Bryce Towsley has this story to tell us about a grizzly attack:
Shortly before I arrived in one northern Alaska camp, a couple of packers were rushing to beat the failing light. Covered with blood and loaded down with packs of caribou meat that doubled the weight pressing on their boots, they were tired and thinking mostly about a hot meal and a dry sleeping bag as they pressed toward camp. As the lead guy pushed through the thick brush along the river a very large grizzly bear stood up and took a swing at his head. Young reflexes saved him, as he ducked and the bear hit the pack, knocking him down. He rolled, drew the .44 Magnum pistol in his chest holster and emptied it into the bear, ending the encounter.
Bryce Towsley goes on to say this about handguns and bears in his article Packing Pistols In Bear Country.
The .44 Magnum is probably the bottom limit. I know, some guys claim the .41 Magnum or even the .357 Magnum is fine, but I do not agree. I have watched them both work on black bears and I was never impressed. The point here is not to prove you can stop a charging bear with a smaller, inadequate cartridge. The point is to survive. So, I think a .44 Magnum is the starting point.
It is also important to use a heavy bullet. The key is to penetrate through bone and through a lot of critter, and for that you need bullet weight: at least 300 grains for a .44 Magnum and that much or more for any of the .454 Casull or heavy .45 Colt loads. I use 400-grain bullets in my .500 Wyoming Express; with some bigger cartridges like the .500 S&W Magnum you can go up as high as 500 grains.
Part of the bullet-weight decision, though, should be based on your ability to shoot the cartridge. A 500-grain bullet at 1450 fps from a handgun is not a load most NPR listeners will want to shoot. I have passed my personal limits with some of the biggest cartridges when used in lightweight, carry-style handguns. For example, in my .500 Wyoming Express I had to back off from the full-power loads. A 400-grain bullet at 1600 fps proved too much for me (and everybody else who shot it) to handle well in a relatively light, 2-pound, 12-ounce Freedom Arms revolver. So I backed it down to about 1300 fps and found the load much more manageable, but still plenty bear-worthy.
Cartridge selection? Well, I really believe any handgun you bring to any fight should have a name that starts with at least a 4. That’s really important if you are fighting a bear! I think the rule of “4-3-1” should apply: a bullet diameter that starts with at least a 4, a bullet weight that starts with at least a 3 and a muzzle velocity of at least 1000 fps.
If you prefer to look into your particular needs check out these sites.
Cabela's 10 Best Bear Defense Guns by John McAdams
Handguns for Protection in the Field by Chuck Hawks