By Kenneth Mollohan |
When I started my series of articles, I promisedto take a crack at some of my own favorite theories. Let me tell you about thetime I discovered what
discovered absolutely nothing new or revolutionary. It just showed me what I'd already known. I found that lighter bullets went faster than heavy ones and that cast bullets seemed to work best with small cases and long barrels, which comes as a shock to nobody. It was almost a total waste of time, but it did manage to focus my attention on those things I knew, but just hadn't thought about very much.
I noticed that the highest successful velocities were reported with bullets having the lowest sectional densities. That was interesting, and some work with low SD bullets gave some very good results. I could add several hundred fps to the speed of my .30-06 and still maintain decent accuracy if I used low SD bullets. Granted, the low SD slugs were of limited usefulness, but they were fast.
I also noticed that I could load a plain-base bullet in a small case to pressures far beyond those practical in a large case. You can load a bullet to 40,000 psi in a Ml carbine yet that same bullet is a total failure at 30,000 psi in a .30-06. You can use bullets from the same mold, cast at the same time from the same pot of metal without changing a thing. I realized that this could not be true if failure was the result of base upset from the pressure. The strength of the bullet would have to depend on the size of the chamber behind it instead of it's own alloy composition, and that is simply not possible! Still, more powerful loads continued to fail, so pressure was obviously involved. But how?
When I added the success of long barrels and small cases, I realized that both of these mean high expansion ratios, and high expansion ratios mean relatively low muzzle pressures. Also, the slower powders that work better with hot loads have a flatter pressure curve which translates to a maximum velocity for a given pressure level. Could it be that failure or success of a cast bullet load depended on the muzzle pressure? A dependency on muzzle pressure would agree with the success of small cases and long barrels and keep pressure as a factor.
It seemed that I was on to something, but I had no way to test my muzzle pressures, or did I need to? Eventually I realized that Lyman had already done the tests for me and published the data in their reloading manual; or almost anyhow. Would it be
I noticed that the highest successful velocities were reported with bullets having the lowest sectional densities. That was interesting, and some work with low SD bullets gave some very good results. I could add several hundred fps to the speed of my .30-06 and still maintain decent accuracy if I used low SD bullets. Granted, the low SD slugs were of limited usefulness, but they were fast.
I also noticed that I could load a plain-base bullet in a small case to pressures far beyond those practical in a large case. You can load a bullet to 40,000 psi in a Ml carbine yet that same bullet is a total failure at 30,000 psi in a .30-06. You can use bullets from the same mold, cast at the same time from the same pot of metal without changing a thing. I realized that this could not be true if failure was the result of base upset from the pressure. The strength of the bullet would have to depend on the size of the chamber behind it instead of it's own alloy composition, and that is simply not possible! Still, more powerful loads continued to fail, so pressure was obviously involved. But how?
When I added the success of long barrels and small cases, I realized that both of these mean high expansion ratios, and high expansion ratios mean relatively low muzzle pressures. Also, the slower powders that work better with hot loads have a flatter pressure curve which translates to a maximum velocity for a given pressure level. Could it be that failure or success of a cast bullet load depended on the muzzle pressure? A dependency on muzzle pressure would agree with the success of small cases and long barrels and keep pressure as a factor.
It seemed that I was on to something, but I had no way to test my muzzle pressures, or did I need to? Eventually I realized that Lyman had already done the tests for me and published the data in their reloading manual; or almost anyhow. Would it be
possible to translate Lyman's peak pressures into a good estimation of muzzle pressures by using the Ideal Gas Laws? There were some factors that I couldn't figure out how to deal with, like frictional losses or heat lost to the barrel, but they seemed likely to be fairly constant from one barrel to the next and I could therefore ignore them safely. Burning rates and similar uncontrollable factors could be held constant by just comparing data for a single powder and buJJet. By golly, it worked! I
couldn't calculate absolute muzzle pressures values of course, due to the influence of unknown factors that I could only deal with as constants; but the Lyman data indicated that cast bullets failed when muzzle pressures exceeded a surprisingly uniform relative value, and that value didn't seem to change much from one rifle to another, or even from one caliber to the next. I even took Lyman's maximum load data for IMR 3031 and their #311291 bullet in the .30-06 and, using the Ideal Gas Laws, I back calculated a maximum load for the same powder and bullet in the .30-30 Winchester. My prediction was less than a quarter grain of powder from Lyman's published maximum load. You just can't get much better than that! I had finally solved the great mystery of what caused cast bullets to fail at high velocities. It was muzzle pressure and I had the scientific proof! I didn't know the how yet, but boy, did I have the what pinned down cold!
