Criminals would steal the polymer, and not pay the tariff.
Your scanner likely won’t scan money, photoshop may refuse to edit or print images of money (https://en.wikipedia.org/wiki/Central_Bank_Counterfeit_Deter...), your printer may black them out, etc (https://en.wikipedia.org/wiki/EURion_constellation), all to prevent the easy counterfeiting of money.
Problem, of course, is that, for guns, the exact looks aren’t important. That’s true for child pornography and pirated movies, too, though, and those are being worked on.
So, I foresee tech in commercially bought 3D printers that attempts to disallow printing anything remotely looking like a gun or gun part, and people circumventing that by tweaking models, say by adding easily cut of images of flowers and kittens.