This article was written by John Alexander and was published in Fouling Shot #212 - July/Aug 2011.
I think it was Dr. Mann that famously said that the rear end of the bullet was the guiding end. This pro-nouncement launched cast bullet shooters on a hundred years of worrying about the base of their bullets. All knowledgeable shooters know that the back of the bul-let must be nearly perfect and perpendicular to the long axis of the bullet to achieve decent accuracy. One way to do this is with a nose pour mould.
Even though a nose pour mould cant produce a pointed bullet and they are expensive to make, and slow to use, thousands have been sold because they make bullets with near perfect bases instead of depending on a perfect sprue cut off technique.
The theory behind our fixation on bullet bases is sim-ple, easy to understand, and sounds reasonable. If the base of the bullet isnt uniform and perpendicular with the long axis of the bullet, when it leaves the muzzle the expanding gases rushing out the muzzle will place an unsymmetrical force on the imperfect base and deflect the bullet from a true path.
However, just because a theory sounds reasonable doesnt mean that it is true. After all, when people had limited information to the contrary, the earth is flat and the sun revolves around the earth were both reason-able sounding theories. However, the theory about per-fect bases sounds so reasonable that it gave me, and a lot of other cast bullet shooters, base-a-phobia.
Since first reading this plausible wisdom many years ago, I have thrown away thousands of bullets because of minor base defects after casting sessions that didnt go too well. Since being infected with this dreaded disease I have noticed other distressing symptoms. My pants dont fit as well as they used to, all my hair has fallen out and I have shrunk in height.
I have also developed an urge to duct tape up the snout of my neighbors barking dog and sometimes display other random symptoms of antisocial behavior.In spite of my advanced base-a-phobia, several years ago I became skeptical that slightly rounded edges on the base of the bullet would degrade accuracy if cov-ered over with a gas check.
Subsequent testing of rounded vs. sharp edged based bullets convinced me that a little rounding if it looked more or less even all around the circumference had no effect on accuracy.
I think it was Dr. Mann that famously said that the rear end of the bullet was the guiding end. This pro-nouncement launched cast bullet shooters on a hundred years of worrying about the base of their bullets. All knowledgeable shooters know that the back of the bul-let must be nearly perfect and perpendicular to the long axis of the bullet to achieve decent accuracy. One way to do this is with a nose pour mould.
Even though a nose pour mould cant produce a pointed bullet and they are expensive to make, and slow to use, thousands have been sold because they make bullets with near perfect bases instead of depending on a perfect sprue cut off technique.
The theory behind our fixation on bullet bases is sim-ple, easy to understand, and sounds reasonable. If the base of the bullet isnt uniform and perpendicular with the long axis of the bullet, when it leaves the muzzle the expanding gases rushing out the muzzle will place an unsymmetrical force on the imperfect base and deflect the bullet from a true path.
However, just because a theory sounds reasonable doesnt mean that it is true. After all, when people had limited information to the contrary, the earth is flat and the sun revolves around the earth were both reason-able sounding theories. However, the theory about per-fect bases sounds so reasonable that it gave me, and a lot of other cast bullet shooters, base-a-phobia.
Since first reading this plausible wisdom many years ago, I have thrown away thousands of bullets because of minor base defects after casting sessions that didnt go too well. Since being infected with this dreaded disease I have noticed other distressing symptoms. My pants dont fit as well as they used to, all my hair has fallen out and I have shrunk in height.
I have also developed an urge to duct tape up the snout of my neighbors barking dog and sometimes display other random symptoms of antisocial behavior.In spite of my advanced base-a-phobia, several years ago I became skeptical that slightly rounded edges on the base of the bullet would degrade accuracy if cov-ered over with a gas check.
Subsequent testing of rounded vs. sharp edged based bullets convinced me that a little rounding if it looked more or less even all around the circumference had no effect on accuracy.
Another of my symptoms is an uncontrollable urge to check to see if the bullet bases are perpendicular to the long axis of the bullets. I compulsively line up the gas checked, sized, and lubed bullets on their bases in a straight row and visually check for any that appear to be leaning or out of line. Since these are often 75 or 85
grain 22 bullets that are over three times as long as their diameters it is easy to pick out those that dont have per-pendicular bases. If the base is skewed, instead of being perpendicular to the long axis, the gap will be wider near the top on one side and narrower near the top on the other. This can be seen with the middle bullet in the picture. That particular bullets base was skewed nearly two degrees.
