In the US this is not illegal as long as they aren't finished recievers.
What are called 80% lowers are legal to buy and sell legally.
Finishing them is also legal for an individual that is making them for themselves, and not to resale.
(...)
The individual wouldn't be finishing the lowers, it would be robots. Basically there would be no human intervention whatever. Robots would be doing everything from procurement to subtractive machining to delivery. There would be no sale, the items would be given away. Technically there would be what would amount to transfers, but government entities would have to go and invent a whole new class of crime since technically a robot can't be prosecuted for transferring without a license. Presumably any potential "crime" a robot might be charged with for transferring a completed lower to someone could be mitigated / prevented by having interested parties make a small deposit (possibly in bitcoin) with the smart property tied to the robot(s) such that it would provably assert temporary ownership of a portion of the robot (and of the code which carries out its instructions) during the limited time the robot is working on subtractive machining on the item which will eventually be sent to the human individual.
Not to mention the fact that in most states, the act of making a lower for one's own personal use without registration and state-mandated S/Ns isn't a crime anyway, that is only planned in the North Korean state of California (AB 857, which becomes effective in July 2018) and it's likely to be contested in court when it becomes ripe to challenge in the year it becomes "effective."
As mentioned, this idea is quite conceptual at this stage, there is nothing in any warehouse that would actually implement the idea - I don't have a horde of robots at my beck and call, but since there are already similar projects underway (e.g. the farmbot with smart property, etc), the feasibility of it is not in question.