80% lowers, anyone?
I was extremely surprised to only see one person (Ian Michael Gumby) mention 80% lowers, and even then it was something like 70 comments in.
Worrying about plans to use a plastic 3D printer to make a gun is pretty laughable. As many have already said, with current materials available for most 3D printers, the guns produced are absolutely terrible, unlikely to fire more than a few times, and have horrible accuracy, tolerances, etc.
What makes it even more ridiculous though, as Ian touched on, is that you can make a metal, production-quality firearm using a so-called "80% lower" along with off-the-shelf parts for the rest of the gun. For those who might not be aware, federal laws related to firearms in the US are tied to the one key piece of any gun--the lower receiver. That is actually the piece that is regulated, must have a serial number, etc. The rest of the gun (barrel, stock/housing, trigger assembly and all the other miscellaneous pieces required to complete the weapon) are NOT regulated, and can be easily purchased from any number of vendors. So, the only thing stopping someone from building their own, unregistered weapon is the need to build that lower receiver. That would normally be a bit of a complex undertaking, which is why even rabid gun control activists have never really focused any attention on the fact that it's perfectly legal for anyone to build one themselves. It was complicated enough that few would have the knowledge, tools and desire to bother.
However, someone can sell you a partially-completed block of metal, which has certain milling to turn it into a lower receiver already completed, but with key portions that are still solid metal, thereby making it far enough from a functional receiver that it's only considered raw materials, and does not fall under the regulations as it is not legally a receiver (yet). These are called 80% lowers, meaning that approximately 80% of the milling required to turn that block of metal into a receiver has been done, but that you will have to do the rest to get it there. Typically the vendors of these get a letter from the ATF confirming that what they're selling does not meet the definition of a receiver, which gives them certainty that they're not violating any regulations.
More recently, companies have started selling (relatively) low-cost CNC milling machines specifically designed for completing the milling on these lower receivers (just Google "Ghost Gunner"). They generally also sell specialty jig sets for particular models of firearm lower, along with detailed instructions, to make completing the lower as simple as possible. So for something like $2,000 you can have a machine that does most of the hard work for you, and simply requires you to re-position the lower a few times during the process. And from that one machine, you could crank out as many lowers as you'd like, for a wide range of handguns and rifles. Then you combine that lower with the other parts like the barrel you can order, which are all high-quality components produced by firearms manufacturers, and you end up with a weapon that is functionally identical to something you'd buy in the store, with none of the disadvantages of plastic 3D-printed weapons.
For those not willing to spend the money on the specialty CNC machine, some companies sell 80% lowers along with jigs that are designed to allow hand tools (files, etc) to complete them, where the jig acts as a guide ("file the metal down until you reach the jig", etc), which takes a LOT more effort, but is far cheaper. And, of course, there's nothing stopping someone from carefully measuring and drilling/filing an 80% lower without any special jigs or machines if they really wanted to take the time and effort to do so.
Really the debate about all of this is a bit silly, since most criminals that make use of firearms in this country are never going to use a 3D printer OR a specialty CNC machine. They're going to buy a gun from a gang contact/friend, which was almost certainly stolen from an original law-abiding owner, or they'll steal one themselves, or get a friend to make a straw purchase for them. I suppose I could see an entrepreneurial criminal setting up shop and cranking out weapons built from 80% lowers to everyone else, but given that it's been possible to do so for quite a few years now, and I haven't heard of anyone actually doing so, it probably means it's just simpler for them to steal them, or smuggle them in from other countries that have lax regulations.
But to return to the actual fight at hand, this isn't about whether someone can, will or should be able to print a gun on a 3D printer. This is about whether a company or person can publish the instructions for doing so. As others have mentioned in these comments, this was pretty well decided after the whole PGP debacle. That while a "thing" or "tool" that actually does the illegal activity can be banned, the instructions on how to build that thing constitute protected speech and cannot be.
That's not to say that the government won't decide to give you a hard time about it though, or that courts won't issue injunctions that would take you years and many dollars worth of resources to get overturned. There was a case of a guy running a company that trained people how to beat polygraph exams. That in itself isn't illegal, but of course the government wasn't happy about it. They sent undercover cops to him for the training, who made a point of mentioning they wanted the training to, say, hide the fact that they had a criminal record when applying for a job with a government agency, or wanted to hide the fact that they'd helped smuggle drugs through the airport they work at. Since he gave them the training anyway, he was charged with obstruction of justice, plus I think mail fraud.
So even though a company might be able to do this sort of thing, and I expect Defense Distributed to (eventually) prevail in the courts here, they'd better be careful that they don't do anything even marginally legally questionable (as a company or as the individuals that run it) as they'll have a target on their back.