I cant figure out what sometimes causes this. I cut off sprues with a gloved hand while pressing the sprue plate against the top of the mold. Although the bases of the bullets look perfect, after the gas checks are in place a few of them can sometimes clearly be seen to list a bit. Fortunately, most lots of bullets all appear to have uni-form gaps and I quit worrying about that particular grem-lin in that lot of bullets.
Recently however I cast a batch of bullets and several failed my parallel gap test. My labor in making this visual check seemed vindicated. I could see that they were definitely skewed but by how much? Being an engineer I wanted numbers so I carefully measured the distance between the tips of the skewed base bullets and their more upright neighbors. It seemed like an impossible task at first, but after a lot of experimenting about out how to brace my hands holding the digital caliper I was able to get repeatable results. Using thetrig functions on my calculator that hadnt been used in a while I found that the worst of the crooked bases were about two degrees from perpendicular to the long axis ofthe bullet I
I speculated on what terrible havoc those skewed bases would have wreaked on my scores if I hadnt dis-covered them. Since I had 28 of the defective bullets as well as plenty of the ones that had passed my gap test,I decided to find out.
I loaded twenty five rounds of the skewed base bullets and thirty rounds of the good bullets for the 223 Tikka T3 Lite, an ugly six pound rifle with a plastic stock I have been shooting in the CBAs new Hunting Rifle Class. The load consisted of a Remington small pistol primer, 5 grains of 700X and my 75-grain Mos bullet lubed only in the gas check shank with LBT Blue. Muzzle velocity was 1,450 ft. /sec. The groups with good and suspect bullets were fired alternatively on the same day under excellent conditions.
When I tabulated the results the good bullets aver-aged 0.96 minutes of angle for the six five shot groups. The five groups with the skewed base bullets averaged 1.28 MOA. This was a 33 percent increase in average group size. These results seemed to indicate that bullet bases skewed up to a maximum of about two degrees degraded accuracy somewhat and were worth checkingfor.
However, differences between the averages of small samples can be deceiving and often result from normal variation, not a real difference. To verify that the differ-ence was probably real, I applied the statistical methodin Joe Brennans book, Cast Bullets for Beginner and Expert. Given the 33 percent difference and the numberof groups fired we can say with a confidence level of over 95 percent that skewed bases degrade accuracy.
That the skewed bases seemed to degrade accuracy was expected, but the relative small difference was unexpected. The test results conformed to the theory about bases needing to be near perfect. What was surprising was that most of the skewed base bullets I had rejected seemed to shoot as accurately as my good bullets.
After I measure the extreme spread of a five shot group I then ignore the hole farthest out and measure the extreme spread of the remaining four holes and write down both numbers. This is just a personal quirk, but I think it may sometimes give me a bit of additional information about whether the load is tending to group or just producing a collection of random holes. It is sometimes useful to know that a two-inch group had four shots clustered in less than an inch.
The average extreme spread for the four best holes in the six groups with good bullets was 0.67 MOA. The similar average for the four best shots in the five groups with the skewed base bullets was 0.70 and the groups were positioned the same as the groups with the good bullets relative to the aiming point. This is less than a five percent difference and didnt seem significant. The statistical method mentioned above indicates that I would have to fire 156 five shot groups with each bullet and still have a 5% difference to be able to say with 95% confidence that the difference was real. That the best four groups were about the same for either good or skewed base bullets intrigues me because it seems to indicate that about 80 percent of the skewed base bul-lets shoot as accurately as the good bullets. Maybe only bases that are skewed about two percent or more are a problem and the easily visible, but smaller, skews dont degrade accuracy at all.
So I have proven something that everybody already knew. Was it a waste of time? Well, not for me and it was my time. I was glad to learn how little the skewed bases affected accuracy. After all 1.3 MOA is good enough for head shots on squirrels at sixty yards and far better than the accuracy I can get out of some of my other rifles. From now on when I am preparing similar loads for hunting, plinking, or offhand shooting, I will con-tinue to try to produce the very best bullets and the truest bases possible. But, I will not throw away any of my hard earned bullets for having slightly skewed bases. I will also skip the laborious checking to see if the bases are true and spend the time shooting. For bullets to be used in competition, I will continue to be as anal as cast bullet shooters are expected to be.
Of course these findings were only on one caliber, bullet, and load. Although results may be different for other situations, I would be surprised. These tests clearly show that, at least for the load tested, excellent cast bullet accuracy can be achieved with the bullet bases skewed up to about two degrees. This means we can relax just a bit about our base-a-phobia except for really precise shooting.
