As stated above, if you think you are going to shoot an unaware bear from 100 yards, then use a .338 or .375 with a scope.
If you have to stoop and crawl in thick brush, and maybe take the first shot inside 20 yards, I would carry a lever action in .444, .45-70 or .405 Win, unless you have a .375 H&H or 9.3x74R double rifle handy.
