What is 3D printing, you ask? 3DPinting.com explains:
3D printing ... is a process of making three dimensional solid objects from a digital file. The creation of a 3D printed object is achieved using additive processes. In an additive process an object is created by laying down successive layers of material until the entire object is created. Each of these layers can be seen as a thinly sliced horizontal cross-section of the eventual object.You can print small things, like bowls, jewelry, and utensils, to large things like houses. Consumer 3D printers are available for $1,000 to $3,000.
... And now you can print guns!
Companies like Microsoft and Google are woking on scanning apps that allow you to scan an object (like a gun!) with your phone, and later print it at home.
The battle to contain guns is about to get a whole lot more complicated.
H/t to Eugene Volokh.