From USPS - Shipping Terms & Conditions - Firearms
11.3 Rifles and Shotguns
Although unloaded rifles and shotguns not precluded by 11.1.1e and 11.1.2 are mailable, mailers must comply with the Gun Control Act of 1968, Public Law 90-618, 18 USC 921, et seq., and the rules and regulations promulgated thereunder, 27 CFR 178, 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 gun is unloaded and not precluded by 11.1.1e.
11.1.1e says:
e. Short-barreled shotgun means a shotgun that has one or more barrels less than 18 inches long. The term short-barreled rifle means a rifle that has one or more barrels less than 16 inches long. These definitions include any weapon made from a shotgun or rifle, whether by alteration, modification, or otherwise, if such weapon as modified has an overall length of less than 26 inches. A short-barreled shotgun or rifle of greater dimension may be regarded as nonmailable when it has characteristics to allow concealment on the person.
I believe that the Gun Control Act of 1968 only covers interstate transactions.
And here's 27 CFR 178:
http://www.access.gpo.gov/nara/cfr/wais ... 78_01.html
I'm not sure if "Sec. 178.31 Delivery by common or contract carrier" from the link above applies or not.
