The use of any other material, other than pure lead, for muzzleloaders, will be impractical.
The very nature in which lead works, is what makes
your rifled muzzleloader accurate.
Lead is soft, heavy ( 11 times the weight of water)it expands after ignition, sealing the bore, and thus rotating down the bore, emparting spin to the ball, or minie ball.
No other commonly available material has these properties.
Wheel weights for example are "mostly" lead. They don't work worth a darn in a rifled gun, and yet they are softer than copper, gold, silver, depleated uranium, tin,zinc.
Changing the regs on hunting migratory birds with lead shot was a wise move, elimiting small lead shot from the water, and wounded birds.
Removing lead from a bullet type projectile will not resolve anything, other than appeasing the concerns of some "TREE HUGGER" that thinks they are saving the world again,
Anybody who studies balistics, or physics understands the use of lead as a projectile in a rifled gun.
Hard lead, copper ball, or steel ball bearing will not effect the accuracy of a smooth bore, but the won't work in a rifled gun.
Best Regards
Old Ford