If you've worked in anything related to technology or complex machinery you'll recognize the phrase, "chain of events." Tragedies with complex systems are seldom the result of a simple failures. Instead they are the result of a series of small failures whose results lie dormant until a final fatal failure causes all of the other minor events to coalesce into the big event.
Space Shuttle Challenger failure? Cold ambient temperatures plus wind plus Go-fever in management plus O-ring brittleness. Any one of them by itself was not fatal. Combine them and seven astronauts die.
The worst accident in aviation history was the collision of two Boeing 747s at Tennerife in the Canary Islands. Confusion plus fog plus an overcrowded airport plus radio problems plus a captain with Go-fever and 585 people die.
This concept of a chain of events occurs around us all the time.
Having your brakes fail is bad. Having them fail in your driveway isn't all that bad. Having them fail while driving down a mountain road is very bad.
Locking your keys in your car is bad. Doing it in when your wife is standing next to you with her set of keys is not so bad. Doing it while the car is running while it's raining at a highway rest area with your cellphone inside at 3AM is very bad.
The gun community seems to not get this when they delve into the endless discussions of gun type, caliber, stance, ammo capacity, shot placement, bullet weight or carry method and the major subject is self defense.
Each of the discussed categories have their own values but none of them by themselves will cause a disaster in a self-defense situation.
Shot placement matters. But good shot placement may not be enough if you use a small or slow caliber.
So does caliber matter? Sure can't hurt to carry something that starts with a 4 but if you aren't hitting center mass even a big round won't help you. A .22LR in the eye socket will almost certainly stop the threat.
So does round count matter? Only if you end up needing them but don't have them. Having more is great right up until the gun weighs too much to carry comfortably.
What about reliability? If the gun reliably shoots 5 rounds without a malfunction that's almost certainly good enough for self-defense. If it jams up after 100 rounds or when it's hot that might rule it out for competition but for self-defense anything other than 5 rounds is nice but not necessary.
Ok, so I should carry a .45ACP in a gun that holds 40 rounds? That would be great if you can (and will) actually carry a gun like that.
Frankly, if physics and the law were no objects I'd carry a belt-fed minigun and dip the bullets in Cone snail venom. Since that's not really feasible anything else is going to be a compromise.
Most self-defense encounters are settled with the expenditure of 2 or 3 rounds so anything that will reliably fire four or more is functionally sufficient for self-defense with a reasonable caliber in a gun that you're comfortable with and carried with you in a way you can access it.
In the end it's going to come down to your own judgement of your skills, your reaction speed, the way you react to life-threatening situations, your size, shape and lifestyle. Those variables will require you to make judgements on the other factors.
Making the wrong choice in any of the factors is unlikely to be THE ONE THING that gets you killed. But if you carry a Raven in a .25ACP in an ankle holster while wearing BDU pants you've probably stacked up the odds against yourself. Add in darkness, cold weather bad enough to make you shake, and a bad guy popping up inside your Tueller limit and there you go, disaster. Ameliorate enough of the negatives and you're ok. Stack up enough of them and you're not.
If you like your Glock in .40S&W and you can hit the target at 7 yards then carry that and be content.
If you like a 1911 in .45ACP and you can hit the target at 7 yards then carry that and be content.
If you carry a .38 snubbie in a shoulder holster and you can hit the target at 7 yards then carry THAT and be content.
If your gun isn't capable of shooting 4 or 5 rounds reliably then get it to a gunsmith or replace it.
And if the guy next to you is carrying something else in a different way shut up and mind your own business.
That means you, Rob Pincus.