Create It Real announced they've developed a system for preventing 3D printing of firearms. The system apparently uses some kind of geometric detection to identify firearm-like 3D models. Once identified, it simply prevents them from printing. But how does it work? They say:
Upon opening a 3D file, the smart software scans the model and tries to match its characteristics with the characteristics of a firearm. If certain features align, the software will not allow the user to view and print the model. For safety reasons, there are no models of firearms stored on the user's computer but rather a list of its characteristics.
We have no idea of this system's effectiveness, but while it may be able to detect (and therefore lock out 3D printing) the current set of firearm designs, there are obviously many geometric possibilities for firearms. It's likely determined firearm designers could develop models that sneak past this test.
Weaponry goes well beyond firearms, however. It's possible to produce all kinds of weapons with a 3D printer because they are simply differently shaped objects. Keeping up with weaponry design could be difficult for a system like this; you may end up subscribing to a detection service, similar to what is currently done for software viruses.