The two important parts of a gun are the barrel and the receiver.
3d printers can produce working lower receivers fairly easily, and even using current technology some quick googling turns up threads like this, where people are talking about (shoddy) upper receivers as well.
http://www.ar15.com/forums/t_3_118/5...it_works_.html
I expect that the time is fast coming where just about anyone will be able to print out a decent quality assault rifle receiver. A decent barrel is probably further away.
What to people think about this? What would it take from a technology standpoint in 3d printing to subvert the effectiveness of gun control with respect to mass shootings?
Maybe 3d printers won't be useful enough to proliferate widely, to the extent that potential shooters have the opportunity to use them without oversight. I'm not really sure.