The harder the punch the better off you're going to be. Nobody can give you a definitive answer. You could kill a bear with a .22 if you're a damn fine shot. If you're worried about bears from a self-defense stand point I'd spring for brenneke black magic magnums. They are pricier but you aren't taking them out for range ammo.
I think Publius is onto something but 12ga 00 buck only has 9 balls in it. I'd maybe even go with birdshot to blind.
Your 9mm could, and in all likelihood will expand. But my .45 will never get any smaller.