Use any bullet you can shoot confidently with - i would use a big heavy slow bullet in 308 - they are not hard to kill with a proper shot. My choice is a 12g slug - under 20yds is Thors hammer on blacks...been there, done that....
Since they can fight back, you're best shot would be a side shot thru front vitals/shoulders - Drops them in their tracks
X2 for the 12ga, this was a defence situation during hunting season a couple years ago, the nervous laughter was replaced with smiles once the situation calmed down as we had a bear tag to fill





















































