Antiques:
Antique firearms are defined as any muzzle loading rifle/shotgun/pistol, which is designed to use black powder or a black powder substitute, and which cannot use fixed ammunition.
They must be:
Mailed between curio and relic collectors only when those firearms also meet the definition of an antique firearm.
Certified by the curator of a municipal, state, or federal museum that exhibits firearms to be curios or relics of museum interest.
from usps site.
