99% of 3D printed guns
100% of 3D printed guns.
Barrels and chambers are quite difficult to 3D print. But springs strong enough to set off primers, cartridge casings, and bullets must be made of metal. You can get clever with all of the above, but a plastic bullet would be laughably ineffective and even if you're going to go with electrical rather than mechanical ignition to eliminate the springs you're going to need metal batteries, metal wires, metal switches...
It is functionally impossible to make an all-plastic firearm. You'd be better off making a Jörg Sprave style crossbow or something.