They aren't really "muzzleloader" barrels - they are essentially centerfire barrels used in muzzleloaders. I think that is what accounts for the .410, .451, .504, and .540 caliber markings. They are bored and rifled as if for centerfire, example would be that a .451 White could be wearing the same barrel (with breechplug threads instead of a chamber)that might be used for a .458 Winchester in another life.