My question would be: What guarantee do you have that you are going to get a one-stop shot from any gun of any caliber?
Yes, if you have five bullets and three assailants you HAVE to make those rounds count but there is no guarantee, even if you do happen to make head shots in every case, that the assailants are going to go down and/or stop fighting.
There was a case of a police officer in Georgia who was shot point blank in the face while preforming a traffic stop. He was aware enough to return fire and survived.
Read the book, "The Year We Disappeared." A police officer shot in the side of the face with a shotgun. He never lost consciousness and was even writing notes about who he thought did it while they were waiting for the ambulance to arrive.
As pointed out, one-stop shots look great on the movies but in actuality we have absolutely no assurance that our bullets or guns are going to make someone stop.
We are all pretty aware that drugs or even desperation can keep someone going despite incredible odds.
People are shot with shotguns, blown up with grenades, rifles, etc, and still survive.
We've all seen the famous Brad Kasal photo of the Marine getting helped from the house in Iraq still holding his M9, finger off trigger.
The man was shot seven times, had taken the brunt of the blast from a grenade, lost half of his blood and was kept conscious by will power alone. Of course he is a good guy but it stands to reason that if one good man can stay alive despite the odds, a bad guy can do the same.
Not to mention that taking the time to stop and assess whether or not your one shot had the stopping effects you were hoping for is precious time wasted that you should probably be following up with more well-placed shots (if you even remember drawing and firing to begin with).
I don't get to caught up in the idea of one-stop shots. I do believe they are possible but if/when they happen I would look at them as a gift rather than something I expected.
One of my instructors once said, "Expect to have to fire your gun empty, reload and empty that magazine, beat them to death with your empty gun and cut their head off with your pocket knife. If the fight ends before all that you can be pleasantly surprised."
I don't feel under-gunned with a lower capacity firearm. I'm not out there looking for trouble. But, were I a police officer or required to go into questionable neighborhoods for jobs, etc, you can bet all you own that I would be carrying the highest capacity available in the largest caliber I can comfortably handle and I'm shooting until slide-lock.