In Alaska the regulations require that you MAY NOT hunt brown/grizzly bear, black bear, moose, bison, elk, muskox, or mountian goat with a muzzle loading rifle UNLESS it is .54 caliber or larger, or at least .45 caliber with a 250 grain or larger slug.

The few people I know who actually have hunted with a muzzle loading rifle have used nothing smaller than .54 caliber. They may have used a .58 or .60 caliber smooth bore gun.