"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. "
http://pe.usps.com/text/dmm300/601.htm#wp1103548
