9" long at 1,800 fps.
If memory serves, a dubious assumption, Joe has confirmed that a 12" twist will stabilize this bullet
By my calculations that requires a GH constant over 200???
Joe, Larry, Frank and anybody else interested do the other methods of estimated needed twist predict this?
John
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John
I've tried to impart in these "stability" threads, as has Frank I believe, that there are other variables involved. We all know joe lives and breaths hard and fast numbers but bullet stability is not hard and fast regardless of how much data is presented because it will then only apply to the bullets/velocity/twist used to obtain that data. Joe's presented data clearly demonstrates that sometimes his loads with that bullet in that twist barrel sometimes marginally stabilize the bullet and sometimes not so marginally stable......
Some apparently have a problem understanding that bullet stability is not like a woman.....she is either pregnant or she isn't. A bullet isn't either stable or unstable because there are degrees of stability which all can have an effect on the obtained accuracy. Also understand there are other variables that effect accuracy that are apart from and have nothing to do with the stability of the bullet. The examples given by joe and Frank illustrate that. Both are good illustrations of marginal or partial stability with a quality bullet where the bullet is flying point forward and giving some semblance of "accuracy".....good, bad or indifferent depending on our individual perspective.
Every bullet can be not stabilized (flying sideways, "tumbling" or reversing and flying base first......btw; few bullets will actually tumble end over end in flight, even from a smooth bore barrel with no spin imparted to the bullet).
Bullets can also be marginally stabilized, fly point forward and give some semblance of "accuracy as seen in examples posted by joe and Frank.
Many bullets with Sg's of 1.4 to 1.5 are actually fully stabilized and have been found to most often give the best accuracy at both short range (100 yards) and long range (out to 1200 yards+).
Bullets can also be "over stabilized" which is actually a misnomer as once a bullet is stabilized that's it......just like a woman, once pregnant she is, no "partial or "over" about it. "Over stabilized" bullets are actually over spun is all. The over spinning can and does create higher RPM which increases the effect the centrifugal force has on any imperfections or imbalances in the bullets. However, while so called "over stabilized" bullets can appear to be very accurate at shorter ranges the "over stabilization" can be problematic for the bullet at longer ranges. Over stabilization may inhibit the bullet to turn nose downward as it arcs down at longer ranges. Over stabilization can also be problematic for bullets with a larger distance between the CG and CP when the bullet enters the transonic phase of it's trajectory.
The higher constants in the formulas is intended for use with longer bullets than was used in the Greenhill formula development. However, unless one is using really long for caliber bullets of the VLD type Greenhill still works well for general use with cast bullets.
LMG
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In a neighboring thread (Trigger work on Ruger #1) OU812 demonstrates that a 12" twist will stabilize a 22 bullet nearly .9" long at 1,800 fps.
If memory serves, a dubious assumption, Joe has confirmed that a 12" twist will stabilize this bullet
By my calculations that requires a GH constant over 200???
Joe, Larry, Frank and anybody else interested do the other methods of estimated needed twist predict this?
John
W/O the nose length, meplat diameter, specs of the BT (if it has one) and type of ogive (Tangent or secant) I can't run it in my program. Those are the things that set the CG of a bullet.
Frank
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I'm a little mentally challenger here - can anyone explain the graphs from the Kolbe calculator?
All my crunching for a .338-06 and 1850ish accurate 247L 1.13" long shows a 14" will work.
Having a tube rebored and don't fancy making a mistake.
I can shoot bullets, at least 1.32 long (The NOE 33 version of the 311365), in my 14.5 twist barrel, at ~1450 fps.
Frank
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I have quite a rebuttal for LG's post but, for some reason, the forum, won't let me post pictures.
( Can any of the monitors address the reason? I've posted many pictures on this forum, even the one that I want to post today.)
