Are there any documented cases of anyone using a handgun to thwart a grizzly attack?
I've just done a rudimentary google search and I can't find anything. Also, there are very few documented cases of bear spray actually thwarting attacks. I can find a few cases where a rifle/shotgun stopped a bear's charge after being wounded while hunting.
Last year my cousin and a buddy had a brown bear sow bluff charge them several times. They inadvertently crossed paths with her cub, who crossed their trail with a salmon in it's mouth. They stood their ground with weapons drawn until she and the cub left the area. He said it was one of the most hair raising events of his life. He was carrying a 454 casul and his buddy had a 44 mag, neither fired a shot.
Years ago a buddy used a paintball gun to harass black bears who were tearing up trash cans at a popular camp site. The paint balls were the "pepper" kind. They had a little sow chase them when they peppered her and her two cubs coming out of a dumpster. Their guns were full auto and they had dozens of hits on the bears, even in the face.
Thankfully I've never had a brown or grizzly encounter. The few times I've hunted in that terrain I always felt safe with just my rifle. I'm looking at a carry gun now though.