US requires only the serialized part of a firearm treated as guns. For the AR-15, which is like PC/AT of guns, it's a nearly cosmetic part of it, sort of a motherboard backplate. Or like, a collar for a dog rather than the heart of a dog. As such, that part reportedly can be printed and used to shoot live rounds fine. Most other guns apart for AR-15 don't even matter, like how an E-ATX motherboard with dual PowerPC hardly matter in any talks concerning a PC - if you'd be wondering what about Raspberry Pi, that would be SIG P320 or something like that.

In most place of the world, including where I am, pressure bearing parts such as the barrel, the bolt that locks onto the end of the barrel to seal it as it fires, the firing pin that ignites the cartridge, the live cartridge containing gunpowder, etc etc, rather than the part that merely carries its nameplate, are controlled. It is illegal in such places to buy or possess functionally relevant parts of a gun, at least without a license, and/or prior approvals. This is more like buying a CPU or motherboards would be controlled rather than cases and faceplates. In some places, what is considered a gun in US hardly qualify as such, even almost slipping through customs(allegedly).

You guys gotta fix that broken classification before trying to offload onus onto the global 3D printing community. Or drop it altogether.