Bird shot might turn a bluf charge but if he had any serious intent it wouldn't have done much good past 10-15 feet. Even buckshot has not been impressive, about the only thing out of a shotgun that works well is brenneke black magic out of a smoothbore or one of the better sabot slugs.
Shotguns are for birds! The are most effective with the fireworks styel shells used to scare bears off before they become a problem. Your best bet is a short compact rifle like a 45/70 guide gun running a good heavy projectile. Any handgun is a poor replacement but is far better than nothing.
The glock 10mm is going to be a lot easier to make repeated hits with in shorter amount of time. It will penetrate well with FMJ or hard cast and poking holes into the right spot is the name of the game. A 44mag, 454 casual, or 500 smith are impressive rounds but you will likely only get one off before the bear is on top of you, so make it count.
The #1 bear defence is situational awareness and prevention. The best way to survive an encounter is to not have it in the first place.
