Not sure if any of the previous posters have actually shot a bear or not. But, I can tell you that shooting a bear in a hunting scenario taught me that if you don't hit it hard and where it will do the most good, you aren't going to stop a bear.
Now mind you, a feeding bear that's unaware of your presence isn't going to be pumping adrenalin, yet at the shot, if not solidly anchored, they can still move very fast and for a considerable distance. In my case it was going away, which is a good thing.
Now amp a bear up on adrenalin, and you now have a critter that is very unlikely to be "stopped" by all but the most powerful of handguns, and then only with a shot that hits either the CNS, or breaks the front shoulders. Yes, I said shoulders, because even on three legs a bear can still move amazingly fast and do you considerable damage. If you manage to get enough shots to break both front shoulders, it should drop and then can be dealt with.
If you are seriously worried about a bear charge (they do happen, and folks do get ate, so it isn't out of the realm of possibilty) a long gun of sufficent ballistic energy, backed up by a minimum of a heavily loaded .45 colt or .44 mag would be advisable.
The only problem, is that this combo is going to be heavy as all get out to carry around all day. And, unless you can sling the long gun, you're going to have to pick it up and set it down all day long. And Murphy being Murphy the one time you're going to need the darned thing, it's going to be out of reach, ergo the large caliber wheel gun.