As flyu2 said a 22 will get the job done if you have the cool to be accurate, ie head shots.

Todays criminals are most likely trying to get money to drugs, so they can be hard to shot. This where mass comes into play. The Colt m1911 45 cal was adopted by the US Army because it spot a drug crazy HUD in the Phillipines.

The other thing have you every heard of a police shooting where they didn't empty the magazine into the Perp? They practice and they also need more than one shot.

If I was going to add one more handgun to my collection it would be the "Judge", 410 shot for two legged snakes and no leg snakes, plus a 44 cal round for stopping power.
GregM