Long guns can be mailed by individuals USPS.
12.2 Rifles and Shotguns
Except under 12.1.1d and 12.1.2, unloaded rifles and shotguns are mailable. Mailers must comply with the rules and regulations under 27 CFR, Part 478, as well as state and local laws. The mailer may be required by the USPS to establish, by opening the parcel or by written certification, that the rifle or shotgun is unloaded and not ineligible for mailing under 12.1.1d. The following conditions also apply:
Hand guns can be mailed USPS from MFG to FFL and between FFL's.
USPS 432.23 Manufacturers, Dealers, and Importers:
Handguns may also be mailed between licensed manufacturers of firearms, licensed dealers of firearms, and licensed importers of firearms in customary trade shipments, or for repairing or replacing parts.