Rapid prototyping is making strides so fast that some things not possible last year are already old hat. Machines that used to cost a quarter of a million dollars are now the price of a family car, and machines that prototype in ABS plastic, for instance are breaking the thousand dollar barrier.
3D printing is a type of rapid protoyping where very small layers are laid down by a nozzle and slowly built up into the shape desired. This can be accomplished by cheap computers and free software and the sky is the limit. The whole idea of printing your own parts threatens a paradigm shift in the way we think of things. Are we a consumer or creator? I guess we'll be both.
Thingiverse is a repository of designs enthusiasts have made and uploaded to be shared by the community of primarily MakerBot 3D printer guys. These designs are mainly made in different grades of plastic, however the same files can be uploaded to companies, such as Solid Concepts, to be made by metal printing machines, if that output was desired. And it doesn't cost huge amounts of money like these operations used to.
King Ludd, a Thingiverse contributor, is working on a M-16 lower receiver. Another guy whose handle is crank built a magazine for an M-16 that did work, however it still has some teething problems. These are early days. This magazine can be built on a MakerBot, about the cheapest way to get into 3D printing there is so far.... and prices keep dropping.
This whole gun making thing is led to a lot of handwringing over at Thingiverse, They don't know if they want their technology to be used to produce weapons. There are also serious legal implications, as explored by Keith Lee and David Daw. However, guns were let out of Pandora's box about 500 years ago, and they are not likely to get put away. And like everything on earth, they can be used for good or ill. I think the whole idea of making your own parts is exhilarating, Anything to empower the little guy.