House foundations is really nothing more than pouring concrete with a robotic arm - not going to help you make a gun.
Printing precision parts is a complete bitch - the plastics warp and change shape significantly over any distance (hence the incredibly short barrel on the Liberator), and the plastic really isn't strong enough for the job (notice how the amazing test firing was only a single shot?). ABS is far more elastic than steel, so it's never going to be possible to get a proper seal around the slug with ABS. Also the seal around the chamber isn't going to be very good with ABS either. I've no idea what the muzzle velocity is, but I bet it's not great. Also there's no rifling, as the ABS can't cut into the bullet, so with a tiny barrel and no rifling you're going to struggle to hit anything you're pointing at. I'd imagine the most likely outcome of firing a Liberator being pretty similar to holding a bullet in a vice and hitting it with a hammer.
As for printed metals, they're currently done by using powdered metals and 'fusing' the particles together with a laser. The result is pretty brittle and the machines are ridiculously expensive, unlike the ABS printers you can get/build for a few hundred dollars.
If you *really* wanted to manufacture a gun at home then all you need is a half decent workshop, a mill and a lathe. Add in a CNC lathe, CNC router, CNC mill and you could automate the process and make something half decent in a week or so.
EDITED: 23 May 2013 17:32 by KOSWIX