But, regarding over stabilization and long Range (LR). The black Powder (BP) shooters, that shoot 1000 - 1200 yards, over stabilize their bullets. In 45 cal, they shoot bullets around 1.5 long, in as fast as a 12 twist, most are in the 16 twist range. That combo (16 twist, 1.5 lg bullet. Miller formula) is 2.76 Sg @ 1200 fps. Their flight is mostly subsonic.
I don't shoot that discipline but, have talked to many and read their input about LR shooting. Today, 16 twist, is considered minimum for their LR shooting.
Regard LR angle. If a bullet travels 1000 yards, it's basic up angle is .229 and down angle the same or .458 deg. if it rises 144" at it's max trajectory and it's point would be skewed less than .012, to the base.
Frank
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Frank sez:
Length .852"
Nose length - .312"
secant ogive
No meplat
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My program gives a Sg of 1.17 @ 2000 fps. With a 6 cal secant ogive.
Frank
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Frank
"I have quite a rebuttal for LG's post but, for some reason, the forum, won't let me post pictures.
But, regarding over stabilization and long Range (LR). The black Powder (BP) shooters, that shoot 1000 - 1200 yards, over stabilize their bullets. In 45 cal, they shoot bullets around 1.5 long, in as fast as a 12 twist, most are in the 16 twist range. That combo (16 twist, 1.5 lg bullet. Miller formula) is 2.76 Sg @ 1200 fps. Their flight is mostly subsonic."
Once again you've come up with basically moot rebuttal points to what I said. Your example of LR BP bullets are subsonic and not coming back down through the trans sonic velocity range when they start back down from mid range. They also have a CG closer to the CP than does any VLD jacketed bullet or one with a long pointy nose such as joe is talking about. A 165 SPBT out of an '06 runs a Sg of 2.75 +/-......hardly "over stabilized".......but then that bullet is not a long pointy nosed one. Now if we put that in an 8" twist we end up with 50k+ more RPM and the Sg goes up to 4.03.....now that's over stabilized/over spun.......Also putting a thin jacketed 55 gr SX or 50 gr Blitz in a 7 - 9" twist .223 at 2900+ fps is why they spin apart inside of 50 yards......because they are over stabilized/over spun......
The blunt nose bullets (that's a relative comparison to the type of HV bullets used at LR) you're talking about which the BP LR shooters use with high sectional density are less prone to buffeting in the transition from sonic to subsonic velocity at long range but, as you mention they shoot them subsonic any way. Compute the angle all you want to attempt to minimize the effect but it does happen to the over stabilized bullets I was referencing.
LMG
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LG,
Miller says that 2.0, is the max that should be used and anything above that, may start the decay in accuracy (paraphrased).
I think you missed my point, I said that 16 twist, was the minimum that they use. They use up to a 12 twist, with good results and some wins. 12 twist is 4.92Sg.
Frank
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My program gives a Sg of 1.17 @ 2000 fps. With a 6 cal secant ogive.
Frank
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Thanks Frank,
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Of course Frank, I'm sure the BP boys will be using 7" twists pretty soon. Been to a several LR BP matches (just observing) the last couple years and most were using 18" twists in their 45s.....they must all be so last century like me......
LMG
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My program gives a Sg of 1.17 @ 2000 fps. With a 6 cal secant ogive.
Frank
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Thanks Frank,
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The BP, LR guy's haven't use a 18 twist, for the top guys in about 6 years, maybe more.
18 was the standard twist for many, many tears, I had a RB built with a 18 twist in 1985 and it had been used for many years before that.
I, myself, don't understand the reason a tighter twist is producing better scores but, it is. They all have their theory's but, know one that I've talked to, really understands it, in real terms. It just does.
Frank
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Frank
"I have quite a rebuttal for LG's post but, for some reason, the forum, won't let me post pictures.
