Black powder muzzle loader rifles, either original or replica, are not considered as being firearms under Federal law. They can be shipped via USPS.
From USPS Regs re. BP guns:
431.3 Antique Firearm
Antique firearm means any muzzle loading rifle/shotgun/pistol, which is designed to use black powder or a black powder substitute, and which cannot use fixed ammunition (except those that incorporate a firearm frame or receiver, any firearm which is converted into a muzzle loading weapon, or any muzzle loading weapon which can be readily converted to fire fixed ammunition by replacing the barrel, bolt, breechblock, or any combination thereof); or any firearm (including those with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured on or before 1898, or any replica thereof, if such replica:
Is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition.
Uses rimfire or conventional centerfire fixed ammunition that is no longer manufactured in the United States and is not readily available in the ordinary channels of commercial trade.