Plastic will never be a suitable material for firearms most plastics shatter under high pressure, A common industrial 'accident' is a company cheaping out and using PVC plastic pipes to distribute compressed air instead of black iron or copper lines, Eventually the PVC shatters like a grenade causing injury to those in proximity. I will not even go in a shop with plastic air piping. (and no OSHA does not allow use of plastic piping for compressed air)
As to 3D printing a weapon in metal that process is known as 'direct metal sintering' but there is nothing magic the same G-code which can 3D print a gun works just as well on a CNC milling machine. Possibly better as any machinable material can be used.