Theory versus reality
As has been pointed out already, polymer based compounds used in 3D printing systems cannot withstand the pressures inherent in modern day ammunition, even something as small as the .22 long rifle. Also inexpensive parts assembled into "Zip Guns" have shown how easy and inexpensive it is to obtain a one use, drop it and leave weapon. During WWII the US government developed the "Liberator" pistols, cheap stamped metal construction in a much more authoritative caliber, the .45 ACP. The idea behind these were to be airdropped behind enemy lines for resistance fighters to use once or twice to obtain a "real" firearm from the enemy in calibers that could be liberated from the enemy supply chains to keep them viable. When Polymer framed pistols were first developed the hue and cry of the press was that these would be used to defeat the metal detectors at security check points. As usual they were so off the mark that it was laughable. While the frames were indeed polymer, the slides, barrels and internal parts were still made from steel, enough so that any metal detector worth its' name would ring off the wall when one passed through its' portal. 3D printing is another one of those technologies that in time may be developed to the point of being used for this purpose, but there are much more effective and inexpensive means of producing the desired end result. So titles such as "3D Printing Producing Firearms" is just another press buzzword title to generate readers for the author rather than actually report a factual occurrence.
Here in Las Vegas we have a small company that is producing polymer lower receivers for AR 15 type rifles and carbines. While these parts have undergone extensive testing by the maker showing that they can handle the conditions they are used in under several thousand cycles of operation and offer a much lighter weight end product when coupled with other similar parts, they are still just a non pressured part that house the parts that deal with the real work of firing a round of ammunition. So until you can come up with a polymer that can act as the barrel and chamber of a firearm to withstand those pressures, this is still just an exercise in theory.
