The 45ACP was developed in response to the MORO warriors, who were severing the heads of soldiers AFTER being mortally wounded by 38 Colt rounds.
Given "ball" type ammo (non hollow point), the 45 has a much greater frontal area than the 9mm, about the same difference as between the 45 and the 38 (remember the Moro warriors?), and as such transfers more energy to flesh, and disrupts more tissue, creating more shock and a larger wound channel.
When you move to hollow point ammo, the difference is even greater. One of the most reliable man stopping rounds is the 45ACP 230 grain flying ashtray. It ranks a VERY close second to the 357 Magnum 125 grain +P hollow point. The 9mm is not often in the top five.
There were four main reasons given for adopting the 9mm over the 45 for military use, most of which are not even valid, some of which the weapon adopted negates.
The first is that our NATO allies use the 9mm.
The second is that the 9mm is more likely to penetrate hardened (kevlar) headgear. Well, rarely do you have people shot in the helmet, because few people try headshots, and even then, you rarely see one made at an angle that allows ANY pistol round to penetrate.
The third is grip size. A full third or more people find the old 1911 Colt grip much more agreable than the Model 92 Beretta. The grip of the 92 Beretta is not small, because it has a double stack magazine, and the trigger reach, especially for the first shot, isn't much shorter either.
Fourth is felt recoil/controlability. Many find the Beretta to be no more controllable than the 1911, because despite being 100 years old, the frame/grip of the 1911 was better designed to fit the human hand and control recoil.
Regarding capacity, I'd rather hit the enemy ONCE and put him DOWN, than hit him three or four times and have him keep coming.
