HaveBlue of AR15.com has managed to create a working polymer AR-15 lower made from a 3D printer, specifically a mid 90′s Stratasys 3D printer. So far, the lower has been combined with a .22 LR upper, with over 200 rounds fired and no issues. A .556 upper was also used, with some feeding and extraction issues, however according to the creator this upper has also had problems with a standard aluminum lower.
A few other articles I’ve seen on this project have instantly jumped at the chance to (incorrectly) claim people now have the ability to create fully-functional firearms using just a 3D printer, so in the name of truth and fairness, I feel it’s important to mention that only the lower was created with the printer, not the trigger assembly and other required components.
AR-15.com thread detailing the project