Not legal advice
This is actually something I have had occasion to consider before and do some preliminary research into. Here's what I've come up with, FWIW.
It appears that postal regulations do not define a handgun as does GCA '68. That is, postal regs define "Pistol or Revolver" as "A pistol or revolver is a handgun designed to be fired by the use of a single hand." This would mean a complete handgun, not a frame or other parts.
The postal regs further say:
432 Mailability
432.1 General
The following conditions apply:
a. 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 C024.1.0.
b. 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 C024.1.0 or C024.2.0.
[See:
http://www.usps.com/cpim/ftp/pubs/pub52.htm]
Right there we can see in subsection "b" that the only restriction applies to parts that can be readily reassembled into a weapon. Obviously, a frame alone cannot be so reassembled.
Next, Exhibit 432.1 ("Mailability Requirements for Firearms") doesn't deal with parts, but "Handguns," which are defined as complete handguns: "Pistols, revolvers, and other firearms capable of being concealed on the person (for example, short-barreled shotguns and short-barreled rifles) are defined as handguns." Moreover, anything in Exhibit 432.1 would only be an exception to non-mailability, and not any additional restriction on mailing parts.
As far as I can tell, nowhere do postal regs define a handgun frame as a "firearm" or a "handgun," and the only mention of "parts" I could find restricts mailing "the disassembled parts of a handgun or other type of nonmailable firearm that can be readily reassembled as a weapon."
Clearly, one could ship a complete upper slide assembly and barrel as parts. That doesn't constitute a "pistol or revolver" under postal regs, nor is it a firearm under the GCA.
Now, if one ships, say, a handgun frame alone, that is not a "pistol or revolver" as defined in postal regs, nor something "that can be readily assembled as a weapon," and thus would be apparently mailable, notwithstanding the GCA definition of the frame, alone, as a "firearm."
Off hand, I don't recall anything in the GCA that would prohibit mailing a firearm as long as the recipient is authorized to receive the firearm (e.g., an FFL, a licensed manufacturer, etc.). I would need to check the GCA to be sure on this point.
So, the issue is: Can one legally mail a handgun by breaking it down into two shipments, one containing the frame, and the other with the remaining parts? It would seem so, but I am certainly not offering a legal opinion or advice on the point, nor am I suggesting anybody act on this discussion without further research and information.
I'm more than open to somebody finding the flaws in my legal reasoning, but I'd appreciate if you could cite some authority (e.g., laws, federal regs, ATF opinions, case law, ect.) for support so I can test my hypothesis. Probably the most helpful next step would be a review of the GCA to see what it has to say, if anything, about mailing firearms other than that the recipient be authorized to receive firearms in interstate commerce.