But, regarding over stabilization and long Range (LR). The black Powder (BP) shooters, that shoot 1000 - 1200 yards, over stabilize their bullets. In 45 cal, they shoot bullets around 1.5 long, in as fast as a 12 twist, most are in the 16 twist range. That combo (16 twist, 1.5 lg bullet. Miller formula) is 2.76 Sg @ 1200 fps. Their flight is mostly subsonic."
Once again you've come up with basically moot rebuttal points to what I said. Your example of LR BP bullets are subsonic and not coming back down through the trans sonic velocity range when they start back down from mid range. They also have a CG closer to the CP than does any VLD jacketed bullet or one with a long pointy nose such as joe is talking about. A 165 SPBT out of an '06 runs a Sg of 2.75 +/-......hardly "over stabilized".......but then that bullet is not a long pointy nosed one. Now if we put that in an 8" twist we end up with 50k+ more RPM and the Sg goes up to 4.03.....now that's over stabilized/over spun.......Also putting a thin jacketed 55 gr SX or 50 gr Blitz in a 7 - 9" twist .223 at 2900+ fps is why they spin apart inside of 50 yards......because they are over stabilized/over spun......
The blunt nose bullets (that's a relative comparison to the type of HV bullets used at LR) you're talking about which the BP LR shooters use with high sectional density are less prone to buffeting in the transition from sonic to subsonic velocity at long range but, as you mention they shoot them subsonic any way. Compute the angle all you want to attempt to minimize the effect but it does happen to the over stabilized bullets I was referencing.
LMG
Frank,
A few of the data points concerning NRA Black Powder Creedmoor above are not likely common or outright in error.
Most of the BPCR Creedmoor ammo has a muzzle velocity between 1300 and 1400 fps arriving at the target between 900 and 950 fps. The LR bullets start their flight in the upper trans-sonic and really don't exit it. However, the flight drops below 1100 fps somewhere near 400yds. So strictly speaking, yes, 6/10ths of the flight to the long line is subsonic.
Bc is dynamic.
Most Creedmoor bullets used in competition are between 1.375" and 1.450" long. There are exceptions.
There are trade-offs in the Creedmoor game.
More twist = more precession.
Less twist, even marginally stable bullets can produce fantastic LR accuracy.
The number of Serious competitors having tried something faster than a 16 Twists, like 14T = a fraction of one percent.
The number of Serious competitors having tried something faster than a 16 Twist like 12 T =
well??? I am sure there was one sometime. Good luck on that.
The wind is your friend at a Creedmoor match. Why, ?? ...anyone can hold center and pull the trigger. Getting flags to point the same direction for 1000yds often does not happen. And when that sheer condition fires up the bullets experience wind sheer. Then more twist is your friend.
Bullets at long range tend to fly nose into the wind. Holes in the target form fully stable ammo often show what is mistaken for tipping.
Two or more sheer or cross winds on the bullets flight is a long conversation. Longer than I am going into here.
In 45 cal Creedmoor rifles18T, 16T and a few 17T barrels are in use. The ODG's (old dead guys) rifles; The 1874 Creedmoor rifles mostly used 20T barrels. Long Island is often known for mild steady wind.
Winning in the Creedmoor matches is a combination of managing your BP rifle in hot condition, being a great rifleman which means reading the condition correctly every shot and last, accuracy. If I were to have weakness in one area and still expect to be competitive, I would elect to shoot a 2MOA rifle rather than have a temperamental - sometimes one MOA rifle.
Enough for now,
Michael Rix
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"I would also check the depth of the lead core, that ill have a big effect on the CG, if it doesn't extend very far from the base.
Thanks for your comments Frank. The bullet was a slightly shortened NOE 27780 SP -- a cast bullet. This remains a mystery.
John
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John, I agree about it being mysterious.
OU812, have you measured the actual twist rate of your rifle?
Michael Rix,
I'm sorry that I was misleading the use of twist rate. As I said, I've never competed in LR. My knowledge, comes from the ASSRA forum, when LR is talked about.
