There are muzzel loaders and there are muzzel loaders. A modern, in-line weapon of at least .50 .cal, particularly with one of the conicals, would be very effective on most plains game. If eyes and open sights are an issue, in-lines can be easily scoped. So set up, such a black powder rig would give you lethal accuracy to at least 150 meters. A traditional black powder weapon, firing a round ball over open sights is, I think, a 100 meter weapon with a bit less margin of error and significantly less penetration than a solid conical. I would stay away from most of the sabot projectiles - many are little more than glorified pistol bullets with lousy penetration characteristics.