So Mr. Ayoob carries both but recommends 45acp for cold weather when people are wearing more clothes and the cold can restrict their bloodflow.
Well, a shot from any round will get their blood flowing again...
The FBI asked this same question a couple of decades ago when it was looking to replace their .38's with either a 9mm or the .45. They were also doing some testing on the 10mm (.40), so they included it in their testing.
They made some interesting findings and came to some interesting conclusions:
1. "Knockdown power" is largely a myth.
2. "Hydroshock" is largely a myth.
3. Penetrating deeply enough to reach the vital blood vessels running along the spine, behind the gut, is a critical factor.
4. Permanent cavity size is a critical factor.
5. Penetration is positively correlated with both bullet mass and velocity.
6. Higher velocities of hollow points actually reduce penetration, as the higher velocities result in significantly greater expansion.
7. The .45's greater mass and diameter results in a larger cavity, but the .40 has greater penetration.
8. The FBI determined that a minimum average penetration of 12" was necessary to increase stopping power.
9. The FBI chose the 10mm/.40 over the .45 due to it's lesser recoil, reduced muzzle flash, greater penetration, and larger magazine capacity.
10. While the 10mm/.40 edged out the .45, both were about 20% better than the 9mm across the board. Don't rule out the 9mm, though, as it's double-tapping accuracy is excellent. Since the FBI also determined most officers miss the suspect 80% of the time, shot placement is critical.