I confused the 12 twist with what has been used in 38 cal. But, as for velocity, I was speaking of the 45/70 and those velocitys are in the 1175 (I meant 1157)-1275 range, depending on what brand and grad of powder and compression used.
Isn't the 45 cal "Money" bullet in the 1.47-1.51 long range?
Frank
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Is there another game afoot leading to inaccuracy other than "stability"?
I shot this target today at 300 yards. It is with pulled M80 bullets (147 gr FMJBT). They were loaded in the .308W for potential use in my 14" twist Palma rifle (27.4" barrel). The M80 bullets were all brass jacketed and had been pulled from various different lots of ammunition. I had sorted them by length with the 12 bullets on the target shown being the longest at 1.170" +. The majority (600+ bullets)were sorted into 3 separate groups by length of > 1.149, 1.150 - 1.159, 1.160 - 1.169. There were only the 12 bullets that exceeded 1.170" in length. Previously fired 20 shot test groups (shot today) with each length showed they were 1.8 to 2.4 moa capable at 300 yards. All test shots were chronographed with an Oehler M35P chronograph.
The load for all 4 length test was the same; Winchester Palma 308W fire formed and NS'd cases, WLR primers, 46.5 gr IMR4895.
Here is the Oehler printout of the 12 shots;
The mean average was 2848 fps at 15' from the muzzle so probably close to 2860 fps at the muzzle. The Sg (Miller) for that out of the 14" twist barrel is 1.26. The bullet is turning 147,000+ RPM. Looking at the 300 yard target we see all the holes are perfectly round, again at 300 yards, even though the Sg is only 1.26. Note 10 of the shots are in a group of less than 2 moa (the bull is 6" or 2 moa). All is apparently fine with those 10 shots. However, note the bullet hole out at 2 o'clock in the 5 ring and the bullet hole down at 8 o'clock cutting the 6 ring.......both were called good shots and if all was well should have been "in the group". Yet they aren't........and the both holes are perfectly round and of bullet size giving no indication of pitch, yaw or tumbling.........
Observing this perhaps it seems another game is afoot with other forces causing inaccuracy......
LMG
FYI; I was tracking each shot and there was no correlation in vertical impact based on the velocity.
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As everyone says, accuracy and stability, are to different thing. I'm surprised that you got as good a group as you did, with the ten shots. The M80, is more of a pray and pray type of bullet, regarding it's GQ. Not to mention that I'm not sold on any HV bullet, with a open lead base. They are also made by several sources and no way of knowing if they all came from the same source.
My guess is a defect in the two fliers.
But, overall this is another instance that the Miller program, ignores the CG of the bullet in it's calculation of Sg.
Frank
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Larry says:
"FYI; I was tracking each shot and there was no correlation in vertical impact based on the velocity."
================
I think this is a valuable piece of information to keep in mind. I have found the same thing with hundreds of groups at shorter ranges.
Do we have similar information, or different type of information, about what happens at 600 yards and beyond?
John
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As everyone says, accuracy and stability, are to different thing.
Frank
My test results suggest that there is a u-shaped relationship between group size and stability-that min group size occurs as mv falls, close to unstable, then rises until Wild Thing..
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Michael Rix,
I'm sorry that I was misleading the use of twist rate. As I said, I've never competed in LR. My knowledge, comes from the ASSRA forum, when LR is talked about.
I confused the 12 twist with what has been used in 38 cal. But, as for velocity, I was speaking of the 45/70 and those velocitys are in the 1175 (I meant 1157)-1275 range, depending on what brand and grad of powder and compression used.
Isn't the 45 cal "Money" bullet in the 1.47-1.51 long range?
Frank
Hello Frank and thanks for posting a reply as well as researching BPCR long range on the other forum. I followed that but did not want to post there. Sorry I have been slow to acknowledge you getting back to me via this post; my excuse is upon returning from the Butte, MT Schuetzen match last week I caught the most wicked cold I have suffered through in 5 or 10 years and have been a slug. I imagine I was exposed to while on the road.
