Actually an AK-47 could be a reasonable bear gun since many 30-30's have taken large griz. A .223?? No, not what I would want with large griz but even that would be better than a sling shot or harmonica.A gun simply cannot kill a bear, they have become resistant to the lead I am told. Their bodies deflect most of the bullets anyway. Pepper spray only makes their poop spicier.
The best thing perhaps would be taking classes in bear communication, where you can learn skills of reasoning, persuasion and discussion with the bears. There was this one guy who hung out with grizzlies in Alaska who was very effective at doing this. Sadly, one day his negotiating skills fell through and well, he ended up in a bear's stomach.
It also doesn't hurt to take somebody along with you and a 22.. Basically, you can outrun the other guy if the communication tactics fail.
An AR for bear defense?? Good luck.. Yeah , I heard another guy who says only to use 45 ball ammo, as you get more shots off.. I'd take his advice over the advice of an AR. I don't like his advice either.
Show me a link where a single guy with an AR took down an 800lb tiger. Grizzly bears have thicker fur and mass than your average jungle tiger anyhow. The Indochinese tigers our soldiers may have encountered in Vietnam, in general, weighed 250-400lbs, about the same as Black bears do here in Oregon and Washington.
http://thefiringline.com/forums/showthread.php?t=424481&highlight=harmonica