Short barreled Rifle with a very big hole in the end or 12GA shotgun with slugs. Forget buckshot or revolvers, they both lack adequate knock down or penetration. Bottom line, Deep penetration, break big bones. Forget the fancy advice to make a head shot. You'll be lucky to hit center of mass.
If you insist on using a revolver, load with only one round.
When attacked feed your arms and legs to the bear as slowly as possible. When the pain becomes unbearable shoot yourself.
Remember this is not about buying a firearm capable of taking a big bear under normal hunting conditions. This is about stopping a large Bear in his tracks that is quite intent on killing you.
Once you've decided on a weapon, PRACTICE A LOT!!!
Then practice some more.
The odds are quite slim, but should you ever find your life is under immediate termination by a bear there will be no time to think. You're mind and body will go into automatic. Time will slow down and every thing in your world will cease to exist except total focus on the bear. You will only survive by doing those things which you have trained yourself to do. With good training there will be no panic, just calm methodical action. There will be plenty of time later to get all wiggly kneed.
Without practice, you’ll just be another excrement covered snack.
Joel.