What's wrong with a good rifle, a 308 or a 30-06?
Forget about the .410, that's really asking for trouble. The little shotgun is probably only going to piss off the bear. Theoretically 410 slugs should be enough to finish it off, but they are so inaccurate that it is virtually impossible to place them into a vital spot of a charging animal. Most guys think a bear is going to stand up right in front of them exposing his chest, but that is often not the case. There's gotta be some guys in here who have experienced a bear charging head on. I know I have. You barely have any (!) target at all, it's a big fur ball coming at you, if you put a bullet in his coat you're a goner. You need to hit his brain or heart, and these spots are only as big as a cantaloupe, seen from the front. Even withe a good rifle you may have a hard time to right spot, not to mention a slug that can't even hit a pick-up truck at 40 yards. A 12 gauge with heavy buck is better as the last resort, but the bear needs to be really close. Shooting at over 25 yards is pointless, it won't drop him. If you figure that out you have about 2 seconds to shot again, because that's how long it takes him to do the 25 yards and get to you. Less than 30 feet is a good distance for buck.