Years ago the army did research on pistol rounds and found that shot placement was more important than caliber and the only places to reliably stop someone was with a shot to the head, the spine or the long bones of the leg. A .45 to the chest would kill the perp eventually but they would still have time to do some damage to you, while a .22 in the brain had the same effect as blowing the head off with a shotgun. So instead of looking for a killer round get something you can afford to practice, practice practice with.
Thanks for taking the time tho!!