I cant figure out what sometimes causes this. I cut off sprues with a gloved hand while pressing the sprue plate against the top of the mold. Although the bases of the bullets look perfect, after the gas checks are in place a few of them can sometimes clearly be seen to list a bit. Fortunately, most lots of bullets all appear to have uni-form gaps and I quit worrying about that particular grem-lin in that lot of bullets.
Recently however I cast a batch of bullets and several failed my parallel gap test. My labor in making this visual check seemed vindicated. I could see that they were definitely skewed but by how much? Being an engineer I wanted numbers so I carefully measured the distance between the tips of the skewed base bullets and their more upright neighbors. It seemed like an impossible task at first, but after a lot of experimenting about out how to brace my hands holding the digital caliper I was able to get repeatable results. Using thetrig functions on my calculator that hadnt been used in a while I found that the worst of the crooked bases were about two degrees from perpendicular to the long axis ofthe bullet I
I speculated on what terrible havoc those skewed bases would have wreaked on my scores if I hadnt dis-covered them. Since I had 28 of the defective bullets as well as plenty of the ones that had passed my gap test,I decided to find out.
I loaded twenty five rounds of the skewed base bullets and thirty rounds of the good bullets for the 223 Tikka T3 Lite, an ugly six pound rifle with a plastic stock I have been shooting in the CBAs new Hunting Rifle Class. The load consisted of a Remington small pistol primer, 5 grains of 700X and my 75-grain Mos bullet lubed only in the gas check shank with LBT Blue. Muzzle velocity was 1,450 ft. /sec. The groups with good and suspect bullets were fired alternatively on the same day under excellent conditions.
When I tabulated the results the good bullets aver-aged 0.96 minutes of angle for the six five shot groups. The five groups with the skewed base bullets averaged 1.28 MOA. This was a 33 percent increase in average group size. These results seemed to indicate that bullet bases skewed up to a maximum of about two degrees degraded accuracy somewhat and were worth checkingfor.
However, differences between the averages of small samples can be deceiving and often result from normal variation, not a real difference. To verify that the differ-ence was probably real, I applied the statistical methodin Joe Brennans book, Cast Bullets for Beginner and Expert. Given the 33 percent difference and the numberof groups fired we can say with a confidence level of over 95 percent that skewed bases degrade accuracy.
That the skewed bases seemed to degrade accuracy was expected, but the relative small difference was unexpected. The test results conformed to the theory about bases needing to be near perfect. What was surprising was that most of the skewed base bullets I had rejected seemed to shoot as accurately as my good bullets.
After I measure the extreme spread of a five shot group I then ignore the hole farthest out and measure the extreme spread of the remaining four holes and write down both numbers. This is just a personal quirk, but I think it may sometimes give me a bit of additional information about whether the load is tending to group or just producing a collection of random holes. It is sometimes useful to know that a two-inch group had four shots clustered in less than an inch.
The average extreme spread for the four best holes in the six groups with good bullets was 0.67 MOA. The similar average for the four best shots in the five groups with the skewed base bullets was 0.70 and the groups were positioned the same as the groups with the good bullets relative to the aiming point. This is less than a five percent difference and didnt seem significant. The statistical method mentioned above indicates that I would have to fire 156 five shot groups with each bullet and still have a 5% difference to be able to say with 95% confidence that the difference was real. That the best four groups were about the same for either good or skewed base bullets intrigues me because it seems to indicate that about 80 percent of the skewed base bul-lets shoot as accurately as the good bullets. Maybe only bases that are skewed about two percent or more are a problem and the easily visible, but smaller, skews dont degrade accuracy at all.
So I have proven something that everybody already knew. Was it a waste of time? Well, not for me and it was my time. I was glad to learn how little the skewed bases affected accuracy. After all 1.3 MOA is good enough for head shots on squirrels at sixty yards and far better than the accuracy I can get out of some of my other rifles. From now on when I am preparing similar loads for hunting, plinking, or offhand shooting, I will con-tinue to try to produce the very best bullets and the truest bases possible. But, I will not throw away any of my hard earned bullets for having slightly skewed bases. I will also skip the laborious checking to see if the bases are true and spend the time shooting. For bullets to be used in competition, I will continue to be as anal as cast bullet shooters are expected to be.
Of course these findings were only on one caliber, bullet, and load. Although results may be different for other situations, I would be surprised. These tests clearly show that, at least for the load tested, excellent cast bullet accuracy can be achieved with the bullet bases skewed up to about two degrees. This means we can relax just a bit about our base-a-phobia except for really precise shooting.