**full disclosure-I've never shot, or hunted bears, and
a conservation officer I spoke to in S. BC once told me you're far better off with bear spray over a gun**
Only time I wish I had a gun with me was in Grizzly country (AB), but it was a young black bear that was a little too interested. Didn't get hurt or anything, but I had no confidence in the fly rod I was holding to help me either. I've had more experienced guys tell me that you can just "tell" when a bear has ill intent, and I never knew what that meant. Had a bad feeling about that bear the second I saw it.
Had a black bear get half-way into our tent here in Ontario last spring, fortunately by buddy who owns the camp is a light sleeper. I woke-up to the sound of someone hitting a spoon on the bottom of a large pot. lol Same guy has an 870 at arm's length all the time, just isn't as nervous as this suburban guy.
I have seen a few Grizzlies in AB, one fairly close while I was in a Nissan Pathfinder. Based on the size of those bears...it's easy to believe the 45-70/12ga slug gun suggestions. It's alarming to see how fast bears can move overall, I think if there is really a concern about bears..you want to be good with whatever you choose, and practice getting shots off fast. Something like a Marlin 1895G would be small/fast-handling if you can find one. You could also leave a round in the chamber and know you're very safe/could be cocked very fast, etc. Personally, I'd trust myself with a 12ga pump more, but mostly due to familiarity/confidence.