While I agree it would be unwise to test this legal question in court, I think it is fair to say this is a murky legal question. The BATFE, the federal law enforcement agency responsible for enforcing US laws regarding firearms manufacture, have a history of arbitrary rulemaking in defiance of simple logic (for instance, in one famous rulemaking BATFE ruled that a shoe string would meet the legal definition of 'machine gun' if used to repeatedly fire a gun with a single pull of the trigger [1]).

Given this environment of legal uncertainty, it's not at all clear that BATFE would not pursue the 3D printer operator who produced the receiver. In this case, charges against the 3D printer operator would likely be unlawful manufacture of a firearm, possibly in addition to charges related to interstate transfer of a firearm. Given the creativity of some US attorneys, perhaps a conspiracy case would be made alleging the buyer of the printed part 'conspired' with the printer operator.

I would also suggest a lower receiver could probably be made from materials weaker than you might imagine. The lower receiver does not bear any gas pressures from the firing of the cartridge, though the holes for the roll pins which hold the hammer and trigger in place would probably start to 'egg' after a round or two. In any case, the legal question has nothing to do with how reliable the resulting firearm is; only whether it meets certain vague and broadly-interpreted legal definitions.

[1] http://www.everydaynodaysoff.com/2010/01/25/shoestring-machi...

> Anybody that wishes to pretend that the legal line has been blurred will find themselves sooner or later in an excellent position to try to prove that to a very unrelenting judge.

The line is blurred, that is tautological considering the complexification of the issue. The argument would be that the blurring is enough to warrant a different interpretation of the law.

> If the legal fiction is that the lower receiver of that gun is the gun, then legally that didn't change so the method of manufacturing it is not relevant.

Likely so.

> if they don't review the orders (hard to do, after all, you can't know each and every prohibited item) then [they] are still on the hook.

In the past making a custom part required a skilled machinist familiar with common machined parts. They would more than likely recognize an unlabelled but common part.

Now a part can be made without a single human involved, or if there were they could be lesser-skilled cleaning technicians only.

In the past they'd notice what you were doing by default, unless it was severely obfuscated. Now they'd have to hire machinists to spy on customer jobs and try to guess your intent to ban 'bad' items. One is trivial, the other invasive and ultimately impossible.

It's a bit moot. They can't effectively pass a law putting the genie back in the bottle without banning 3d printing and rapid-turnaround prototyping - something which would drive the last of our development businesses away.