The articles I've seen have said that min for black bear is 30-30 or ballisicly equal. And after my bear I would want a good bullet in a 308win or bigger. There is just something about a animal that can clean your clock running out you that I would feel a little over kill in a gun is ok (as long as u can handle it)
I know my grandfather shot a charging sow with a 30-30 and killed it and it's cub with one shot. He always felt bad that it came to that.