Minimum 44 Mag in a revolver. No offense to anyone, but, if my butt is on the line, I would not carry an automatic....and, I like auto's, especially the Colt 45 ACP! If you choose an auto it should be thoroughly tested, and you should have 100% faith that it will function reliably under all conditions!!! A Ruger Super Blackhawk, Redhawk, Super Redhawk, Freedom Arms, and I am sure there are others out there, that can handle stout heavy bullet loads is all I would put my life on the line with, and heck, that might not even work!!! There has been much discussion on this forum over the years about what it takes to stop a charging bear, and the difference between what a cartridge can do under "hunting" conditions, and what the same cartridge is capable of under a "charge" condition. By listening to what other folks have shared on this forum (yes, I do listen and learn), it is my belief, that a rifle in the "proper" caliber, or a shotgun with some of Gates slugs would be my first line of defense. The handgun is only a last ditch effort if the rifle/shotgun doesnt get the job done.