A black powder muzzle loading rifle does not satisfy the federal definition of "firearm" and may be sent directly to the buyer via US Mail.
Suggest you attend some local black powder meets (rendezvous). They're a lot of fun and you'll learn to roll your own (powder horn, hunting bag, knives, clothing, even guns, etc.).
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!