3D-printed AR-15 lower

[Complication]

I just ran across this: http://www.thingiverse.com/thing:11669

The short-version is that, with a 3D printer, you can manufacture your own lower at home, no background checks, no nothing.

The functionality issues are almost as interesting as the legality issues. It seemed to me to be a subject a few here would be interested in discussing.