There's an old pickup bed I was taking shots at one day. Hollowpoints and FMJ in 9mm and 40 S&W would penetrate the side (two layers of metal) but 45 acp just made big dents in the first layer. So I purchased some FMJ Semi-wadcutters and they made it through both layers just fine. 230 grain for all three types of 45 acp. Anyone care to speculate why the the hollowpoint and round nose bullets didn't penetrate but the semi-wadcutters did?
My best guess is that the bigger diameter bullet spends more of it's energy on impact than the smaller rounds. And the smaller rounds having a more narrow diameter focus that energy in a more centralized spot rather than spreading it out. The more conical shape of the 40 S&W FMJ and 45 FMJ-SW being tapered may be why they penetrate just as well. But it doesn't explain why the 45 round nose didn't penetrate or why the HP 40 S&W did.
My best guess is that the bigger diameter bullet spends more of it's energy on impact than the smaller rounds. And the smaller rounds having a more narrow diameter focus that energy in a more centralized spot rather than spreading it out. The more conical shape of the 40 S&W FMJ and 45 FMJ-SW being tapered may be why they penetrate just as well. But it doesn't explain why the 45 round nose didn't penetrate or why the HP 40 S&W did.