1) Ok, so I think when you say barrier you are referring to the sound "barrier?" /EDIT: I think I missed saying something pretty important for the laymen here, about the "sound barrier." The "barrier" is in fact not a barrier at all. Yes, you get an increase in the drag coefficient for a body around Mach 1, but it's not really that much of a "barrier," especially when you're talking about ballistics. Although everyone made a big deal about Yeager breaking the sound barrier, it's not like the engineering community just didn't realize that bullets and other objects travelled faster than the speed of sound (as most everyone here no doubt realizes there are plenty of supersonic blackpowder rifle loads). It was just a little more challenging to push a lightly built self-propelled aircraft faster than Mach 1 than it is a solid bullet shot from a barrel. This was in part because of compressibility impacts on the controllability of the aircraft with the then state of the art, and also with the principle mode of propulsion at the time (the propeller). Props rapidly loose performance and tend to get vibration problems as portions of the blade become supersonic (which inevitably happens as the blade rpm increases and/or aircraft speed increases). /EDIT
Bullets are not very stable regardless of the sound barrier. The effect of supersonic flight is to make them less aerodynamically stable as the center of pressure moves forward (and you want the center of pressure behind the center of gravity for aerodynamic static stability).
We spin long bullets really fast for two principal reasons... To keep the launch end forward despite bad aerodynamic stability, and to average out radially asymmetric aerodynamic forces from bullet surface geometry imperfections.
I think my point regarding the relative magnitude of forces due to ambient gas pressure vs. base pressure still stands. The 64 psi number I stated before was a maximum bound for pressure at the stagnation point on the nose for standard conditions in freeflight. I calculated the "total pressure" as an upper bound, but really it is too high as the total pressure assumes the freestream is isentropically brought to rest, and the gas at the nose of the bullet has experienced shock(s). I'm not even sure though that the nose of a "supersonic" bullet is really moving faster than the freestream until it flies through the first few calibers/gets through some muzzle blast after leaving the bore. You see, the bullet has been pushing air ahead of it down the bore and there is also probably some gas leakage, and so the mouth of the bore is "blowing" prior to the bullets exit (I'm not sure how fast, typically, but it could theoretically be a supersonic jet of gas right before bullet exit /EDIT: Well, actually without leakage gas velocity would have to be as fast as bullet velocity right around exit... with leakage gas velocity blowing out the bore would be higher than bullet velocity immediately prior to bullet exit /EDIT). Then when the main muzzle blast occurs with the uncorking of the bore, the gas around the bullet is traveling faster than it is as the bullet is overtaken by the expanding gases!
ANYWAY, I'm really pretty confident that with a reasonably sealed bore the freestream forces at bullet nose exit (before the tail of the bullet is released by the bore) are negligible compared to those induced by the escaping gas in the muzzle blast when the bore is uncorked. The freeflight/ambient pressures just shouldn't be of the same order of magnitude as those resulting from the pressures in the barrel.
Sorry for going on... Did I correctly understand what you meant by "barrier"?
2) by accuracy zones do you mean the useful velocity range that provides accuracy in a given caliber (and given tailoring of powder to cartridge)?
3) Do you propose this is true for similar designs of cartridges/firearms/bullets? That is, if I took a 50 bmg rifle (instead of a 45 long colt) and shot it with a similar design bullet do you think I'd have significantly different results in terms of improving accuracy with hardness vs velocity than I would in 22 caliber?
4) I don't understand what you mean... Please rephrase? What is an "accuracy point"?
5) neat! This may or may not be the same thing, but this sort of result is VERY well known in the penetrator impact engineering literature. Do the following: calculate the dynamic pressure of your test medium as 1/2 (medium density)*(velocity)^2, for all your test point velocities, and compare the pressures you get with the yield and ultimate stresses (in the same units) for soft lead as you were using. I'll bet your dynamic pressure crossed over the yield and/or ultimate stress for the bullet metal. What happens is you get increasing penetration with increasing velocity up to a point, and then the fluid flow pressures generated by the impacted medium start destroying the penetrator (bullet) which decreases penetration.
I'm not sure penetration here had anything to do with accuracy or the intactness of the bullet edge? It does sound like you were stripping the rifling, though, which ain't good for shooting groups for a bunch o' different reasons.
Thanks BA for expanding on your original post.
Best regards,
DrB