It doesn't take a cannon to kill a Black Bear.
I have laid many, many blackies low with chamberings starting at the 6mm Remington right up to the 338 Win Mag.
Personally, If I was going after a specific big blackie, I would probably grab my 8x57JS, using that great 200 Accubond.
I have shot a couple with this combo, and one with the 8mm - 180 Ballistic Tip, a pretty tough bullet in 8mm.
All were passthroughs, and all were dead within 30 meters.
Obviously the 270, 7mm's, and any reasonable 30 cal is plenty of medicine for them.
Eagleye.