Well, I imagined this would happen at some point...someone has now posted the Print-on-demand 3D printer file for both an AR magazine & a lower receiver.
Mag: AR-15 Rifle Magazine by crank - Thingiverse
Lower: AR-15 Lower Receiver by KingLudd - Thingiverse
3D printers are really becoming more of a reality now, and they are doing more than just plastic parts, some of the industrial type machines can do metal parts too. They can do fairly precision parts now, here is a great example:
High Quality Pliers by russrobot - Thingiverse
The coolest thing is that in theory, with the plans/parts file a machine like this can print out the parts to make duplicates of itself(minus the electronics of course).
In case you are wondering, the printers are still a bit expensive from $1200 for a full kit to ~$2250 and up ready to run. Some are starting to be made in China and thus the price for an at home model is coming down fast. (see this one: Personal Portable 3D Printer
Here it is in action:
+ YouTube Video