Handguns are mailable between dealers and other parties as indicated in C024 of the USPS manual. When shipping long guns by the USPS they have the right to inspect the gun to ensure that it is unloaded.
PART 178-COMMERCE IN FIREARMS AND AMMUNITION
Federal Firearms Act. 15 U.S.C. Chapter 18.
Firearm. Any weapon, including a starter gun, which will or is designed to or may readily be converted to expel a projectile by the action of an explosive; the frame or receiver of any such weapon; any firearm muffler or firearm silencer; or any destructive device; but the term shall not include an antique firearm. In the case of a licensed collector, the term shall mean only curios and relics.
C024 Other Restricted or Nonmailable Matter
Handguns
1.2
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 1.3 and 1.5 after the filing of an affidavit or statement required by 1.4 and 1.6.
Authorized Persons
1.3
Subject to 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 1.3c through 1.3e.