https://news.yahoo.com/atf-enforcing...103043563.html
There are lots of rules about making and selling guns in this country: If you sell guns regularly as a business, you need to get a license and conduct background checks on your buyers; each gun a manufacturer creates for sale needs a serial number; etc. However, it’s generally legal to sell firearm parts without following those rules.
The exception is the “receiver.” Federal law treats this part — the frame that holds the gun’s guts, basically — the same way it treats an entire firearm. It needs to have a serial number and so on even when it’s sold by itself, preventing people from evading the law by simply buying and selling firearms piece by piece.
A guy named Joseph Roh illegally manufactured and sold AR-15s and other guns through a slapstick scheme to avoid the law. A judge issued a tentative order against Roh — but in the process held that lower receivers are not firearms under current regulations, thus acquitting Roh of some of the charges. The government decided to let Roh off with a slap on the wrist rather than pursue the matter further, to prevent the order, as CNN puts it, from “becoming permanent, drawing publicity, and creating case law that could hamper ATF enforcement efforts.”
WTF????
defining what's a receiver seems to be a problem for ATF.