Originally Posted by
yeahbub
If the PP boolit isn't large enough or doesn't obturate sufficiently to seal, there will be considerable blow-by which will act as a torch on paper or lead. In fact, there was a fellow at the range some years ago who had a jacketed bullet stuck in the barrel throat that did not seal and the resulting blow-by eroded the copper jacket and to a lesser extent, the barrel steel, leaving a hideous divot when viewed through the boroscope, so your surmise about damage can be correct when there's enough leakage to subject the exposed area to sufficient heat. A lot of fire went out through that spot. The specific heat of gasses is actually very low compared to other materials like metals, hence, to transfer enough heat for damage to occur, there has to be a lot of gas rushing past the spot in question. This is why the base of a boolit shows no evidence of melting, but the sides of an undersized boolit will show what looks like torch cutting on the portions which should have been sealed against the bottom of the rifling grooves.