couldn't calculate absolute muzzle pressures values of course, due to the influence of unknown factors that I could only deal with as constants; but the Lyman data indicated that cast bullets failed when muzzle pressures exceeded a surprisingly uniform relative value, and that value didn't seem to change much from one rifle to another, or even from one caliber to the next. I even took Lyman's maximum load data for IMR 3031 and their #311291 bullet in the .30-06 and, using the Ideal Gas Laws, I back calculated a maximum load for the same powder and bullet in the .30-30 Winchester. My prediction was less than a quarter grain of powder from Lyman's published maximum load. You just can't get much better than that! I had finally solved the great mystery of what caused cast bullets to fail at high velocities. It was muzzle pressure and I had the scientific proof! I didn't know the how yet, but boy, did I have the what pinned down cold!
That is the "logical and reasonable" part of my pet theory. The but wrong" part came when that darn Col. Harrison flushed all my hard work and fine ideas right down the drain with his work with paper patched bullets in the .300 Magnum. He got good accuracy without gas checks at muzzle pressures far above what I'd proven was the maximum that cast bullets could stand, even with gas checks. His results destroyed my theory about muzzle pressure just as completely as it wiped out the guys who prattle on about loss of accuracy being due to base upset.
Where did I go wrong? I still haven't figured that out exactly. I worry about it sometimes, but just haven't had time to go back and dissect it all over again. My suspicion is that muzzle pressure is the right basic idea, but I misread the significance of what I discovered. I assumed that muzzle
Where did I go wrong? I still haven't figured that out exactly. I worry about it sometimes, but just haven't had time to go back and dissect it all over again. My suspicion is that muzzle pressure is the right basic idea, but I misread the significance of what I discovered. I assumed that muzzle
pressure was somehow deforming the base of the bullet as it exited the muzzle, or variations in the muzzle blast were just kicking the slug a trifle off course by a few degrees of angle.
My data was good, the logical sequence seemed unassailable, and my mathematical calculations were right on the money. The fact that the theory works so well for lubricated, gas-checked bullets shows that it does have some merit; but, the fact that it doesn't work worth a hoot for patched bullets proves that its basic explanation is obviously wrong; and history has a way of discarding obviously wrong ideas without worrying too much about why they're wrong.
In hindsight, I believe my mistake was failing to consider other explanations for my data. For example, I now realize that my critical muzzle pressure failure point could also be read as the pressure level above which gas blow-by and etching of the bullet in the bore becomes serious enough to destroy accuracy. It seems likely that Col. Harrison sidestepped that problem by preventing etching with paper patches, and thereby showed that I was no better than the ancient fusiliers who believed in infernal propulsion. Like them, I had a detailed explanation for my observations that was perfectly logical and wrong.
So I have learned to be very careful indeed, not to confuse "reasonable" or "logical" with "right' any more. But doggone it anyhow, that was the first time since I was a teenager that I really thought I had it all figured out!
My data was good, the logical sequence seemed unassailable, and my mathematical calculations were right on the money. The fact that the theory works so well for lubricated, gas-checked bullets shows that it does have some merit; but, the fact that it doesn't work worth a hoot for patched bullets proves that its basic explanation is obviously wrong; and history has a way of discarding obviously wrong ideas without worrying too much about why they're wrong.
In hindsight, I believe my mistake was failing to consider other explanations for my data. For example, I now realize that my critical muzzle pressure failure point could also be read as the pressure level above which gas blow-by and etching of the bullet in the bore becomes serious enough to destroy accuracy. It seems likely that Col. Harrison sidestepped that problem by preventing etching with paper patches, and thereby showed that I was no better than the ancient fusiliers who believed in infernal propulsion. Like them, I had a detailed explanation for my observations that was perfectly logical and wrong.
So I have learned to be very careful indeed, not to confuse "reasonable" or "logical" with "right' any more. But doggone it anyhow, that was the first time since I was a teenager that I really thought I had it all figured out!