There is no firearm that is 100% plastic.
Springs, firing pins. Barrels. Casings. Plenty of metal to detect. Reinforcement metal on stress point areas.
Not quite. The liberator is 100% plastic except for the firing pin, which is a nail.
https://3dprint.com/73842/download-3d-printed-gun/
That being said, it is illegal to create one and assemble it without inserting 3.7 ounces of metal into the grip.
18 USC 922(p) has been the law since 1988.
The ABS Liberator will only be good for 5-10 shots before it fails.
With a metal insert, these can last longer.
Not to mention that totally non-metallic ammo isn’t a very common commodity.