It's well established that there's no such thing as a "brush buster" load. All projectiles, including 12 gag slugs, are easily deflected by the "wrong' sort of impact with twigs. Naturally, the closer to the gun that impact occurs,a nd the further away from the target it occurs, the less likely you will be to get a good hit/decent penetration. I"d say that a 12 ga shotgun, with 000 Buck,. using 3" shells in an autoloader, should do ok vs big bears. You need brain hits to stop charges. So just wait until he's inside of 10 yds, and get off 3-4 really good shots at his head. Bears often make "false" charges, from a distance, to see what you will do. Once you wound them, tho, then their charge will be to the death. So to start hitting a charging brown bear (in the body) with a rifle, at less than 50 yds, with an autorifle, or less than 100 yds (open country) with a bolt action, is a mistake.