The ATF Headquarters has released a video series showing the testing of a 3-D Printed Pistol based on the Liberator design, which was first on the scene. They have utilized two types of materials in manufacturing the guns including ABS and Invisalign. Clearly not all 3-D Printed guns are created equal and one moral of the story is to utilize the proper type of material if you are going to bother at all.
This test is very limited in scope and shows very little in terms of long term durability, however it has been shown that there already is a continual evolution to the development for these types of firearms including metal components that can be "printed", so it is only a matter of time as to when a long term fully functioning handgun, shotgun, rifle, etc. will be created simply from a 3-D Printing Device.
The first two videos show the gun manufactured with ABS material. This is one of the more common plastics utilized for its durabile and lightweight properties.
The other material utilized for this test's sake was VisiJet, which was developed specifically for 3-D Printing of prototypes and tooling. You can quickly ascertain, which of these two materials would be a better selection.