What people are doing with "3D printed" guns is printing the receiver or frame components that are otherwise serial numbered and federally regulated, and populating those with metal barrels, slides, upper receivers, trigger assemblies, pins, springs, etc., as appropriate from the genuine item. These can be quite functional and durable, because the majority of the gun is, in fact, still made from "real" gun parts.

Clever individuals have gotten quite far in managing to print most of the required components, but several critical parts simply can't be made with consumer level printing technology. At present it is impossible to fully print a gun out of plastic and actually have it work.

The way federal law works, the ATF has identified and decided what constitutes the minimum identifiable major "gun part" of a given model of firearm, which is the part that must bear the serial number and is the component that cannot be sold without a background check through an FFL. For the Armalite platform, for instance, it is the lower receiver which is a component that can be 3D printed. The upper receiver is not a regulated part. For many polymer framed pistols like Glocks, the grip housing and frame is the FFL component. These can typically be 3D printed as well. But some guns, like the PTR/HK 91 and Sig P250 it's not the frame, it's the trigger assembly that's the FFL item. You can't effectively 3D print one of those -- although you could probably manufacture one with a milling machine pretty easily.