I have the Back-Half of a De-Milled/Sheared M16 that I keep on my desk as a paper weight.  I originally bought it from NESARD/Sendra, back in the Mid-80's at the Lake County (IL) Gun Shows.

When I originally purchased it, it was almost complete.  It had the GI Auto Sear, Receiver Ext. Stub, Dimpled Rear Take Down Pin and Safety still in it.  I got rid of the Auto Sear a long time ago, I think I gave it to an SOT/CL3.

I kept the Dimpled Take Down Pin, and it is in a build I have going now.  The Safety, I neutered, to keep myself legal.

BUT, now I wonder, would it be LEGIT to put it back to the condition that I originally received it in, without any Constructive Possession or Intent, and still keep it as a Paper Weight/

I would guess that your paperweight would be a low priority to BATF, whatever the rules are for chomped parts. It is not a receiver anymore technically, and it was bought legally in the 80's.

But you need to ask yourself, how much trouble is a paperweight worth to you?

I think the problem with constructive possession is the auto sear. All the other parts (technically, not necessarily legally) can be used in an AR-15 without firing fully automatic . (except a "full house" which causes full auto slam fires when the selector is in the full auto position) A full house is the M16 hammer, trigger, disconnector, selector lever, and bolt carrier. The auto sear serves only 1 purpose. To release the hammer upon bolt closure in full battery in full automatic mode. It can't be used in an AR-15. So, I would stay away from the auto sear. -W

The big question is when was the last time a Firearms agent of the ATF was in your office?

Its not a gun and even if the ATF went Bat S_it crazy they would just take it.