I really love to stir things up in the shooting community (especially the internet portion of it) so I’m just going to say it, “.45ACP is outdated.” That’s not to say it isn’t effective, or that it isn’t reliable; just that it’s outdated. The .45ACP round was developed by John Browning in 1905 and was later adopted by the United States military because it was deemed more effective that the .38 Long Colt that was currently used by many service pistols. For many years it was deemed a superior pistol round to the majority of alternatives available. Because the military is an institution that has historically been slow to make drastic changes, the .45ACP cartridge was kept in service for a long time before 9mm was adopted as the standard.
The .45ACP cartridge is large and takes up considerably more space in a magazine than a 9mm round does. The largest of handguns are mostly limited to 13 rounds per magazine (15 in some FN models). When it comes to the 9mm, there are pistols capable of capacities upwards of 17 rounds. The relevance here is that the less ammunition you have in your gun, the more magazines you have to carry and the more often you have to reload; when you’re reloading, you’re not engaged in the fight. It has been long understood by any military worth its salt that volume of fire trumps accuracy every time when it comes to direct engagements. Rounds down range, keep the threat’s head/s down and prevents them from shooting at you; this allows you to maneuver (close with and destroy) or employ accurate fire and eliminate the threat. Having more bullets makes the whole process a whole lot easier.
The .45ACP has a lot of myth surrounding it as well. Stopping power is not necessarily a thing; at least not in the context people are generally think of. In terms of ballistic coefficient the .45ACP bullet is very similar to the 9mm. However the 9mm bullet travels at a far greater velocity giving it a flatter trajectory and better penetration as far as terminal ballistics go. Wound cavity is subjective to the rounds dimensions and design. Shot placement will always be the determining factor here though. If a 9mm and a .45ACP fmj (full metal jacket, ball ammo standard to the military) were both fired at a human heart they would achieve similar results in cavitation but the 9mm would get there faster and penetrate better.
Again, the .45ACP is not a bad round; it’s effective and reliable. It’s outdated because it travels at a slower velocity and can not be carried in the abundance the alternative can. The majority of the world’s militaries employ the 9mm round and even the United States Special Operations Command has gone back to the 9mm after years of employing several .45ACP pistols. The truth of the matter is, shot placement and capacity beat round size every time.