Back on topic, these conversations about stability, accuracy, twist rate, sustained rotation and rate of decay and all the formula that goes with it have been beat around the shooting forums for as long as the net has made them possible. (that is a good thing!) Long range shooting provides a great view of what bullets do. Shooting to 200 or even 300 yards only begins to reveal what (sometimes) becomes greatly exaggerated by the longer distance and flight time of the 800, 900 and 1000 yard match events. In other words, some of this stuff really shows up at the longer distances.
Having put in a lot of time shooting different bullets (mold of the month club) checking Bc with two chronographs, reading highly technical books by specialists in the field (choking through some of it) inputting my data on the old McCoy based website Eskimo if you remember that one, the lay-rifleman like me only gets an impression of what is going on. It is VERY hard to make concrete statements based on best guessing. I think we can do fairly well in this area but empirical evidence with the limited tools we have can only be so good. I like to say we get an impression of what is going on.
Those of us that do proclaim to know "everything" make me smile when there is just too much noise in the data.
I've made the US Creedmoor team four different years and won the World Creedmoor championship in 2002 and placed second in 2004 and won the NRA nationals in 2009 and gotta tell you that there is a lot I don't know! But I have had what seams like endless email strings and late night hotel lobby conversations trying to sort through what works and what keyboard experts proclaim. Sorry if that sounds harsh. I don't mean it to.
Isn't the 45 cal "Money" bullet in the 1.47-1.51 long range?
I am not sure what you are asking here. However, I can tell you the earliest two molds cut are 1.465" long for the 45 cal and 1.375" long for the 40 cal. These were made by Paul Jones.I better quit typing, I'm pooped. More if you want, later.
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Michael Rix
Your posts are much appreciated. Hearing from one who has shot LR BPRC as successfully as you have brings a lot of factual information to the discussion on what works and what doesn't. I certainly concur with your statement' "Long range shooting provides a great view of what bullets do. Shooting to 200 or even 300 yards only begins to reveal what (sometimes) becomes greatly exaggerated by the longer distance and flight time of the 800, 900 and 1000 yard match events. In other words, some of this stuff really shows up at the longer distances." I have been attempting to impart that for some time here as it applies not only to LR BPCR but to other cast bullet and jacketed bullet shooting also. Thanks for your valuable input.
LMG
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"But I have had what seams like endless email strings and late night hotel lobby conversations trying to sort through what works and what keyboard experts proclaim. Sorry if that sounds harsh. I don't mean it to."
Truer words were never spoken. I am always leery unless I see "I don't know for sure.." or "I think.." in the paragraph. And I am a guilty as most about not saying it more often. "This is what works for me and what I think is happening."
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Michael,
Again, as you know, I'm not a LR competitor but, I do lesson to what is said, regarding it.
My point in my first post, was that LR guy's, over spin their bullets, in comparison to shorter range shooters.
I was in error saying that 16 twist is the minimum they use but, there are a lot of guy's using 16 twist, in 45 cal and some think, that twist is superior to 18 twist. Again I was in error about the 12 twist, in 45 but, you'll have to admit that 14 twist, has been used. BTW, 14 is the standard twist rate for the 458 Winchester Mag.
Yes, Paul Jones, invented the "Money" bullet but, there are many versions of it, today. I think Paul, also made longer version of it, too at customer request.
But, there again, I was generalizing what is used, i.e 1.5 bullet long bullet At 1200 fps.
Using your bullet twist info, the Sg of the Money bullet is 3.05, more than 50% over the Miller recommendation. The shorter bullets, even higher.
Many guys have shorten there bullet, because they are tipping at 800-100 yards and beyond, increasing the Sg, at that distance.
Frank
Edit:
What I was trying to communicate, is that LR cast bullet shooters, use a higher Sg than any other cast bullet shooters, that I know of.
