no there are no "ceramic guns" you can buy in the US, and unless you can tell me of one that is actually manufactured (including the barrel), they don't even exist

and there is no fully plastic gun, even polymer striker-fired guns have metal components (notably, barrel, slide)

please provide links

What does viable mean here? So no springs, a plastic firing pin and a plastic barrel? Then there is the metallic case required to make it do any work. I guess you could use black powder and a wooden match. Don't forget the plastic projectile.