As long as it is a percussion or flintlock pistol, revolver, or rifle that is considered an antique (made before 1898) by the ATF it can be legally shipped via the US Postal service, UPS, or Fed Ex. This includes original antique firearms and replicas. You don't even need to declare what is in the box. You cannot ship ammo, powder, or percussion caps in the same box, even with a hazmat certification.
This is the basics, check for yourself as I can't guarantee the accuracy since I'm not a shipping expert.