The real stupidity of this entire business starts to sink in when people start to grasp the realities of the basic M4/M16/AR15 rile.
The M16 is a simpler weapon than an M14 or a FAL rifle. The ONLY thing about an M16 which is even a tiny bit difficult to manufacture is the barrel. Unlike a FAL rifle whose upper receiver has to take the stress of firing and which has to be hard forged and whose barrel is fitted to the receiver with something like 130 ft-lbs of torque, the only thing which takes any stress in an M16 is the barrel, the bolt carrier locks into the breach of the barrel and not into the receiver.
Also unlike the case with a FAL, the lower receiver of an M16 which, for all intents and purposes could be made of plastic, is the legal part of the gun i.e. carries the serial number. Ordering a complete upper receiver and barrel for an M16 is legally the same as buying a sack of potatoes, don't take my word for it check it yourself:
Anybody with any sort of a machine shop could manufacture M16 bolts, bolt carriers, triggers and springs and what not. No joke. My next door neighbor builds race-cars and he ends up needing parts which don't exist in the world here and there... No problem, he draws what he needs up on AutoCad, takes the AC file to a shop with computer controlled machines and voila, and it doesn't even cost much.
Very shortly, people will be making M16 lower receivers, butt-stocks, and magazines (the evil 20 and 30 round types) with inexpensive 3D printers and the only way you could get past all of these problems would be to ban and outlaw the M16 altogether, which would leave our military standing around with, in the immortal words of Santino Corleone, just their dicks in their hands.