Amazon.com has AR accessories

[BjornF16]

.. It's just if it has a barrel length of less than 16" (16.1" to be safe), it must never have been sold as a rifle.

I don't believe that is correct....

From ATF ruling:
Therefore, so long as a parts kit or collection of parts is not used to make a firearm regulated under the NFA (e.g., a short-barreled rifle or “any other weapon” as defined by 26 U.S.C. 5845(e)), no NFA firearm is made when the same parts are assembled or re- assembled in a configuration not regulated under the NFA (e.g., a pistol, or a rifle with a barrel of 16 inches or more in length). Merely assembling and disassembling such a rifle does not result in the making of a new weapon; rather, it is the same rifle in a knockdown condition (i.e., complete as to all component parts). Likewise, because it is the same weapon when reconfigured as a pistol, no “weapon made from a rifle” subject to the NFA has been made."

http://www.atf.gov/regulations-rulings/r...2011-4.pdf