A bullet that expands creates a larger wound cavity. The more expansion, the more tissue that is damaged, destroyed or traumatized. Multiple tests have shown a correlation between large wound cavities and bullets that expand at least 1.5 times their unfired diameter. However, over-expansion and/or fragmentation to the point the bullet loses a lot of weight limits penetration and penetration is important. The problem is that deep penetration without expansion is notorious for slow incapacitation. Hunters and gunfighters will confirm this and the reason is that bullets that penetrate really deep and expand very little damage small amounts of tissue and cause minimal pain. Still, if we hope to penetrate the vitals as a failsafe to the possible lack of our attacker’s ability to feel pain, “sufficient penetration”—like Aagaard alluded to—is important.
And then there’s impact velocity. When you combine high impact velocity with expansion, wound cavities get very large. This should not come as a surprise. The .38 Spl. and the .357 Mag. shoot the same caliber bullet, but the .357 Mag. pushes it much faster and has a much better record for stopping bad guys. When comparing their performance in 10 percent gelatin using similar bullets, the difference is obvious.
So, should you carry the combination you think will cause the most pain or the one you think will penetrate through to the vitals no matter the shot angle and regardless of what gets in the bullet’s way? Common sense should tell us that the failsafe, penetration, is our first priority; if pain fails to stop the attack—and it might—we have to rely on the bullet’s ability to drive through vital organs. The ability to inflict the maximum amount of pain should be our second goal. This makes choosing the handgun/bullet combination simple—you want the combination that penetrates to a sufficient depth and damages the most tissue in the process.