LEO, tell me where, how, and with whom a rifle is registered as a rifle? The answer is "it isn't." It is not a rifle until it is built into a rifle. A receiver is a receiver is a receiver. A factory-fresh, never-built-into-anything receiver does
not fit the legal definition of a rifle, period. I posted the citations but I'll also say that if teh law itself doesn't convince you, go look at a Form 4473...an over-the-counter firearms transaction record...the form you fill out when you buy a gun from an FFL. There three categories: "Handgun", "Long Gun", and "Other Firearm (frame, receiver, etc.)." (See section 18,
http://www.atf.gov/press/2008press/100308f4473_pt1_aug2008-sample-rev.pdf
It is perfectly legal to buy a stripped, virgin, never-assembled-into-anything receiver and build it into a handgun or even an AOW.
That said, a completed rifle that was returned to a stripped receiver is still lawfully considered a rifle. Likewise, one cannot take a rifle, chop it into a handugn and have a handgun. In that instance it's a short-barrelled rifle.