The hardware and software for 3d printers is actually much simpler than that of regular printers. The only specific requirements for a 3D printer is basically just an Arduino and some stepper motors with drivers. Everything else can be (and was back when it started) made from off the shelf parts. Even the filament can be made at home from soda bottles.

It's relatively easy to detect modern US currency, it doesn't vary by much, and if you were to vary it a lot, it'd be an obvious counterfeit. Firearms don't operate like that. They are all wildly different designs with different complex parts. Even if you wrote something to detect the current models that are out there, they could be easily changed to be identically functional but unrecognizable.