This article is talking about "disappearing vehicles," but you can't make a vehicle out of plastic. The best you could do would be to have a vehicle that decomposed into a pile of batteries, motors and wires.
You can print a plastic device, insert one nail (needed for a firing pin), and fire bullets that can kill a person.
Is it accurate? No. Durable? No. A crappy gun? Definitely!
But I think that this meets most people's idea of a meaningful concept of a gun. A weapon with a barrel and a trigger. Pull the trigger, there is a bang, and a high speed projectile goes where you pointed it at.
The design that I was talking about was 2013. Since then there have been legal issues back and forth. Entirely plastic designs probably exist, but are not legally available.
I could use a 3D printer to create a solid block of plastic and then hand carve it into a doll. But, nobody would call the doll 3D printed even if one of the steps was 3D printed.
Also, I don’t think a gun needs a firing pin. You could have a gun where the trigger release a spring that brings two chemicals together that react violently, thus firing the bullet.
It had 2 metal parts. The first was a nail for a firing pin. And the second was a useless hunk of metal whose only purpose is to make metal detectors go off. (Thereby avoiding being illegal because it is concealable.)