3D printers are not commonplace yet, but they are out there. They used heated plastic to form shapes that are 'printed' from a computer graphic.
article found here (http://countryplans.com/8301-17938_105-57583039-1/3d-printed-liberator-gun-fires-first-successful-shot/?tag=nl.e497&s_cid=e497&ttag=e497)
Basically this guy designed a handgun that is all plastic except for the firing pin. And in his case a six ounce piece of non functional metal that is required by the Undetectable Firearms Act to make it legal. He has test fired it. No mention of caliber.
It's legal under US law as long as you make it for yourself and do not sell it.
Cool technology. It also goes to show that there is always a work around for gun bans.
another article (http://countryplans.com/8301-11386_3-57499326-76/you-dont-bring-a-3d-printer-to-a-gun-fight-yet/)
Metal guns are easy to make with basic machine tools. I could probably make a functional, if not pretty gun with nothing but files and a drill press. Look up 'zip guns' to see how guns have been improvised over the years. Now, when they can 3D print ammo, we might have something.
3D printing is a cool technology, though. We had one in the shop I worked at. Draw up a 3D part in CAD on the computer, then send it to the printer. The customer has a real, 3D part to hold in their hand before committing tens or even hundreds of thousands of dollars on permanent tooling. One of the samples the salesman had was a functional gearbox that turned and moved. All of the parts were printed in different colors, too.
Reminds me of "In the Line of Fire" where John Malkovich's character builds a plastic pistol to assassinate the President.