This letter from the ATF was posted in another forum on this site:

"This refers to your letter of March 19, 2001, in which you ask about the status of certain semiautomatic assault weapons which have been altered to another configuration.

As defined in section 921(a)(30), of Title 18, United States Code (U.S.C:), the term "semiautomatic assaultic weapon" includes certain named weapons and certain semiautomatic rifles, pistols, and shotguns that have a combination of enumerated features. Title 18 U.S.C. section 922(v)(1) prohibits manufacture, transfer, and possession of semiautomatic assault weapons; however, section 922(v)(2) provides that any semiautomatic assault weapon that was lawfully possessed under Federal law on September 13, 1994, is excluded from the prohibition.

A frame or receiver of a semiautomatic assault weapon, meets the definition of a "firearm" in 18 U.S.C. section 921(a)(3); however, a firearm frame or receiver alone, without the additional qualifying features, does not meet the definition of a "semiautomatic assault weapon" in section 921(a)(30). Therefore, a firearm frame or receiver does not meet the exemption in section 922(v)(2).

We have also determined that a semiautomatic assault weapon in knockdown (unassembled) condition consisting of a receiver and all parts needed to assemble a complete semiautomatic assault weapon are subject to regulation if the parts are segregated or packaged together and held by a person as the parts for the assembly of a particular firearm.

You describe an AR15 type rifle that met the definition of a semiautomatic assault weapon and was lawfully possessed on September 13, 1994. At some subsequent time the rifle was temporarily reassembled in a configuration such that it no longer had the qualifying features of a semiautomatic assault weapon. You asked if the original components could then be lawfully reinstalled on the rifle.

Provided that the original components were held by the owner and reinstalled on the rifle, it is our opinion that the rifle would still qualify as an exempted semiautomatic assault weapon even though it had been temporarily assembled in a different configuration. We note, that mere disassembly of a semiautomatic weapon by an owner would not remove the firearm from the definition of a semiautomatic assault weapon nor would the reassembly constitute manufacture of a prohibited semiautomatic
assault weapon.

[b]Your second question concerns a semiautomatic assault weapon that also meets the exemption in section 922(v)(2). However, this firearm was disassembled and the receiver, without other components, was sold. Since the receiver is no longer possessed with all parts necessary to assemble a complete semiautomatic assault weapon, it no longer meets the definition of a semiautomatic assault weapon. The receiver does not meet the exemption in section 922(v)(2) and assembly of this firearm in the configuration of a semiautomatic assault weapon would be prohibited under section 922(v)(1).[/b]
[red]Basically this is saying that if you take a pre-ban AR-15 and split the upper and lower receivers and sell the lower receiver alone, it is considered a POST-BAN lower from that moment on.[/red]

CONTINUED...

If you are interested in determining the status of a particular receiver or semiautomatic assault weapon, you should contact the manufacturer or importer and ask about the date that it was manufactured and the configuration at the time of sale. It may also be necessary to contact subsequent dealers and owners who possessed the firearm.

Sincerely yours,
Curtis H.A. Bartlett"

Actually, it's VERY likely that it would hold up in court.  It is consistant with ATF's previous (restrictive) rulings.

The part you are missing, Ponyboy, is that the law has defined the definitions of "rifle" and of "assault weapon", and those definitions are VERY different.  Thus, restrictions on each will vary, and that's perfectly okay under the law.

Under the definition of "assault weapon", it must be a complete gun capable of firing...  Note the difference between the definition of a rifle, which by law can be just a receiver.

-Troy

No, there are BATF rulings in force that specifically allow the *original manufacturer* to "repair" a rifle, up to and including replacing the receiver (and destroying the original).  This does require a variance from BATF, but BATF has given plenty of them out, and doesn't appear to be abandoning that.

-Troy

So if someone had a grandfathered AR-15 with a telescoping stock and pistol grip on it and sold the complete lower, it would still fail the ATF's scrutiny even though the lower was serialized and had the 2 "evil features" required to make it an AW?