Antique guns, including C&B pistols, are not restricted for mailing within the U.S.
From the USPS website:
DMM Revision: Mailing Firearms ”” Clarification
12.1.1 h. Antique firearm means any muzzle loading rifle/shotÂgun/pistol that is designed to use black powder or a black powder substitute and that cannot use fixed amÂmunition (except those that incorporate a firearm frame or receiver, any firearm that is converted into a muzzle loading weapon, or any muzzle loading weapon that can be readily converted to fire fixed ammunition by reÂplacing the barrel, bolt, breechblock, or any combinaÂtion 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:
1. Is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition.
2. Uses rimfire or conventional centerfire fixed ammunition that is no longer manufactured in the United States and that is not readily available in the ordinary channels of commercial trade.
Enjoy, J.D.