I've never even seen a black bear in the wild. You shouldn't worry about bears unless you wander around Sitka or Yellowstone while wearing bacon-flavored underwear. But since you asked, I'll give you an opinion as worthless as any other guy's thoughts whose never shot a bad bear either. Seems to me if you have to shoot a bear he's going to be very, very close. You don't really have to kill him on the spot. This would be ideal, but if you hit said bear full in the face with a load of heavy buckshot at 15 feet, I suspect he'll not only loose his sight, but his hostile intent if not his brains too. Single lead buckshot pellets at 40 yards are about like .22 rimfire bullets. But a swarm of them clustered like a fist will break facial bones, take out eyes, teeth, noses and generally f*** up any bear enough to let you get away. The pellets might even get through his skull enmass and get to the brain. I would choose a buckshot load over a slug as the slug (if not centered) might glance off the skull and mearly cut a channel along the flesh. What I've outlined is all a guess on my part. If a bear attacked me, I'd pull a model 36 S&W and shoot myself in the right temple. It might work out better in the end.
John