Think about it, we are going from tech geeks and designers wanting these, to gun fans, which there are a lot of. Also, the venn diagram of the two groups isn't close to overlapping, so the tech is going to spread, and fast.
Hi, I'm a tech geek and a gun fan. That Venn Diagram does overlap. The desire for this tech may still spread quickly, but not because this particular Venn doesn't overlap. (Because, obviously, it does.)
They have been trying to stop digital child porn since the early days of the net, and that is a clearly winnable war.
Clearly? Unfortunately, that's about as winnable as the war on drugs.
I am not too concerned about criminals getting (more) guns, but I am worried about your average slob with poor judgement being empowered like this, since there are far more of them with plenty of good intentions.
What makes you think that the "average slob with poor judgement" will even care about having tech like these printers? When I think of someone like that, I think of the lazy people who called me to fix their DSL and, oh, while I'm at it, to set up their wireless routers. If those people get printers like these, I'm willing to bet they won't be able to find the files needed - or be willing to shell out the money for the building materials - to make these things. On top of that, the important parts of firearms - the slide, the firing pin, the barrel, etc. - are still going to be metal for a long, long time, and even when it's possible to print metals at home, there's still the cost of the raw materials.
No, the "average slob with poor judgement" will not have easy access to unlimited firearms via 3D printers.