It needs a metal firing pin, but technically it can fire with a plastic barrel.
But, of course, this means that the barrel is pretty huge, in order to not just blow up in your hand… Which makes the “undetectable” gun a giant blocky thing. And it still has a decent chance of just blowing up in your hand.
In order to actually make reliable firearms, you aren’t using some kind of maker bot 3d printer. You need pretty hard core stuff, to be able to print stuff flawlessly enough with materials that can handle the stress.
At which point you could just buy a gun for a trivial fraction of the effort and price of 3d printing one.
Folks are freaking out over crap like “instructions for printing an AR15!”
It’s literally a high resolution laser scan of an AR15. You can’t print that in plastic and have it work. You would need a metal printer, which costs a fortune.