In 1994, Willie "The Cigar Man" Clinton, decided to pass legislation making assault weapons illegal to manufacture unless they were for law enforcement or military use. Any weapon that was already an AW before the law was grandfathered in. So, a lower receiver off of one of these pre-ban AW's can still be used to make what meets the definition of an AW. According to the legislation, an Assault Weapon is defined as a rifle that is capable of using a high capacity magazine (greater than 10 rounds) and has more than one of the following evil features: 1) collapsible or folding stock, 2) pistol grip, 3) bayo lug, 4) flash suppressor or threaded to accept a flash suppressor, or 5) grenade launcher. So, if you have a post-ban lower receiver (one that was made after 1994) you have to use a barrel that does not have a bayo lug or flash suppressor. Since the AR15 has a pistol grip, that is the one feature you are allowed to have. So, no collapsible stock, no bayo lug, no flash suppressor, and no grenade launcher. If I left anything out, my esteemed colleagues will fill in the blanks.
Charles Tatum
Alamo Professional Arms