You actually don't need any kind of gun at all to survive a grizzly attack. I worked for a lady who, on duty in a park in Alaska, played possum with an attacking griz and emerged unscathed. Her partner suffered significant wounds and they aborted their mission.

I could have suggested that she was simply too tough and ornery for the grizzly to stomach, but, since she was my boss at the time, I held my tongue. Just for the record, she was one of the better supervisors I have experienced. Grizzlies just have no taste whatever....