There is not only ammo to think about but also strategy:
What strategy can you expect when you are accosted at night by an assailant intent on taking your purse, car or your life? When your attacker is at arms length you won't have time to consider shot placement. At this point your best hope for is to react as quickly as you can. Fire six shots and pray...and a .22lr just ain't going to get the job done (IMHO).
If you are serious about self defense you might want to consider something else. I would not trust a .22 to protect me from a large dog attack, much less a large Meth-head. Just my two cents.