Would you feel confident in using a .22 in a defensive situation of that was all you had available at the time? Obviously it's not ideal but is there any reason it wouldn't do the job if you could hit what you needed to? Personally, I don' know. I think I'd do sort of a modified Mozambique drill. Fire 2-3 shots center mass as quickly as I could get the front sight on target and then if he didn't drop or give up, attempt a head shot, which would probably also turn into a small volley of fire, rather than a single shot. That's the main advantage of a .22. There is almost no recoil so you can shoot a lot of rounds very quickly and accurately.