I've been charged by a bear in the bush (geologist, charged by a black bear), and I don't think many people fully realize how fast a bear is. Or how little time you may have to process and react. Paying attention to your surroundings is vital, something I wasn't doing as it was heavy bush. I was charged from about 10-12 yards, and it took about 2 seconds for the bear to reach me. We didn't have a rifle with us, but I had no time to pull the bear spray from its holster on my belt before it was on me. The only thing that kept me from being mauled was my field assistant standing 4 yards behind me. Bear jumped me, looked up and heard/saw her losing her #### close by and fled.
One of my other assistants wounded a black in camp that same summer, and it came back the next night in a foul mood. The assistant shot it at point blank at the entrance to our kitchen tent, terrified with a 303, and hit the damned thing in the leg. Shot placement is a tough thing to get right when rushed and scared. I get the rationale for thinking a big bore would be handy in that situation, but I'm of the opinion that carrying something you're intimately familiar with may be better, smaller caliber or not. But if you're not paying attention to your surroundings, and/or don't have time to react, it may not matter.