You drop somebody into the woods who knows little about bears and guns with his 9mm in front of an angry bear, I'm betting on the bear.
Of course, if the guy did not have a 9mm, the odds are stacked even more in the bear's favor.
If you were to sit down with God and design a critter to resist gunfire while coming at you to kill you quick, you would come up with something that looks a lot like a bear. Not that it's impossible, but I'd rather be in Philadelphia.
But all of this is irrelevant when it comes to the right to keep and "bear" arms. Most guns are designed to be effective against humans, and that is why the right is guaranteed. Using a gun designed to be effective against humans against bears is selecting the wrong tool for the job.
The old joke is that the word "ducks" does not appear in the Constitution. Of course, the word "bear" does, but not in that context.
A few years ago a fisherman killed a chargeing brown bear with one shot from his 9mm any gun is better then no gun.
More likely a cape buffalo or a gaur but bears would be high on the list. About half of the critical target area of the giant bovines is covered by the boss of those horns which even the most powerful of rifles can't always penetrate.