I'm a believer in Massad Ayoob teachings in self defense.

Largest caliber in the heaviest lead faced bullet you can handle in a given firearm under stress.
To some that is a .22lr to some that is a .500S&W.

My personal is a 200 grain at 1300ft/sec. More and I become recoil sensative. Less and I feel it's not enough from me.