[Ouderkirk]

There are several businesses that will sell you an 80% receiver and all the drill bits and milling tools to make it a full automatic.

One needs the sear to make it work.

The auto-sear and the possession thereof with an AR-15 lower (milled to accept it or not, installed or not) is possession of a machine gun.

I believe that machine guns are covered by the "shall not be infringed", but there are others who are not of that same opinion.

[gandalftb]

They sell you the sear casting and tooling to make it yourself.
...........
You need some machinist training and a milling machine to get it right.