You might be unpleasantly disappointed, wildcard, if you just settle for 12ga. slugs off the shelf. Breneke shotgun slugs are the best choice for dangerous game:
http://www.african-hunter.com/Rifle_Choice_4_Dangerous_Game.htm

I know.. we're not in Africa hunting with the shotgun. We're just trying not to be eaten by the bear. But these guys have been there.. done that.. I think they make sense.

I'm still sticking with my bearspray.