I've spoken to this topic before, but since my titles, while sometimes clever, don't always indicate the content of the post, and to save anybody looking from having to scroll through a couple thousand pieces, I'll speak to it again.
If you are writer who has characters using handguns, you need to know about stopping power.
There are two main schools of thought, and I won't get into them in depth, but basically, one school uses ballistic gelatin, measures things like stretch cavities and penetration and such, and gives various calibers and bullet types a number. Higher the number, more likely it is to do an instant stop. The other school uses stats from actual police shootings–how many people got shot with a particular caliber and stopped what they were doing right then.
Which is how a "stop" is defined. The bad guy trying to stab or shoot you instantly ceases this activity when you plug him. Drops the gun, falls over, desists all hostile activity, whatever.
Whether he dies or not isn't what matters here. If you were justified in shooting, that's the legal and moral coverage. You aren't trying to kill him, you are trying to stop him killing you.
Stopping power is related to, but not the same as, killing power. More people have been killed with .22s than any other caliber, at least that's what I found last time I did the research. But a .22 LR out of a handgun will only stop somebody cold 30-40% of the time with a single hit to the body. So he might wander off and croak on the morrow, but have plenty of pep to shoot you in the head right now.
This is what is known as a pyrrhic victory.
If you are an LEO or soldier or civilian protecting the citizens, your family, or yourself, what you want to happen is for the bad guy to not-shoot-not-stab-not-do-anything-lethal after you shoot him. And yes, unless you are using a dueling pistol, you can shoot him more than once, but the odd thing about people is that if the first bullet doesn't do the job, the second or third might not do it, either.
A lot of police shootings take place on tachypsychia autopilot–once the action starts, it doesn't stop until the gun runs dry, and since this might take all of a second or two when the hormones are roaring, the conscious mind's decision to cease fire might not get there in time:
Gun! Gun! Shootshootshootshootshoot–!
And the brain kicks back on, the tunnel vision clears, the ears click in–after the incident is done.
The link takes you to a place where you can key in a caliber and even a brand of ammo and see how it fared in actual one-shot stops. Sometimes the total numbers in the sample aren't very big. Not a lot of police use mouseguns. And some calibers don't show any stops, because nobody has admitted to shooting somebody with them. I don't see a lot of .454 Casull or .500 Reeder shootings, though I'm pretty sure that a gun that will drop a charging brown bear or a rhino will cause most people to reconsider their actions, too ...
The best commerical centerfire handgun rounds run to 96% one shot stops, usually in .357 Magnum JHP, or .40, maybe a couple of the 10mm rounds. The worst, usually .25 ACP ball, will do it less than 25 or 30% of the time. Ten guys coming at you, you shoot them all center of mass with a .25 ACP, six or seven of them keep coming. Not so good.
Ten guys, a .357 Magnum, nine-and-a-half of them stop. Much better, but still, the other four-tenths guy might eat your lunch. It has happened.
So even with the best commercial handgun round, your revolver or semi-auto or single-shot pistol is not a magic wand that you can wave and guarantee that all will fall down.
The surest instant firearm stop is a big fast bullet to the brain or cutting the spine up high, and rifles are better than handguns. A handgun shot to the heart or lung or liver will probably be fatal a lot of the time, but not instantly so, and it only takes a moment for the shot guy to shoot back.
There are people who fall over dead that fast with a single shot by somebody's nine, or Saturday Night Special in .32, from shock, but they are the exceptions and not the rule. If your good guy is cooking, you need to keep that in mind.
People can be fragile, but they can also be most durable.