Not me, some of the calibers would be totally ridiculous and your time would be better spent running away. There are people that bring .22s, .22mags, .32s, .380s, 9mm, and other worthless calibers into the wilderness because it's their "carry gun" or they have some totally unrealistic expectation of getting a moving target in the eye while they are scared out of their mind. The 5.45/5.56 is at least moving fast enough that the small fragments WILL penetrate, it's just going to take a lot of those fragments to cause the massive damage necessary to take down something like a bear. I've thought about what it would take with my AR, and the local story the other day of the bear shot with the AK74 is pretty much what I expected, take a lot of rounds to cause the kind of damage necessary. It's not going to go through the skull intact and with enough energy due to how thick it is and the fragmentation, but enough fragments (rounds) and enough damage will be done eventually. Far from optimal and why people don't generally use ARs around here for bear defense, very poor choice.

Saw some huge moose just a few min ago, I swear one was as big as a car. Luckily it was chilling as we sped by on our bikes, but those are the kind that can charge you if they are protecting their calfs.