Neither the bullets from BPC rifles nor the round ball are going to be noticibly misshapen. That would have to occur after the projectile has left the bore and that ain't gonna happen due to the obvious pressure drop. Projectiles can only expand so much inside the bore with black powder pressures. The ball stops at the patch and bullets stop at the grooves. At most we're talking a few thousandths and that isn't going to be noticible, at any high film speed.
I'm confident that with smokeless powder one could drive a round ball, even a very hard cast one, hard enough, quickly enough to shape it into almost a cylinder, semi-wadcutter like, but with smokeless we're talking considerably higher pressures. If you've shot cast bullets over smokeless much you can see this if you drive them too fast or with a powder that burns too quickly. They will be ......bent at the nose.....for lack of a better word. With bullets that have bore riding noses, they're kinda wasp waisted between the nose and the driving bands, you can measure that the waist has actually disappeared, expanded into the rifling, when driven at high cast bullet velocities.
I'm also confident that if you cast a ball or bullet from linotype, or alloy it hard enough, that it would not expand into the patch or the rifling when using black powder. Linotype generally goes about 22 on the BHN scale and the Hawk, jacketed bullets I use in my double rifles measure 25 on the BHN scale. There isn't a lot of difference between the two. Unless I'm mistaken pure lead, or as pure as we're likely to use, goes about 6 on the BHN scale.
I'm always amazed that this relatively simple concept is often so difficult to understand or believe.
Vic
