But it's not hard for the terrorist to make a single-use gun now. You could probably make a pretty lethal and reliable single-use gun from $20 dollars in parts from your local hardware store.
What? I'm finding it hard to believe that an odd-looking 3d-printed single-use gun would be useful to terrorists (or criminals in general, actually). What you need as a mugger, for example, is a recognizable threat. The gun doesn't necessarily need to work but it does need to look like a gun. Terrorists have it even worse, as they're generally looking to threaten a whole lobby full of people; what they're waving around had better look really automatic. And had probably better work. I can't imagine that a single-shot weapon that looked unrecognizable would suit their needs.
A single-shot gun might be useful to a murderer, but hardly an ideal choice (what if you miss)? And it might be useful in a self-defense scenario, but the same caveat applies.
In fact, they actually make single-shot guns. (http://en.wikipedia.org/wiki/Derringer) And they aren't used by terrorists! They're used by pirates in movies. And in real life, generally, women carry them in purses.
Which, to come full circle, means I would support the availability of 3D-printed guns, since I perceive the most likely use would be in a self-defense scenario as a "well, it's better than nothing" option.
It's very unlikely that it will be possible to make a safe, practical, reusable gun from a 3D printer in the foreseeable future.
I'm sorry if I came off as aggressive in my criticism. It just struck me as silly that terrorists -- you know, the guys that buy military hardware from shady international arms dealers and have their own supply and manufacturing chains for explosives -- would be significantly enabled by crappy 3D-printed handguns.
My second point was an attempt to point out that it's probably cheaper and easier to create a single-use weapon now without the use of the 3D printer.
How do you figure that?
They should - anybody trying something out of the AC is likely to kill themselves in the process:
i.e. buying a 3D printer plus material and sending a blueprint file to it is an order of magnitude simpler than acquiring and learning to use a milling machine.