Comment by imtringued

There are two parts to a gun.

The "explody bit" that needs to survive intensive pressures, which boils down to a carefully machined barrel (rifling is optional) and a firing pin.

The "comfort features" that make it easy to hold that barrel in a human hand, that direct the recoil to parts of the human body that can better handle it, that hold the ammo in a way that automatically feeds the next bullet (spring magazine) and that makes it easy to press the firing pin against the munition (the trigger).

The latter can easily be 3D printed.