My own opinion is that a .44 Mag should be minimum, only because it gives you a margin for error in case the shot you wanted to put in the lungs gets into the shoulder instead.
I've only killed one bear myself (68 lb. Hoyt throwing a 2216 with a 125 Magnus head
), but I also know a lot of guys who have, and it seems that in a hunting situation
a black bear is no where near as tenacious of life as a whitetail. Put a good solid kill zone hit on them and they die pretty quick.