[mrerick]

The USPS definition of a firearm, handgun, etc is here:

http://pe.usps.com/text/pub52/pub52c4_008.htm

431.1 Firearm

A firearm is defined as any device (including a starter gun) that is designed, or may readily be converted, to expel a projectile by an explosion, a spring, or other mechanical action, or by air or gas pressure with sufficient force to be used as a weapon.


- - -



One could make the argument that a frame, receiver or other gun part is a device that could "be readily converted...".


But it's further defined in:



http://pe.usps.com/text/pub52/pub52c4_009.htm


432 Mailability

432.1 General

The following conditions apply:

1. Pistols, revolvers, and other firearms capable of being concealed on the person (referred to as “handguns”) are nonmailable in the domestic mail, except as permitted in Exhibit 432.1 and DMM 601.11.1.
2. The disassembled parts of a handgun or other type of nonmailable firearm that can be readily reassembled as a weapon are nonmailable, except as permitted in Exhibit 432.1 and DMM 601.11.1 or 601.11.2.
3. Unloaded antique firearms sent as curios or museum pieces are generally permitted, as specified in Exhibit 432.1 and DMM 601.11.2.
4. Unloaded rifles and shotguns may be mailed if the mailer fully complies with the Gun Control Act of 1968 (Public Law 90—618) and
   18 U.S.C. 921. The mailer may be required to establish, by opening the parcel or by written certification, that the gun is unloaded and not excluded from mailing because of the restrictions in 432.1b and c.

The DMM is at:



http://pe.usps.com/cpim/ftp/manuals/dmm300/601.pdf


Firearms are actually discussed in section 12, rather than 11.



It now clearly states this:



12.1.2 Handguns
Handguns and other firearms capable of being concealed on the person are
nonmailable unless mailed between the parties listed in 12.1.3 and 12.1.5 after the
filing of an affidavit or statement required by 12.1.4 and 12.1.6, and are subject to
the following:
a. Firearms meeting the definition of a handgun under 12.1.1b, and the definition
of curios or relics under 27 CFR 478.11 may be mailed between curio and relic
collectors only when those firearms also meet the definition of an antique
firearm under 12.1.1h.
b. Firearms meeting the definition of a handgun under 12.1.1b which are certified
by the curator of a municipal, state, or federal museum which exhibits firearms
to be curios or relics of museum interest may be accepted for mailing without
regard to 12.1.3 through 12.1.6.
c. Air guns that do not fall within the definition of firearm under 12.1.1a that are
capable of being concealed on a person are mailable; but must include adult
signature service under 503.8.0. Mailers must comply with all applicable state
and local regulations.
d. Parts of handguns are mailable, except for handgun frames, receivers or other
parts or components regulated under Chapter 44, Title 18, U.S.C.
e. Mailers are also subject to applicable restrictions by governments of a state,
territory, or district.