It's not clear to me if a rifle part printed from plastic at home is usable.
I wonder how the manufacturing of these thing are regulated normally: Can you have a professional place make 2-3 of these for you without telling them what it is?
Reminds me of the case where Phil Zimmerman noticed that exporting crypto was a form of arms export, but exporting books was protected by the First Amendment. So he published the source code to PGP and shipped it.
And let's go even further into the hypothetical future where we've made great advances in AI. Also, for whatever reason, a court has found weapons CAD files to be illegal. What will the law say when a machine is intelligent enough to decipher the same human-targeted AR-15 repair manual and reconstruct one from that information?
It's not hard to imagine a Congresscritter raising a fuss over concerns of "children printing machine guns" or some such nonsense, and thereby introducing legislation to regulate such materials, including executable scripts for 3D printing machines. It's even possible to imagine such legislation passing the House and Senate. Our current president would likely sign any such legislation he was given. However, it is very difficult to imagine the legislation surviving a Supreme Court challenge on First Amendment grounds.
Then again, I said the same thing about the McCain-Feingold Campaign Finance Reform Act, and SCOTUS upheld that 5-4, so what do I know.
Next month is the TEN YEAR anniversary of the USA PATRIOT act, the legislation that abolished the fourth amendment. Checks and balances only work when they, well, balance.
Unfortunately that isn't the case any longer.
As with most things firearms related, the offense occurs with the action. In this case, the transaction. You can manufacture a firearm with standard machinery and suffer no penalty. The same applies for a printer capable of printing a firearm (or parts). It is when you sell those firearms (or parts) that you have broken the law.
If it is against the law to possess a firearm in your location, then it doesn't matter how you made it. Possession remains illegal.
It wouldn't surprise me if it were perfectly functional. The lower receiver on an AR houses the trigger group, buffer tube, magazine, and grip. Not really a lot of high-stress parts there. The barrel, bolt, gas tube, etc that have to withstand 60k+ PSI of pressure are all part of the upper receiver.
Legally, only the lower is classified as a firearm in the United States (meaning they have to be transferred through a licensed firearms dealer if shipped); the upper and the rest of the gun are simply parts and can be shipped directly by UPS/FedEX/USPS.
Printing full metal parts is possible but there are some penalties, relatively low tensile strength is one of those.