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.
Hiring someone to produce it while obfuscating the intent is not gray, it puts you in the docket. The printing house is acting on your explicit instruction, and if they don't review the orders (hard to do, after all, you can't know each and every prohibited item) then you are still on the hook.
Trained machinist or 3D printer operator the change is only in the material of the produced item.
As for not being able to whittle that part from wood, there are many kinds of wood that are much stronger than the ABS wire that a 3D printer uses.
And if it can't be made out of those kinds of wood then it certainly can't be made out of plastic printed ABS (if you expect the gun to function).
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...
And I doubt they will declare the manufacturer of the lower receiver is manufacturing a weapon, if it's done in such an automated way. They will just change their rulings so it's a crime to order such a part. If they can't do that, they will work with whatever federal agency regulates telecommunications, and it will suddenly become a serious crime to order certain gun parts from a 3D printer.
Laws don't always change faster than technology, but they change pretty quickly when there's the right kind of pressure.
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.
Yes, some jurisdictions aside you can make one AR15 lower receiver on your own without any paperwork, fees, checks, or permission. Understanding is that nobody going thru that much effort is going to waste it by doing something stupid (if there's criminal intent there are many far easier ways to acquire one).
The issue here is: where's the line on making one? You can make one yourself legally, but if you hire someone else to knowingly make it they'll need an expensive big-hassle manufacturer's license. The key is "knowingly", as up until recently it was pretty obvious to the machinist what he was being asked to do. Now, with "printing", you just upload the pattern for what is otherwise just another odd-shaped chunk of metal, a computer & machine crank one out, and the result gets sent back to you with a minimum of comprehension by any humans involved - are _you_ "making" it? or is the printing house "making" it and, in effect, selling it to you? The legal line has been blurred.