I've read a lot of articles about this and what I've learned is that shot placement and practice comes first. Shoot the highest velocity ammo you feel comfortable with. For times where you may have to shoot through a jacket I would use the CCI FMJ/TMJ.

The only consistently expanding/fragmenting ammo that I know of is the Critical Defense 45gn, Speer Gold Dot 40gn, and the CCI TNT 30gn. I've heard good things about the Winchester JHP but haven't tested it yet.

I also would advise against the spring swap. Rimfires are already fickle and you don't want light strikes. My recommendation is to have a gunsmith smooth out the action, or do it yourself with oil and a scotch Brite pad. Then get some snap caps or used 22 WMR ammo and dry fire it a lot. Do not dryfire without used casings or caps, and double check you aren't using squib or live ammo. After about 1000-2000 rounds I've heard the trigger gets very smooth.

SVT28