You may want to seriously look into whether black powder ownership is legal in your state, in many states it is legal to own as long as it meets the definition of an antique. Antique meaning
A. Any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898; and
B. Any replica of any firearm described in subparagraph (A) if such replica
is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or
C. Any muzzle loading rifle, muzzle loading shotgun, or muzzle loading pistol, which is designed to use black powder, or black powder substitute, and which cannot use fixed ammunition. For purposes of this subparagraph, the term antique firearm shall not include any weapon which includes 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.
As to your original question about "air rifles" like others have said, it would be very helpful if you were to let us know which state you reside in. and to your