Yeah but, supercracker is talking about ROUND BALL and the only way to get more weight / penetration, is to go bigger in diameter.
I would think that a .58 pure lead RB would be quite sufficient for Buff with good placement. You should be able to get 1400-1600 FPS from the Muzzle loader.
I have a .52cal cap & ball, that I made back in the early 80's, and my load is 85g 2f giving 1650 FPS. The pure lead ball weighs 200g and really penetrates well.
While I was typing this two more answers came in.