Those aren't ideal rounds, but you're getting piled on so I'm gonna point out to all the pilers-on that 20+ rounds of 5.56 at bear-attack range is sufficient to kill any bear. It's not what I'd want, but we are talking about 20-30 holes in an animal. Not to mention the arterial damage from the impact shock. Anyone that's shot an animal with a fast round knows just what that can do to a circulatory system.

As someone that routinely arms themselves against grizzlies and has had hostile encounters where I have scared them off with gun shots, I basically refuse to carry anything smaller than 10 mm with Underwood Dangerous Game rounds, and even that is a tradeoff. There are plenty of stories of people killing grizzlies with smaller rounds, but what there aren't are stories of people shooting grizzlies with very big rounds, and the grizzlies not dying. Whereas 357 magnum, 9 mm, etc, all take a lot of rounds to kill on average. 44 magnum is pretty much "sufficient" and if you go to rifles, center mass from a +P hardcast 45-70 will drop any grizzly in one shot. Even out of an 18" barrel. The Marlin 1895 has grown a reputation among bush pilots for that reason.

These debates rage on endlessly because they're entertaining, but ultimately the human with the gun is 50%+ of the equation...not the gun.