Many authority's , say that if a bullet is stable when it leaves the muzzle, it will still be stable, before it hits the ground.
So, Sg's that LR shooters use, are curious to me. I do know that you guy's need them but, I don't know why.
In addition, I remember reading a old test of the Trap Doors, shooting, I think, at least a 1/4 mile, with I assume, standard 22 twist/500 gr loads.
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Now, since I'm back to being able to post pictures. My rebuttal to LG's assumption that my tipping bullets, caused be transonic conditions, were on the verge of being unstable.
This target was shot from the same rifle. The bullet is the NOE, 33 caliber version of the 311365, 1.32" long. .13" longer than the bullet, that is tipping so much in the target I posted, that bullet is 1.19 long.
This bullet is heaver by 10 gr and the powder charge, is a full 1.0 gr less at 12.0. I did not chronograph the load but, it has to be going through the 200 yard target, at below the speed of sound.
Pictured are the two bullets and the target fired with the 1.32 long bullet.
Frank
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Well, I was able to post only two more pictures, I'm not able to post the target but, the target has a 200 yard group, about .8 tall and 2.5 wide. Edit; I measured it and it's actually 1.99 wide.
I will post it ASAP, when the forum let's me.
Again, monitors, what is up with not being able to post pictures?
Frank
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Frank
Your assumption as to "LG's assumption that my tipping bullets, caused be transonic conditions, were on the verge of being unstable" is incorrect.
What I said was; "Bullets can also be marginally stabilized, fly point forward and give some semblance of "accuracy" as seen in examples posted by joe and Frank."
I did mention some bullets can lose stability when dropping back through the trans sonic zone in a later paragraph. Perhaps you misconstrued one with the other in making the incorrect assumption?
LMG
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Ok, we'll use "marginally stabilized".
If my 1.19 long bullets are, marginally stabilized. How is it that I can shoot a .13 longer bullet, at a lower velocity and still have "good" stability?
Still can't post a picture. Monitors HELP!
Frank
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Frank
As I said earlier, stability isn't hard and fast........you have 2 different bullet designs there......different nose profiles, different lube grooves, and different locations of the CG to CP, the meplat on one (moves the CP forward), different BCs, etc.
Have you the actual chronographed velocities? With that knowledge we can compute where the bullets (within a reasonable distance) could have been buffeted or otherwise adversely affected in the trans sonic zone. Without that knowledge we are only making assumptions which, as we've seen, if based on "I don't know for sure.. or I think.....or I assume...." most often aren't correct. Thus if you have the actual velocities ?
LMG
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Larry,
Regarding your (damn good BTW) target with the 2 "fliers" - what were mirage and/or wind conditions like? Out beyond 200 yards anyway wind, particularly wind near the target that the shooter may not be aware of without good enough wind flags, can move shots around quite a bit. Great work that you tracked the velocity and know that velocity variation was not the "root cause".
This thread should be re-labeled something like "Twist effects for the Advanced Shooters and Thinkers".
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frank .. don't know but if the pic is " too big " it is rejected. ... too many pixels ...... you might need to take less dense pics or edit it skinnier in a photo editor.
or send it to my email, i will give it a whirl. [email protected]
ken
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Frank, I am thinking, ….dangerous, I know, that you have developed great interest in black powder long range shooting and you could test your theories buy barreling your old Roller with the 458 -14 twist and shoot the matches next summer in the PNW. That would be a good lead into the MT match in September 2019. I may try to make that one if I don’t draw a BP ML elk tag. We can ask to get squaded so that I can pull and paste your targets in the pits.
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Michael,
First, if you where a member of the 2002 US Creedmoor team, you must have known Bill Crane. We shot together in the Springfield, Oregon Schuetzen matches in the 80's and early 90's. I believe he obtain the barrels for those rifles.
If you've read the ASSRA thread, you know that I've had Rotor Cuff surgery (rt shoulder, of course) I tore it shooting my Borchardt 45/70, in a BP match in 1995/6.
