Remember the fear mongering about Glocks being plastic and able to skate past airport metal detectors? I think John McClain might have started that nonsense. This is similar. You can already buy the upper receiver that he printed without many questions. If you buy a used assault rifle form a second party, which is already perfectly legal, it's no problem to turn it into a full auto. There's really no threat here to registration of ownership, since it's fairly bogus anyway. Another problem is that full autos aren't particular practical, as all of us gamers can attest. There's a reason why the military use their weapons in a select fire mode (Bosk can correct me if I'm wrong). Assholes like Aurura dude are better off using standard assault rifles than machine guns. Moreover, there are still parts you won't be able to print. I'm somewhat up on 3d printing, and I've heard nothing about printing metals. A plastic barrel isn't going to help. Springs? Frankly, this is just being alarmist over something that won't matter for a variety of reasons.
As for 3d printing, groovy stuff. The more fascinating implication will be what it does to the world job market. For some reason, shoes are the thing that always gets cited as an example. What happens to Nike, Reebok and Adidas when everybody's printing their own shoes? Yeah, I'm gonna pay $120 for something that can be printed for $5. LOL. A well known torrent site already added a category for printable, BTW. If they can make conductive components printable, then there goes a huge chunk of the electronics industry. You'll be able to download an opensource motherboard and suddenly a million guys at Foxxcon are SOL. In fact, an Xbox shouldn't be too hard to print up, except maybe the drives, and the opensource MOBO's would allow you to use off the shelf parts, unlike those bastards at M$.
I'll also point out that plenty of people are building their own 3D printers. The technology isn't all that complicated, and plans are readily available. You can find plenty of DIY versions on youtube.