I was thinking some more about that recent ATF airsoft seizure and came up with the following…
For your information, per provisions of the Gun Control Act (GCA) of 1968, 18 U.S.C. Chapter 44, an unlicensed individual may make a “firearm” as defined in the GCA for his own personal use, but not for sale or distribution.
The GCA, 18 U.S.C. § 921(a)(3), defines the term “firearm” to include the following:
… (A) any weapon (including a starter gun) which will or is designed to or may be readily 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 silencer; or (D) any destructive device. Such term does not include an antique firearm.
Source – ATF.gov
Another paragraph of interest…
Individuals manufacturing sporting-type firearms for their own use need not hold Federal Firearms Licenses (FFLs). However, we suggest that the manufacturer at least identify the firearm with a serial number as a safeguard in the event that the firearm is lost or stolen. Also, the firearm should be identified as required in 27 CFR 478.92 if it is sold or otherwise lawfully transferred in the future.
Source – ATF.gov
So… According to the ATF’s citations of the Gun Control Act and U.S. Code, you can make your own firearm and you do not need a FFL.
The ATF does not specify what material you have to use, and it does not put any restriction on making a firearm out of an object that previously had another use. Hell, if you wanted to make a firearm out of a toaster, from what I understand you definitely could.
By that rationale, if you buy an airsoft gun that “Needs Machining” to complete its transformation to fit an AR-15 lower parts kit, is it not only considered a firearm once you complete it?
If you cannot drop a lower parts kit in those airsoft receivers without any machining then I think that is a VERY good argument that they are not firearms, and should never have been confiscated.