The only way that I might compete, is with a 38 and that, I'm investigating. But, it seems that the 38's (other than one or two) aren't doing well past 800. It would be a great teaching experience though.
LG, it's you that use and promote the Miller formula. It gives no allowance to bullet shape. That something that I'm try to help people understand, with real time experimentation and my stability program.
I do have empirical documentation of the 13.0 gr load. No, I did not chronograph the 12.0 gr load, it had to get to the 200 yard target, with it's 10 gr of extra weight, at a much lower velocity and all your assertions of CP, would make it less stable at any range and velocity.
I have another target at 11.5 gr and a 1.21 long bullet, showing stability at 200 yards. I'll post it too, when I can.
Frank
Got it. It wasn't the size but, I changed the file name. It seems (maybe) if the system rejects a file, you can never post it again. I don't know why it would have been rejected, in the first place?
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This 11.5 gr target, above, was my first, 200 yard target, after sighting in, when I first built the rifle. The mold was on loan to me. I kept going up in my powder charge as I progressed and at the Modesto match, I made my final powder charge increase, to 13.5 gr #9. My next 10 shot group target, at my home range, was 1.44 @ 200. Then a few months later I shot a .463, 5 shot group @ 200. I haven't change the powder charge since. All with the 1.19 long bullet.
Frank
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Frank
"LG, it's you that use and promote the Miller formula. It gives no allowance to bullet shape. That something that I'm try to help people understand, with real time experimentation and my stability program."
Again, you're making an erroneous assumption and then make an incorrect statement based on that assumption. I use several twist formulas including Miller's. I do not "promote" Miller's formula for cast bullets because the constant in it is based on long for caliber VLD type bullets. I have stated that in these threads. The Greenhill and Miller formulas were the ones mentioned in these threads by joe, YOU, myself and others.
In case you've not noticed I have been supportive of your contention that the on target results don't always coincide with the formula's answer, especially when the constant, which considers the shape of the bullet, is not correct. Suggest you take rmrix's advise and try some real long range shooting. You might find it interesting and instructive.
LMG
FYI; I'll add that many programs also require a BC which does consider bullet form and is used in the down range ballistics part of the programs (includes stability in some). The BC is calculated or entered using a "standard form" of which there are several. The G1 form is most often used with most bullets. It's use gets you very close in most instances if the other input data is correct. However, for use with long VLD type bullets the G7 form is considered more accurate.
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i don't have a dog in this flight
( see what i did there ? ... flight ... heh )
but about 20 years ago i bot a program that used REAL bc, then was advised that different bullets have different bc at different velocities and humidity etc., so
you need both the horse AND the cart to forecast what is going to happen to your bullet in respect to trajectory ... you need some of the results to improve your prediction .... apparently the same holds true for effects of spin stabilization ...
*****************
these threads on stability reminds me of watching the olympic bobsled runs .... starts out high, easy at first, then shortly starts bouncing from side to side, and all the time going faster and faster downhill ...
but really fun to watch, and just as we hit bottom , we discover that we have gleaned something from the effort after all ..
****************
oh, and just like the bobsled run, i choose to go the safer route and be a supportive observer rather than a participant ... ( g ) .. thanks for the show ...
ken
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Michael,
First, if you where a member of the 2002 US Creedmoor team, you must have known Bill Crane. We shot together in the Springfield, Oregon Schuetzen matches in the 80's and early 90's. I believe he obtain the barrels for those rifles.
Hello, Sorry, I don't know the name Bill Crane. He was not at the match held in September of 2002, Whittington Center, Raton that I know of. No one provided rifles or barrels to us. All competitors provided our own personal equipment and ammo.
You must be confusing two events.
You mention stability and the Springfield rifle (45-70) at a 1/4 mile? I imagine you might be referring to the US Army shooting trials conducted sometime in the 1870's at Sandy Hook. Do a search, it is interesting. The 45-70 was shot to maximum range and the impacts were plotted for angle, distance and more. A good read!
