When the Crime Bill was enacted in September, 1994, there was significant confusion about what would still be permissible. A year later, some of this confusion still remains. Hopefully, this FAQ will clear up some misconceptions that have been floating around since then, particularly with respect to AR-15-style firearms and parts.
One of the provisions of the Crime Bill is that "it shall be unlawful for any person to manufacture, transfer, or possess a semiautomatic assault weapon." 18 U.S.C. sec. 922 (v) (1). However, the code exempts previously-owned "assault weapons" from this prohibition. 18 U.S.C. sec. 922 (v) (2). In the case of a rifle, a "semiautomatic assault weapon" is
A semiautomatic rifle that has an ability to accept a detachable
magazine and has at least 2 of--
i. a folding or telescoping stock;
ii. a pistol grip that protrudes conspicuously beneath the
action of the weapon;
iii. a bayonet mount;
iv. a flash supressor or threaded barrel designed to accomodate
a flash supressor; and
v. a grenade launcher . . .
18 U.S.C. sec. 921 (a) (30) (B).
Traditionally, the frame or receiver of a firearm has been legally equivalent to the firearm itself. 18 U.S.C. sec. 921 (a) (3). This has led some people to believe that a pre-ban receiver, that is, one manufactured before 9/13/94, is the same as a pre-ban "assault weapon." Unfortunately, this is not correct.
Since a lower receiver, by itself, does not have any of the five listed features, BATF doesn't consider it an "assault weapon." This is a good thing, since it means that manufacturers may still make the receivers. However, this also means that a pre-ban lower receiver is not an "assault weapon otherwise lawfully possessed under Federal law on [9/13/94]," and therefore not exempted from the ban. 18 U.S.C. 922 (v) (2).
In essence, there are three categories of AR-15 lower receivers: those manufactured before 9/13/94, which were assembled into "assault weapons" on that date; pre-ban lower receivers which were _not_ assembled into "assault weapons" on 9/13/94; and post-ban manufactured lower receivers. The first class can be re-assembled into an "assault weapon," and can be changed (say, from a fixed-stock model to a collapsing stock). The latter two classes may not be assembled into an "assault weapon."
DISCLAIMER: I am not a lawyer (yet). This information is offered in the belief that is accurate, and the hope that it will be useful, but I cannot guarantee the accuracy or applicability of this information, and I will not be held liable for any damages incurred on account of reliance on this document.