If you follow the manufacturing process back far enough, an AR15 lower starts out as simple aluminum billet, and nothing in the BATF’s charter gives them purview over simple blocks of aluminum. They have pinned the point in the manufacturing process at which (by their definition) it ceases being a mineral deposit and starts being a firearm component at 81%. So a lower finished to just 80% of completion is not yet a firearm. You can buy 80% lowers just like any other block of aluminum, no Form 4473, NICS check, or any other paperwork required.
In addition to 80% lowers (now commercially available for 1911s & Glock handguns, as well as AR15s), they’re also selling molding kits so you can cast 100% finished polymer AR lowers in the comfort of your own man-cave. All they need — at most — is trimming off the odd sprue or a little flashing from the mold imperfections. The holed and all the “working parts” already are located correctly and to the proper dimensions. And since you do the entire process, from liquid to finished product, you become the lawful manufacturer of a lawfully unserialized firearm.
There’s also a Cad-Cam milling machine being sold commercially that has everything included (software, hardware and tooling) so a completely unskilled machinist can clamp in a properly-sized aluminum billet and the mill, switch it on, and it turns your billet into a 100% finished AR lower. But it’s a one-trick pony; all it does is mill AR lowers, by CAD-CAM, completely automated and unsassisted. Again, because the maker and the end-user are one in the same, it is completely legal to take it all the way to 100% finished.
3D Printers controlled by your home PC to print whatever gun component suits your own peccadilloes (except barrels, for now) can’t be far behind.
The only drawback is that all the (unserialized & homemade guns) are (technically) non-transferrable. I guess that when you die, it’s supposed to go in the hole in the ground with you. But then again, if it’s unserialized, who’s to know that wasn’t your surviving heir who made it to begin with? ;-)
So the libs can get over it. The toothpaste is out of the tube, and there ain’t no way to put it back.
Thank you for this well written and illuminating overview.