If the gun is a one-shot gun, could the gun not include a bullet also, as well as a receptacle for gunpowder? Could a 100% plastic gun with plastic bullet be lethal up close? Could one place the gunpowder during printing and seal it? Could the gunpowder be ignited by static electricity for firing the gun? Answer these questions and metal detectors become useless.
There is the possibility of manufacturing caseless ammo.
An exotic replacement for the firing pin isn't strictly necessary. A firing pin is too small to activate a metal detector. At least, the government thinks so, because guns are required to contain a minimum amount of metal for this purpose.
The current stories are neglecting to mention this, but an earlier Forbes article reported that the Liberator contains a 6 ounce piece of steel to remain compliant with the law on undetectable firearms.
There are 350 million real, fully functional, STEEL barreled firearms in the US today.
The epitome of 750 years of tweaks and improvements.
SURELY one of those would be easier to obtain (and safer to fire)if you wanted a gun.