the only 3 times I've mailed disassembled guns, I've always declared my assembly of gun parts as just that--"gun parts", and showed the clerks (and supervisors) this portion of the regs; no one has ever questioned it. Since my shipments have been disassembled, they fall specifically under 1.1.c, and not 1.1.a nor 1.1.b (which deal by inference of 1.1.c), with assembled weapons.
BATF does interpret things a little differently, but US mailing laws don't seem to overlap in the 'FFL definition' of a "receiver".
Thanks for posting that particular reg. I disagree with your conclusion that a dissassembled firearm is NOT a firearm per the USPS because it is NOT able to be "readily converted....to fire a projectile." I think you are in error there and that your belief that per the USPS that a "dissassembled gun is NOT a gun" is an awfully thin thread to hang a defense on.
Look at Section 1.6 "Certificate of Manufacturers and Dealers." I can't get cut and paste to work so you'll have to read the whole section yourself, but I especially want to point out the phrases "...that the parcels containing handguns (or major component parts thereof) are customary shipments...and that to the best of his or her knowledge or belief, the adresses are licensed manufacturers or dealers in firearms"
The "Or major component parts" language jumps out at me. If a frame is not a "major component part," then what is? A real strict interpertation would mean that only FFL holders can receive ANY major component part, such as a slide or maybe even a barrel, but I think the intent was to signify that even if the firearm is not complete, as long as the "major component part" (i.e. frame) is included, mailing is restricted to FFL holders.
I'd also like to point out section 4.0 "Legal Opinions on Mailing Firearms." That explicitly states that "Postmasters are not authorized to give opinions on the legality of any shipment of rifles or shotguns. Contact the nearest office of the Burea of Alcohol, Tobacco and Firearms for further advice."
This establishes that the USPS does defer to the ATF for firearms related legal matters. This also makes sense from a common-sense point of view as well. If the ATF considers the receiver the firearm, why would the USPS NOT consider the receiver a firearm? Postal Regulations do NOT supersede Federal Law and the legal defination of a receiver as a firearm is Federal Law.
I'm honestly not trying to bust your chops. I just don't want anyone to get in trouble through what I consider to be inaccurate advice.
The best advice, as always, is to consult with an attorney familiar with Federal Law regarding firearms and have them review the law and regs and give you a real legal opinion. All this net wrangling is ultimately just amateur speculation.