There are a number of companies producing unfinished lower receivers that are then sold to consumers to complete the machining on. Lowers that are only 80% complete are not classified as “firearms” and thus require no FFL transfer. While a CNC machine is ideal, the kits can be completed using tools as rudimentary as a drill press or a router. Polymer lowers are also available and can be completed using a something as simple as a dremel tool. 80% Glock lowers are also available and are quite popular. Firearms made in this way are perfectly legal though they cannot be sold or transferred as they have no serial number.

You can register the receiver with the ATF and receive a transferable serial number. IIRC you have to have possessed the receiver for a certain amount of time and you can only transfer so many per year before they consider you to be a gun manufacturer.

3D printing the lower receiver for net gains in a government buy-back program is a hilarious idea.

You don't even need to do that for gun buybacks. People routinely make firearms with $10 of homedepot parts and turn them in. In theory you could turn in a "Zero Percent Lower" which are about $20 or so...

I suspect they'll fail to see the humour in it after you show up with the second box full

The only buy back I have to reference is Australia's and it wasn't very much money.

I read an article saying that Australia bought back almost 1 million guns and imported more than a million. That was a good waste of tax money, practically doing a refresh of the private owner gun stocks with tax money.