If you choose a "slug" round for black bear, avoid most of the current crop of Foster type rifled slugs with a "hollow point." These soft lead thimbles quickly turn into a soft lead pancake with the center missing and often break into multiple pieces - severely limiting penetration.
For those who prefer buckshot, I suggest using the largest, hard cast buckshot round commercially available, the 12 gauge, 3 inch Dixie Tri-Ball Buckshot round. This buckshot round is in a class by itself, with three 60 caliber, 315 grain hard cast pellets at 1100 fps. Forty yard patterns from my 870 with a full choke tube usually run under 4 inches!