HOW ABOUT A TAPERED BUMP DIE ?

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  • Last Post 08 February 2019
R. Dupraz posted this 06 February 2019

While presently being locked into the Arctic's grip for apparently another thirty days, this feeble mind has been hard at work dreaming up projects to hold the world class boredom at bay. Therefore the question is...…

Is a tapered bump die where the taper matches the leade angle of the rifling an effective tool in improving the accuracy of the  lead bullet ? If so, does it produce noticeable results?  Or not worth the time and effort. 

 

R.

 

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RicinYakima posted this 06 February 2019

Well, I have used an Egan one and half degree tapered nose first sizing die for about 10 years and have been happy with the results. But I never did a side by side test. Plus all of my bullets are oversized and don't need the bumping part, just the taper of the first band to a completely seal of the throat.

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Maven posted this 06 February 2019

Is a tapered bump die where the taper matches the leade angle of the rifling an effective tool in improving the accuracy of the  lead bullet ? If so, does it produce noticeable results?  Or not worth the time and effort. 

Years ago I purchased one from Ed Wosika (The Hanned Line) made to measure for my M 70 Win. (.30-06).  It has consistently turned so-so performing CB designs into m.o.a. shooters.  Btw, it works with both plain base and gas checked CB's + it flattens the base of either very nicely.  In short, it was well worth it for me and my rifle.

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onondaga posted this 06 February 2019

How about picking a mold that just fits as cast?

Gary

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Scearcy posted this 06 February 2019

Several years ago I was shooting a stock 308 in the matches. Accuracy was ordinary. I ran RCBS 200 gr Sil bullets through a bump die which actually had too little taper (3/4 degree per side) for a 308. I was bumping these bullets up so a good deal of pressure was applied. Accuracy improved dramatically. Flatter bases? more uniform bullet? not sure what but something was dramatically better. Since then I have been a little skeptical of the need to EXACTLY match the taper of the bullet to the taper of the throat - at least in an over the counter rifle.

Jim

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BudHyett posted this 06 February 2019

How about picking a mold that just fits as cast?

Gary

There are circumstances where this happens involving luck. I have ordered custom two-cavity molds providing chamber casts and neither cavity come close to the other cavity, let alone matching the chamber. I am not faulting the mold maker, there are too many variables in machining tight tolerances when boring small holes. My first .30 caliber mold was a RCBS 30-180-SP that produced bullets withing a .005 grain range from both cavities and yet only one cavity shot well.

A custom mold will get you closer to the precision that you desire, but this mold still might need a bump die to get the final precision you seek. 

The RCBS 30-180-SP mentioned above shot the one cavity well in .a selection of .308 and .30-'06 rifles that Ed Doonan and I owned. The other cavity shot well in a Springfield I bought in California that would not shoot any cast bullet well. (It liked Sierra 165 grain SPBT and IMR 4895 powder.) In desperation, I tried the other cavity of the RCBS mold and the rifle produced and it shot well. Cast bullets will always be a mystery to me.

Country boy from Illinois, living in the Magical Pacific Northwest

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Ken Campbell Iowa posted this 07 February 2019

some wiseguy once queried::  which is faster ... darkness or light ??   

answer::   darkness, of course, because it is already there ...

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and so it is with the taper fit between the bullet/throat  ::  we do want the bullet already there when we bust the cap ... right ?? 

i used to think ::  wow, just like a morse taper tool shank !!  complete guidance, no chance to deform !! ..

but then i noticed the rifling is only 0.004 deep per side ... oh ... well, one degree is 0.017 per inch so at 1 degree per side taper that 0.004 can only support 1/4 inch length of the bullet on the tapered section, with light seating pressure.  and 1.5 degrees per side ( common in factory chambers ) only supports 0.2 length  ...   and that is the maximum, if the bullet and throat are both tapered the same.

so, since the back of the bullet is in the case and totally unsupported and just hanging in mid air ... when ignition happens .... don't we want to support as much up front as we can ?? 

1/4 degree taper per side would support a whole inch of that same tapered bullet ... and be so gentle at the big bang ....  1/2 degree is a good compromise ... support half an inch ...

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the above wonderful shallow angle throat scenario is of course if you have a custom chamber throat .... and remember that throater reamer can be used to cut your matching squisher die ... or vica-versa   ..  heh ...

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.... and now to your original question ... my experience is that bump/squisher dies do improve my lyman/lee/rcbs castings shot in my factory rifles ... in simple dies i think it is about half better bases, and half getting a good snug groove diameter throat fit ... in my rifles/molds i haven't had a nose problem that needed bumping .. just lucky ... but almost every casting groove diameter and for darn sure the bases got better in the squish die.

so my vote is :: yes, go for it, if even without the matching tapers.   that taper is tricky, especially for 22 to 6.5 ... it can be bored from the compound, or use a throater reamer ...  and for a factory 1.5 or more angle per side, you will get about as good a match by just giving things a running start when you chamber the bullet .. ( g ) ...

if you make your own dies, that system used by OU812 with the lee expander is kinda neat ... and besides you can then make your own better expander buttons for your own use ...  and i have some 1/2 degree throaters for 22 and 243 if that would help.

above all let us hear what you wound up doing ... hey, spring is almost here ...

ken

 

 

 

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Eutectic posted this 07 February 2019

My bumped bullets never gave me an additional advantage over a great (Eagan) mold and a rifle throated to match the bullet. The advantage of bumping is you can use many different bullets. Bumping did improve the accuracy of other 30 caliber bullets, and this gives you different bullet weights to experiment with. 

This was in production rifles, there may be a greater advantage if you have a custom bench rig. Others can comment on this, bumped bullets have been winning at CBA matches for years.

Steve

The problem with dreaming up projects is they always intersect with reality at the bank.

 

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shootcast posted this 08 February 2019

I started making taper dies simply because some bullets couldn’t be seated out of the case far enough. The lands would push bullet into case on closing. Sometimes small changes make a big difference . Sometimes they don’t. Like anything else with cast bullet shooting. It may work fine in one rifle and not in another. It’s another variation to play with. Sometimes it does pay off. 

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