Hard to Print Metallurgical Properties
Most low-end 3D printers work with plastics or resins as raw material. High-end industrial devices may also work with metallic powders. None of these are suitable for the critical parts of a firearm -- chamber, bolt, maybe also barrel -- that must resist the explosive force of the cartridge when discharged. (Ignoring some variations of firing mechanism that don't affect my point.)
(This is why the world didn't end when Glock came out with their "plastic gun", despite oceans of often hysterical ink spilled by Luddite firearms opponents. Metal barrel, bolt, and guide rails for the slide -- all clearly detectable within the polymer frame.)
Even hobbyist grade computer-driven milling machines only go partway towards making this technology a means of firearms proliferation. Milling out a receiver or even the parts for the bolt only get you so far. These still need (in most designs) heat treatment to achieve the correct balance of hardness, resilience, and strength.
The WWII "victory" one-shot pistols were intended to be used in the equivalent of a mugging, to take real weapons from an enemy soldier. They were only one step up from a zip gun made out of plumbing supplies -- but even so, most of us would be hard pressed to make one in our workshop.
