The first I've heard about 3D printers was a couple years ago when I was on a tour of the Academy of Arts. They were showing the students examples of 3D models that students created from Autodesk Maya and other 3D animation software. The models were printed as figurines from the school's 3D printer. When I first saw it, I was amazed at what it can do.
But also, 3D printers are very expensive. It shouldn't really come as a surprise considering how much you can do for 3D printing. An easier alternative though is to use 3D printing services online. I thought about doing that through some of the 3D models I've been working on. I would also probably use it as a prop in a film I'm working on.
At this point it would be impossible to create a gun, unless you 3d printed every piece individually, and even then it's far from possible. Plus, the materials used for 3d printing would not work for such a weapon.