Wiki Weapons is an effort to produce a 3D-printable gun. They recently tried the first prototype (and it actually wasn’t a full prototype but an add-on to an existing gun) which cracked aftersix rounds. This would mean that I, as a 3D printer-owner, would be able to download a set of files and print my gun, assemble it, and fire it without registering with any government body. This is, arguably, quite possible right now and the average tinkerer can, without much fuss, build some sort of firearm with a drill press, milling machine, and some steel. There are plans available for Zip Guns online and, while I would fear for my reproductive capability if I shot this from the hip, there is little to stop explorers or miscreant from building a gun right now.
The 3D printed gun, then, is little more than a by-product of a specific technology coming into its own. The actual act of printing of said gun will be the culmination of the technology. The subtext, however is far more interesting. You could say that the primary benefit a 3D printer offers is permission to fail. You can tinker, build, tinker, build, ad infinitum and as long as your material holds up and your printer does not break you could, in theory, perfect your gun through design, trial, and error.