I lean with the large bore crowd. In some parts of NA, the black bears run HUGE. Around here, not so much, however my buddy's wife just shot a B&C bear locally. The hide squared 7 1/2 feet and skull was over 20 1/2", so there are a few larger bears - probably closer to the 450 pound mark, than 300. However, 300 pounds is still a large bear!
I could use my .50 for bear, but it's not a hunting rifle, more of a show/rendezvous plinker. My 14 bore (.69) is for hunting, including bear, but I could be happy hunting them with a .54 or .58 as well. If there is a .6 at the front of the calibre number, that's even better.
I prefer a flat butt, myself - Edward Marshal is great in a Penn. rifle in a flinter, imho, or an English gun in cap lock cannot be improved upon.
I do not like the Hawken's buttplate Hooks in calibres that kick. I doubt you would either.
My flat butt English gun is VERY comfortable with a light plinking load of 3 drams (82gr.), and shoots quite nicely with double that amount of powder, however most guys here who have shot her, do not agree with that statement.
My 14 bore's butt.
Dan's 16 bore's butt.
Taylor's J. Lang's butt.
A pair of S.Hawken's butts.
A couple more Hawken butts - both .62's - ouch.