Comment: Pepper-box firearms (Score 1) 846
You don't need to have plastic strong enough to withstand dozens or hundreds of firings, you need plastic strong enough to withstand one firing.
I can imagine 3D printing a shape with multiple one-shot barrels, adding some propellant, electrical detonator and a projectile to each barrel, then mounting the assembly on some kind of (probably 3D printed) grip. When all the barrels have been discharged, throw them away and mount another on the grip.