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Michael,
The event that I'm speaking of was shot in the UK. The US team won, I believe.
I did read the article, on that US Army event, probably 20+ years ago
Frank
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LG,
What I'm trying to point out, is that you stated or inferred that my 1.19 long bullet had marginal stability.
What I'm trying to show, with my targets, bullets and powder charges, is that the 13 gr powder charge, shown stable at 100 yards with the M43, was still gyroscopically stable, while being push around by transonic forces at 200 yards and the transonic forces, where causing the tipping, not instability. It would have recovered after it left the transonic range.
Frank
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That's all well and good Frank because that's exactly what I've been agreeing with you about.....and trying to support......
"What I'm trying to show, with my targets, bullets and powder charges, is that the 13 gr powder charge, shown stable at 100 yards with the M43, was still gyroscopically stable, while being push around by transonic forces at 200 yards and the transonic forces, where causing the tipping, not instability."
Exactly what I've been saying and my posted targets demonstrated also. Yes, there is other games afoot besides loss of stability that can cause the bullet to pitch, yaw or tumble.......or fly off away from the intended line of flight.
LMG
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Larry,
After reading the NOE information several times, could you (or someone) explain how to do the cg and cp balancing acts. Why do we need a scaled up cardboard cutout for cp?
If we hollow point a bullet, do cp and cg move the same direction or opposite?
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Larry,
After reading the NOE information several times, could you (or someone) explain how to do the cg and cp balancing acts. Why do we need a scaled up cardboard cutout for cp?
If we hollow point a bullet, do cp and cg move the same direction or opposite?
THIS IS KINDA INTERESTING.
Cp is explained on the net, serious math involved, not for the feint of heart'
BUT,
https://spaceflightsystems.grc.nasa.gov/education/rocket/rktcp.html
includes
Mechanically determining cp
For a model rocket, there is a simple mechanical way to determine the center of pressure for each component or for the entire rocket. Make a two dimensional tracing of the shape of the component, or rocket, on a piece of cardboard and cut out the shape. Hang the cut out shape by a string, and determine the point at which it balances. This is just like balancing a pencil with a string! The point at which the component, or rocket, is balanced is the center of pressure. You obviously could not use this procedure for a very large rocket like the Space Shuttle. But it works quite well for a model.
which is an explanation of how to find Cg, not Cp, so even NASA is having trouble with Cp. As far as I've ever found, there's no simple way to find Cp, which changes for a bu7llet as it goes through the air.
To estimate Cg more easily make a copy of the bullet drawing, draw a line thru the middle lengthwise, and measure from the cut to edge of the bullet at various measured points a lotta times, sum them up and get the avg, that's Cg.
Or, measure your half bullet dwg. length, (Ex 5"), get excel to write a list of random #s, "Y coordinates, (Ex 2.1, 4.1, 1.3, …from 0 to 5"), measure the center to edge, avg both and that's Cg.
Or, just understand that for any bullet-shaped item, Cg is ALWAYS < 50% of total length from the base. And, that knowing Cg is never going to be of use in the shooting world.
However, talking about such things lends one an aura of knowledge and sophistication that fools some of the people, some of the time.
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After looking at NOE bullet diagrams and using a design which is offered in solid and hollow point, I found the answer. The cg moves closer to the base in a hollow point; the cp stays the same.
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the cg has to do with the center of mass in the bullet ... the mass is in the bullet .
the center of pressure has to do with PRESSURE on the bullet by outside forces, such as the thousand mile per hour headwind the bullet sees on it's trip to the target.
if you hold the bullet in your hand it doesn't have a center of pressure ( except from your hand ... and that pressure depends on where you pick it up ) .
***************
with all this turmoil, it is a wonder the dang bullet ever gets all the way down to the target. amazing ...
ken
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joeb33050 4
