The automobile and aerospace industries use so-called 3-D printers to make prototypes of complex parts. In simple terms, the "printer" is a pot of liquid polymer which uses two converging beams from a laser to harden the plastic where the beams intersect. The polymer is transparent to the laser, but enough heat is generated where the beams cross to trigger the chemical bonding process. The solid part is made from the bottom up, much like an inkjet printer applies dots of ink to make a photograph. The plastic is similar to that used for plexiglas windows.
Some intrepid experimenters used the "printer" to make parts patterned after an the receiver of an AR-15 rifle, which were then assembled and used to fire several rounds before falling apart. The news media picked up on this immediately, and raised an alarm that plastic guns made in this fashion would be undetectable by security magnetometers used in airports and government buildings, without background checks and waiting periods. If that were not scary enough, it was the same type of rifle used by the shooters in Aurora, Colorado and Newtown, Connecticut.
You can file story this along with cold fusion, perpetual motion machines and winning strategies for Tic-Tac-Toe. There is no plastic capable of withstanding the pressure generated by even low power ammunition, much less the 60,000 pounds per square inch or more generated by an AR-15 round. The key working parts, such as the barrel and bolt, must be made from steel, which is easily
There are many firearms which have non-critical parts, like the grip and trigger, made from plastic. A notable example is the Glock pistol, used by more police and civilians than any other single brand. This was described as "undetectible" when it first appeared, by such excitable people as The New York TImes and Mayor Daley of Chicago. A Glock pistol, of course, has over a pound of steel in the barrel, slide and working parts, which is easily detected. Enormous pressure was brought to bear against the distribution of Glock pistols in the United States, much of which was fomented by domestic manufacturers whose products would be overshadowed by the inexpensive and extremely reliable Austrian imports. Needless to say, the sky didn't fall, and all domestic firearm manufacturers followed suit.
Media people are so gullible when it comes to technology, as are politicians eager to step up in order to protect humanity from evil scientists, with all the publicity that would generate.