http://pe.usps.gov/text/dmm300/601.htm#wp1064962
"11.1.2 Handguns
Pistols, revolvers, and other firearms capable of being concealed on the person (referred to as handguns) are nonmailable unless mailed between the parties listed in 11.1.3 and 11.1.5 after the filing of an affidavit or statement required by 11.1.4 and 11.1.6.
11.1.3 Authorized Persons
Subject to 11.1.4, handguns may be mailed by a licensed manufacturer of firearms, a licensed dealer of firearms, or an authorized agent of the federal government or the government of a state, territory, or district, only when addressed to a person in one of the following categories for use in the person's official duties:
a. Officers of the Army, Coast Guard, Air Force, Navy, Marine Corps, or Organized Reserve Corps.
b. Officers of the National Guard or militia of a state, territory, or district.
c. Officers of the United States or of a state, territory, or district, whose official duty is to serve warrants of arrest or commitment.
d. USPS employees authorized by the Chief Postal Inspector.
e. Officers and employees of enforcement agencies of the United States.
f. Watchmen engaged in guarding the property of the United States, a state, territory, or district.
g. Purchasing agent or other designated member of agencies employing officers and employees included in 11.1.3c. through 11.1.3e."
"11.1.5 Manufacturers and Dealers
Handguns may also be mailed between licensed manufacturers of firearms and licensed dealers of firearms in customary trade shipments, or for repairing or replacing parts."