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?

And it's sits fine with you because you are the one who wouldn't pay the price for this "simple image recognition capability". Except you would pay of course, indirectly but at least you wouldn't know for sure so your conscience would feel at ease.