The term of a mangum caliber is just a play of words. The best four combat pistol cartridges are from the smallest to largest.
1) 9X19mm Parabellum
2) .357 Sig
3) .40 S&W
4) .45ACP
1) The 9mm cartridge is actually a very fine cartridge. The problem with it is that currentrules of warfare limits the use of a full metal jacketed round. If a 147Gr. JHP was used, it would put someone down just as fast as a .45ACP.
2) The .357 Sig is a hot little cartridge and is currently gaining popularity in the US law enforcement community. It fires a .36 Cliber (9mm) projectile from a .40 caliber casing. What that means is that the bullet travels faster because the cartridge casing has more space for powder. Right now it's been the standard issue cartridge of the US Secert Service and is a personal choice of many law enforcement officers.
3) The .40 S&W in my opinion is the best cartridge on the market for all uses. It is becoming the number one issued police caliber in the USA. Right now, Glock holds sixty percent of the police firearm market for duty issue pistols. And it's model 22 and 23 is it largest seller. The reason is because you can still get a high capacity platform with the ballistic power of a .45ACP cartridge. This little round was born from the creation of the 10mm Auto cartridge. Back in 1989. there was a shootout in Miami Florida. A couple of FBI agents were killed. They were carrying their issued 9mm and .38 Special sidearms. THe scumbags had .45ACP caliber pistols. Well, the FBI did a study and created a new cartridge. The 10mm, well. Many of it's female agents couldn't handle the cartridge. So the FBI created the 10mm FBI load or what some people call the 10mm Lite. Well, Smith & Wesson took the 10mm FBI load and shortened the case. And There you have the birth of the .40S&W. I carry a Glock 23C every day chambered in that Caliber. And during my time in Afghanistan, I saw PMC (Private Military Companies) use the .40S&W cartridge. It's far better than the 9mm and .45ACP.
4) THe .45ACP is a much a part of America as the .30-30 Winchester cartridge is. It's a proven man stopper. And even when used as a FMJ round. It will stop a man and put him on his @$$. The only catch with that caliber is it's size. If you want to carry more than 8 rounds, you need to use a double stack magazine. The Problem with that is that the grip of the pistol is too large for most people. That is way the 1911A1 style pistol is still the favorite platform for the .45ACP and now the 10mm cartridge.
If NATO or the US Militay was to switch to a different cartridge. I'd go with the .40S&W cartridge. But please remember. It's not the caliber, it's also the projectile that is used. if you need to stop a human with a 9mm cartridge. You need to use a expanding bullet. But since the Houghe Covention limits the use of ammountion to the FMJ. The 9mm will over penetrate the target.