3D printed gun? You still buy a steel gun barrel, metal spring, because if you fire a 38 out of plastic you wont be able to set off the firing ring and if you can the gun will explode in your hand.

The truth is there is nothing special about 3d printing the assembly a lot of guns have non metal parts, wood stocks and so on. The reality though is that what makes a bullet fly forward is that the bullet is in a tight metal barrel that contains all the energy of the bullet explosion and only allows one escape route for the bullet. If the barrel was plastic it wont work because the plastic can not contain the energy of the explosion.