Here is the text of a BATF reply concerning this topic:
Document Control Numbers: 903050: AVH, 3311/2003-659
Dear Mr. *****:
This is inresponse to your letter of July, 2003, to the Bureau of Achohol, Tobacco, Firearms and Explosives (ATF) requesting clarification reqarding the legality of assembling firearms from stripped lower receivers marked "Restricted Military/Government Law Enforcement/Export Only." You refer specifically to a AR-15 type receivers manufactured by Colt.
The Gun Control Act (GCA) enforced by the ATF, per Title 18, United States Code (U.S.C.), Chapter 44, Section 921(a)(30), defines the term "semiautomatic assualt weapon" to include a semiautomatic rifle that has an ability to accept a detachable magazine and has at least two of teh following features:
1. a folding or telescoping stock;
2. a pistol grip that protrudes conspicuously beneath the action of the weapon;
3. a bayonet mount;
4. a flash suppressor or threaded barrel designed to acccomodate a flash suppressor; and
5. a grenade launcher
It is unlawful for a person to manufacture, transfer, or possess a semiautomatic assault weapon except as provided in 18 U.S.C. Section 992 (v). However, Section 922 (v)(2) provides that a semiautomatic assault weapon which was lawfully possessed on the date of enactment of this subsection (September 13, 1994) may still be lawfully possessed and transferred.
Although a pre-ban or post-ban AR-15 type receiver is a "firearm" as defined in 18 U.S.C. 921(a)(3), a receiver does not meet the definition of a "semiautomatic assault weapon." However, and assembly or configuration of an Ar-15 type receiver into a pre-ban AR-15 configuration rifle would be restricted to military or law enforcement only.
The above determination applies exclusively to existing Federal law. It is suggested that you contact your local authorities to determine if there are any State or local ordinances that may regulate the weapon in question.
We thank you for your inquiry and trust that the foregoing has been responive.
Sincerly yours,
signature
Sterling Nixon
Chief, Firearms Technology Branch.