But that's really not the standard for a self defense piece. Many a man has been killed by an opponent who was already seriously wounded -- witness the FBI shootout in Miami. The bad guy who did all the killing was actually killed by a bullet fired by the FIRST FBI agent to die.
The first criteria for a self-defense gun is reliability -- it has to go bang! every time you pull the trigger. The .22LR cartridge doesn't meet this standard.
The second is shootability -- and here a .22 might shine. Most of us can shoot a .22 as well as anything.
The third is power -- given a hit, how likely are we to put the bad guy down? Here, the .22 takes a back seat.
The fourt is concealability -- and there are plenty of powerful self-defense guns as concealable as any .22.