Monel,
You are incorrect on a couple of matters--the "Registered Auto Sear" is more correctly called a "registered drop in auto sear"--> DIAS. It does NOT require modifications to the receiver, which ARE illegal (i.e. no drilling for a sear pin), as the sear comes in it's own box with a pivot pin already in place. That is why they are so expensive.
The ATF will NOT allow new manufacture of machine guns for civilian sale (since 1986). Drilling a hole in an AR is considered making a new receiver. Having M-16 parts (ANY of them) in your possesion with an AR is considered "constructive possesion"--i.e. they can charge you claiming you have the intent to make a machine gun. This came from (and it is a pile of BS) the fact that they got a gun with some of these parts installed to fire more than once with one trigger pull--it may have (an ATF agent is on tape bragging about his ability to do this) come from greasing the gun up so heavily that it slam fired. HK's DO NOT do that, so you could have a full auto carrier in an HK 91 and you would not get into trouble.
There are a FEW limited exceptions--my full auto FNC had the receiver modified to accept the S & H sear, but that was done with ATF approval, as it is a proprietary sear design, and regular factory full auto sear's won't work in the modified receiver. Another screwed up part--a "Clip-On Registered Receiver HK" has the receiver registered, and the trigger group modified to make it work. The ATF won't let you change the receiver (registered part) to a push pin (factory design for full auto) as they consider it making a new machine gun (even though it is registered already).
You had to have the gun or receiver registered with the ATF before the May 1986 cutoff. It can have no further modifications to make it work. That is why Vector Arms is still selling new UZI's. They bought a huge lot of Group industries receivers (just the receivers) AFTER the 1986 ban and have been making them since with new parts. It would be illegal to have registered the AR receiver before the ban and drilled the holes afterward, but if you drilled the holes first (i.e. NO modifications needed done), registered it, and assembled it later, it would be legal.