I've seen multiple comments over the years that state that handguns are really barely marginal for self-defense and that a handgun is just a tool so you can get to your rifle.
Hence the caliber war stance that since all handgun calibers are so close it effectiveness, that it really doesn't matter what caliber you go with.
But then I've also seen multiple comments about the 10mm and other larger calibers being too powerful for self-defense due to penetration concerns.
So which is it?
Well, it's kind of both.
If you look at kinetic energy (which is just one measurement and not the be-all end-all, but it's not useless either, so I'm rolling with it). there's just not that much difference between calibers. 380 ACP is about 200 ft-lbs, 9mm is about 350 ft-lbs, and 45 ACP is about 400 ft-lbs. By comparison, a 223 carries 1200 ft-lbs of energy at the muzzle, and a 30-06 carries 2800. There's other factors that go into it - the diameter of the bullet matters more at low velocity when you're just punching a straight hole, penetration depth is still crucial - but in the grand scheme of things, "9 Sillymeter" and the "hand of God 45 ACP manstopper" just aren't that different. Not
nearly as different as internet caliber wars play them up to be. So that's the root of "pistol calibers don't matter" - for carry sized semi auto pistols, it's generally true.
When you start getting up to the magnums, things start getting more interesting. 357 magnum carries about 600 ft-lbs of energy, and the 44 Magnum carries about 1200. The energy comparisons get more and more strained as the calibers become less and less alike, but the salient point is that those rounds are carrying a
lot more juice than something like a 9mm. Those are more than powerful enough to ruin any attacker's day, but then the question of size and controllability comes up. Are you going to pack an N-frame around like Dirty Harry, and if you do, how fast can you shoot it? Can you get 5 shots off faster with a 9mm or a 44 magnum? Which caliber can you
hit more of those shots with, because misses aren't worth anything.
So basically, the size of the pistol and the amount of recoil you can shoot well with sets an upper bound for an SD gun. The lower bound is where the rounds just aren't effective enough to stop a man anymore. There's disputes of where that line's at - obviously a 22 LR or 25 Auto isn't going to be anyone's first choice for an SD gun, but is a 32 ACP okay? 22 Mag? That's an argument that can go back and forth, but the usual standard is that you need a caliber that can penetrate 12" of flesh - the idea being that no matter what angle you hit a normal-sized man from, the bullet will penetrate far enough to hit
something vital.
So 12" of penetration minimum up to as big as you can shoot quickly. Any caliber in that range would be a reasonable SD choice.
Overpenetration is a concern, but having a caliber you can handle is usually a more pressing priority. While something like a 44 magnum is going to penetrate a lot further than a 9mm, the unfortunate truth is that any caliber that can travel through 12" of Bad Guy can also travel through drywall and plenty of other things you don't want it to.