Muzzleloaders need a soft lead bullet that is undersized to fit the bore .I think the gas check would be detremental to accuracy because a ml has to bump up the bullet into the rifling .The 501 bullet would probably work if the gas check was not on the base.I dont think they would be dangerous .Heck ,if I had some I'd try 'em , you can't ever tell it might be more accurate than anything else.