Greg Ellifritz has written a very good piece analyzing stopping power. He compiled his own data and has some pretty interesting conclusions.
His work reinforces some beliefs I've held about handgun calibers but with data to support it.
When I first learned to shoot handguns, revolvers were the standard. Double action semi-automatics seemed like a solution to a nonexistent problem. Why shoot something that could jam when you had a trust revolver? Why shoot a pip-squeak 9mm when you could shoot a .357, .41 or .44 magnum?
Today, no self-respecting gun enthusiast would carry anything than a double-action semi-auto. Now the 9mm is the standard (with either .45ACP or .40 S&W coming second). The preference is due to higher capacity (15 rounds in many of the larger frames) and better expanding ammunition.
Ellifritz's work uncovers some trends that support these preferences. Most shootings he profiled involved more than a single shot from the weapon. This has more to do with the speed with which the weapon can be fired than actually stopping power.
I had a similar revelation when I discovered my favorite 158gr HP load caused more muzzle flip than did the 125gr HP load. Even if the first attacker dropped, I became concerned how long it would take to engage a second target. I then started to look more to the .45ACP instead of the .357 as my preferred caliber. Having to engage multiple targets as a potential convince me that your weapon needed to fire quickly.
Special forces were some of the first to embrace the concept of the double-tap. Ellifritz's work supports this based on what he calls "psychological stop" or basically the target not wanting to be shot anymore. The injuries may not actually be enough to stop the attack but the attacker has had enough. Two hits in this case makes it less likely an attacker will continue.
The notion of "psychological stop" seems to occur regardless of bullet type. Much has been made about the improvements in expandable bullets but it would appear hitting your target is far more important than what you hit it with.
9mm and .45s seem to have a better success rate than the .40. Ellifritz doesn't analyze this aspect but I suspect this has to do with the tendency of .40s to have more muzzle flip than the other two calibers. The shooter is thus able to fire more quickly and accurately resulting in slighter higher stops.
One data point not covered by Ellifritz is the wound channel and its effect on stopping. It would seem that larger calibers with heavier bullets create larger wound channels, however Ellifritz's article does not cover this. To me it is why I still like the .45 over 9mm (even his data shows they are more similar), a bigger bullet more a bigger wound. But the .45 is not for everyone.
Based on his article and my own experience, I would tend to carry a 9mm over a .40 or even .45 simply because of the speed of firing subsequent rounds. It is consistent with why John Browning designed the P-35 with a double stack magazine with the intent of firing two rounds at a target. In a concealed carry weapon, a 9mm seems to be the best compromise of weapon size, speed and power.
It is easier to shoot a 9mm than a .357 yet for law enforcement I might choose a .40 since it would give an officer they ability to shoot through a barrier (early 20th Century police officers carried .38 Supers for that very reason).
Ellifritz article also supports my contention that bullet expansion is much to do about nothing. You want a guaranteed one shot stop? Pick a rifle or shotgun. Otherwise, you should plan on shooting your attacker (s) more than once. You should then pick a caliber/weapon combination that reliably allows you to do that anytime and every time.
An Alternate Look at Handgun Stopping Power
