The group Defense Distributed has successfully created a lower receiver for an AR-15 that withstood the firing of over 600 rounds, that was printed using stereolithography. The previous model they demonstrated last year failed after 6 rounds.

Note that this lower currently takes 9-12 hours to print and costs $150 to make. Also note that 3D printing technology roughly follows a Moore's Law – style of trend; so think about drop in price and rise in quality of regular 2D computer printers over the last 20 or 30 years. We are going to see something similar with 3D printing / additive manufacturing.

Background article on Defense Distributed:
Shots Heard ‘Round the World: The 3D-Printed Gun Revolution Begins, by Kyle Chayka. AnimalNewYork.com, November 12, 2012.