Back in 2015 I started working on an idea for a three-piece AR-15 lower
receiver kit that would be made from extruded aluminum blanks and finished by
a hobbyist. I shared the idea with Weapons Guild and uploaded some screenshots
of the 3D models on Flickr.

It wasn't until 2019 I finally 3D printed some samples (I planned to send them
to the ATF Technical Bureau for a ruling), and I think I finally assembled the
receiver in 2020.

At the end of 2021 I pulled the rifle out of the vault so I could take it
shooting at least once before the new year. Because it's plastic, I fitted it
with a .22LR upper from CMMG; I don't trust it with full-power 5.56mm
ammunition, but I'm pretty sure it can be used as a rimfire rifle.

The receiver can be easily dismantled and none of the three individual
receiver parts is a firearm, so far as I can surmise without an ATF Technical
Bureau ruling.

I also included a photo of what a kit might look like (silver_kit.jpg). The
three receiver parts would either be fully machined as suggested here or
simply extruded aluminum blanks the end user machined himself. The assembly
hardware is in the baggie at upper right and that's a lower parts kit at upper
left.

The brass threaded inserts are only for the 3D printed plastic prototypes; the
actual parts would be aluminum and wouldn't need inserts.