There was an article that was done not to long ago that basically said that there is not that much difference between the .45, .40, and 9mm. The two things they were looking at was expansion and penetration. It did give the edge to the .45 though.
http://www.firearmstalk.com/entries/What-round-is-More-Effective-9-45-or-40.html
He's ignoring one critical component of the FBI test, linked below.
The FBI states that HPs expand only 60-70% of the time and as such the shooter can only rely on the unexpanded diameter, as that is the only consistency.
They also state that the size of the
permanent wound cavity, which in the picture on that link is essentially just before and just after the 12" line, is what is important from a blood loss incapacitation standpoint.
The temporary wound cavity, which is the big one in the gel picture, really "wows" in the picture but is absolutely useless in a human being.
There are only two ways to stop a determined adversary; disrupt the central nervous system, which means putting a round through the brain or upper spine, or incapacitation via blood loss to the point that they simply can't continue. Blood loss is the more likely occurrence, but it's not an instant stop; only a CNS hit can accomplish an instant stop.
The FBI also states that a larger diameter bullet may hit blood vessels that smaller rounds barely miss, which leads to faster blood loss. They state, very plainly, that larger calibers provide an advantage, even if it's a small one, and that any advantage should be taken.
The other thing he ignores is that 12" penetration is the
minimum the FBI considers acceptable. 14-18" is the ideal. So if you're shooting a 9mm that just makes the 12" mark, it's barely acceptable.
The FBI says, in order of importance, it's penetration, shot placement, caliber. And yes, they say bigger is better.
People have been killed by a single .22; others have survived 6 rounds of .357 magnum. You can't discount any caliber, but the faster blood loss happens, the sooner the assailant will stop. People have taken a .45 to the heart and continued to fight for several seconds.
I've personally seen a guy who was shot in the chest with a .38 who was still up, walking around, when we got to the scene, a good 10+ minutes after the shooting. He died a few hours later, at the hospital. Considering the hospital was 45 minutes or so away, that's not a big surprise.
It's not a matter of if the round kills your assailant; it's a matter of how fast it takes them down. The only "one shot stop" outside of a hit to the CNS, is a person who falls down when shot; that is a person who is not a determined assailant. We, as a society, have been conditioned by Hollywood to believe that people should fall down when they get shot.
Here's the FBI report. It's only 19 pages and a bunch of those are only 1/4-1/2 pages, so it doesn't take long to read.
http://www.firearmstactical.com/pdf/fbi-hwfe.pdf