Ideally Id pick a modern double barrel (O/U prefered) Howdah in 45-70 or larger, something with say 8 inch barrels. For now that doesnt exist yet.. (I working on something). Since its currently vaporware; if I was forced to pick something in my safe, Id opt for a 4" Freedom Arms in 454 Casull. If funds were available I might choose the S&W 500 with a 4" barrel.
Im not convinced more than two shots are necessary or preferred. Ive been charged three times. Once by a Black Bear that I was stalking and twice by Wild Boars. I suppose I could have shot each of them from a greater distance but I chose to get closer, for the thrill of the hunt. For the Bear I was only carrying a rifle (30-06 Mauser Straight Pull), since Canada didnt like the idea of me bringing a handgun. The first Wild Boar dressed out at 300lbs. My buddy had wounded him with an arrow and he chose me to charge. My weapon was a T/C Contender with a 14" barrel in 44Mag loaded with Black Talons. I remember thinking Black Talons were so great in cop guns, as it turned out I found out earlier in the hunt that I would have been better using lead solids. When the Boar got to about 6 feet I fired and it hit him like the Hand of God. The second was when I went to check bait and a Boar came in fast. Im not sure that one counts as a charge but I was between him and the bait. I had left my rifle in the blind (of course right?), so I was only carrying my FA 454. Again I shot when it was about 6 feet away. This one was only about 150lbs but the 454 went from left shoulder and exited the right ham. I actually count it as a bad shot, since I was aiming for his head. Im not sure if it was all me of if he started to turn when I hit him--either way that was some great tasting pork. We celebrated that night with Schnapps around a fire.
A note on BEAR spray; Ive never used spray on a bear; but I did have an occasion to use pepper spray on a Rottweiler, I wasnt impressed. While it did make him turn, Im not convinced it was the pepper as much as the fog. OTOH I did TAZE a Pit Bull on a search warrant. It was a text book TAZE. The dog flopped SH*T and took off to parts unknown. If I were forced to go unarmed in Bear country Id opt for a TASER (not a stun gun).