Welcome to the dark side of 3D printing.

The hobby is best known for creating colorful toys and trinkets, but some enthusiasts are working on design files that would allow anyone to print a working gun. These don't exist yet, but some believe it's only a matter of time.

Why would a 3D-printed gun be appealing? For one, it could potentially be cheap. You can buy a preassembled 3D printer for about $500. A spool of ABS plastic to print with goes for $50. Depending on where you shop, you can buy .38 Special ammunition for 30 cents a round. The plans will undoubted be distributed free like so many MP3s.

In fact, plans for working gun parts already exist. They can be found on a site called Thingiverse and on similar sites, alongside thousands of free plans for toys, jewelry, tools, and design equipment.

Thingiverse is a creation of Brooklyn, N.Y.-based MakerBot and its CEO, Bre Pettis. Pettis and his company have become the de facto faces of 3D printing thanks to regular appearances in mainstream and tech media talking about how 3D printers democratize manufacturing. Pettis usually demonstrates this idea with brightly colored remote-control cars, robots, and other toys made with MakerBot printers. MakerBot and Pettis don't really talk about files related to gun parts.

That doesn't mean the issue has gone unnoticed, with the intersection of 3D printing and firearms having made the news a few times this year. In June, Michael "HaveBlue" Guslick reported on his blog about successfully test-firing a homemade gun whose key component, the lower receiver, he made from ABS plastic on a '90s-era Stratasys FDM 1600 3D printer.

And in August, Forbes' Andy Greenberg wrote about a group called Defense Distributed, which has some lofty goals. In practical terms, their immediate aim is to create a design file for what they call a Wiki Weapon, a functional, 3D-printed firearm...