I think Colorado has it right w/ .50 cal. being the minimum. I previously used a .50 cal., 410 gr. Buffalo Bullet and if a conical is used, the length cannot exceed 2X the dia. Have since gone to a .54 PRB and of the elk I've shot, there's no difference between the BBs and the PRB asre "killing power". Went to the PRB because of better trajectory....less mid-range height. Who's Toby Bridges and why are people "bad mouthing" him?.....Fred