We will make it real simple. Handguns are nonmailable (with the exceptions of FFL to FFL and some other extremely limited circumstances). Handguns are firearms. Federal law defines a firearm to include the frame.
Federal law regarding mailing:
Section 1715.
Firearms as nonmailable; regulations
Pistols, revolvers, and other firearms capable of being concealed on the person are nonmailable and shall not be deposited in or carried by the mail or delivered by any officer or employee of the Postal Service. Such articles may be conveyed in the mail, under such regulations as the Postal Service shall prescribe, for use in connection with their official duty, to officers of the Army, Navy, Air Force, Coast Guard, Marine Corps, or Organized Reserve Corps; to officers of the National Guard or militia of a state, territory, commonwealth, possession, or district; to officers of the United States or of a state, territory, or district whose official duty is to serve warrants of arrest or commitments; to employees of the Postal Service; to officers and employees of enforcement agencies of the United States; and to watchmen engaged in guarding the property of the United States, a state, territory, commonwealth, possession, or district. Such articles also may be conveyed in the mail to manufacturers of firearms or bona fide dealers therein in customary trade shipments, including such articles for repairs or replacement of parts, from one to the other, under such regulations as the Postal Service shall prescribe. Whoever knowingly deposits for mailing or delivery, or knowingly causes to be delivered by mail according to the direction thereon, or at any place to which it is directed to be delivered by the person to whom it is addressed, any pistol, revolver, or firearm declared nonmailable by this section, shall be fined under this title or imprisoned not more than two years, or both.
18 U.S.C. 1715
18 USC 921
"Sec. 921. Definitions
(a) As used in this chapter--
(3)
The term "firearm'' means (A) 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; (B)
the frame or receiver of
any such weapon; (C) any firearm muffler or firearm silencer; or (D) any
destructive device. Such term does not include an antique firearm."
(29)
The term “handgun” means—
(A) a firearm which has a short stock and is designed to be held and fired by the use of a single hand; and
(B) any combination of parts from which a firearm described in subparagraph (A) can be assembled.
Domestic Mail Manual
http://pe.usps.com/text/dmm300/dmm300_landing.htm
601 Mailability
11.0
Other Restricted and Nonmailable Matter
11.1 Pistols, Revolvers, and Other Concealable Firearms
11.1.1 Definitions
The terms used in this standard are defined as follows:
a.
Handgun means any pistol, revolver, or other firearm or device the mailing of which is regulated by this standard.
b. Pistol or revolver means a handgun styled to be fired by the use of a single hand and to fire or otherwise expel a projectile by the action of an explosion, spring, or other mechanical action, or air or gas pressure with enough force to be used as a weapon.
c. Firearm means any device, including a starter gun, designed to, or that may readily be converted to, expel a projectile by the action of an explosion, spring, or other mechanical action, or air or gas pressure with enough force to be used as a weapon.