A self defense firearm should stop an assailant and should stop quickly. A 22 can work, but a .45 can too and will work quicker...most of the time. You will not find a large body of experimental evidence on using 22 for self defense since there are so many better choices. You will find a lot of anecdotal evidence of 22 being used both sucessfully and unsucessfully.

Bullet selection doesn't really matter at such low velocity and the difference between HP and solids isn't a really big deal when talking about 22's.

The key is shot placement, and with a 22, you want headshots, not chest/center of mass shots, to get the most reliable "stop" IMO.

YMMV.