If shot placement is the only determining factor, then just get a .25ACP. As long as you can shoot the BG in the eyes, the bullet will mess around with his grey matter and he will not bother you any more.
Go with a caliber that gives you confidence and practice a lot. Oh yeah, also need to grow a pair so when you point the gun at the BG, your hands don't shake like Deputy Barney Fife.
