3d shape classification is different from matching a set of well-known, mostly fixed patterns (like eurion constellation) necessary to detect currency.
With 3d shapes of non-governmental origin this is at best difficult and at worst intractable. Consider the fact that many parts of a gun can be split into multiple printable pieces to be later assembled, making it very nontrivial to decipher the role of the shape.
With currency, the government has the controls for the supply of the target shape (it can encode hidden signals onto banknotes) and effectively controls the relroduction side (through the pressure on printer manufacturers). But it cannot control the supply of gun-part-shapes (it is not the only source for it), and since the problem is likely intractable - neither can it enforce the control on the 3d printing side.
Paper money being almost non-fungible is a great achievement, but is it as easy to make any mesh nonfungible as well?
It's certainly harder, I agree. We have highly sophisticated, non-deterministic image recognition. We don't have to be perfect to have a significant impact, and to stop the 99.x% of amateurs.
> Paper money being almost non-fungible is a great achievement
Going off on a tangent: Many people in technology and in the public look at cash as backward, boring, even socially embarassing technology. I think few it's amazing technology, an incredible hack: tech we struggle to implement in computers is implemented highly successfully and reliably in a piece of paper.
We also don’t have to forcibly insert nanny software into every 3d printer in the first place.
Not doing anything and preserving maximum agency is an entirely valid choice.