We were playing with 3D Stereolithography for prototype cutting tools back around 2001 and thought then that if a heat setting polymer with high enough tensile strength characteristics could be developed it would revolutionize traditional manufacturing.
A center-fire gun however would need a material capable of over 65,000 PSI at the chamber...and that is still a long way off. However that would only be needed for a small number of parts: Barrel, bolt, firing pin. The rest of the low strength parts would be relatively easy.
The concept is sound if the materials can be developed.