The issue is not posession, but whether or not Colt can LEGALLY build TDP correct civilian sale rifles. They cannot. Lifted shamelessly from ARF.com
The BATF has interpreted the law such that parts which would convert a firearm into an NFA firearm are subject to registration including:
Any combination of parts designed and intended for use in converting weapons into machineguns;
Any part designed and intended solely and exclusively for converting a weapon into a machinegun;
Any combination of parts from which machineguns can be assembled if the parts are in the possession or under the control of a person;
(see ATF FAQ for more details)
Due mostly to ignorance on the part of dealers and the general public, there are many rifles out there that are violating the law unknowingly. Most will have a couple of parts, and though the rifle will not be capable of full-auto fire, will still be in violation of the law. It is important to check your rifles and those you consider purchasing, since the penalties can be harsh:
From: [49 U.S.C. 781-788, 26 U.S.C. 5861, 26 U.S.C. 5872]
Violators may be fined not more than $250,000, and imprisoned not more than 10 years, or both. In addition, any vessel, vehicle or aircraft used to transport, conceal or possess an unregistered NFA firearm is subject to seizure and forfeiture, as is the weapon itself.
Needless to say, the risks are not worth it so it is important to be able to identify the AR-15 vs.. M16 parts; included here are the trigger, disconnector, hammer, selector, bolt carrier, and auto sear. While it is unlikely that you will ever find the auto sear in a non NFA weapon, any combination of the others is possible.
The AR-15 and M16 parts are very similar, and unless you know what you're looking for, would most likely not notice. Many of the AR-15 parts have been made by modifying the plentiful M16 parts. Colt has made a number of modifications over the years to their AR-15 lower receivers to minimize the possibility of "misplaced" M16 parts, but on most non-Colt rifles, M16 parts will fit in place of the semi-auto parts. Let's look at these in detail:

