I've shot brown bears with 12 gauge, .375 H&H, and 9.3x62. I cannot tell the difference between the .375 and the 9.3 on them. I never intend to use a 12 ga. again! I used foster slugs, which was an utter disaster. Thankfully there was another guy with me using a .375 who really solved the problem. I've seen them sorted out with 12 ga. Brennekes, which would be the only shotgun load I'd consider even half suitable, but even then a proper rifle is the better option based on my experiences and observations. I do most of my deer hunting with the 9.3 just because it works so well on both bears and deer.
