That's a relief... Call it a misconception. It certainly LOOKS like there's a lot of them about. So, is there a large proliferation of semi's that LOOK like the real deal or something? And what about your "NEW" comment- Please explain.
The M16 and AR-15 are were built on the same platform. One's auto, the other is semi-auto. There aren't many visible exterior differences.
Since the National Firearms Act of 1934 (NFA 1934), all "Title II" weapons have required registration. Title II weapons include machine guns, suppressors, short-barreled rifles and shotguns, grenades, bombs, etc. Purchasing a Title II firearm requires extensive background checks, permission of local authorities, lots of paperwork, and a $200 tax stamp.
The NFA was revised in 1986 to make the manufacture of Title II firearms for the general public illegal. They were [somewhat] smart and decided not to go door-to-door collecting Title II firearms from their owners, as that would have ended badly. As thus, Title II weapons which were on the Title II registry before May 19th, 1986 are legal for citizens to buy, own, and sell. Anything newer can only be owned by law enforcement, military, etc.