With counterfeit currency, the whole point of the counterfeit is to look indistinguishable from the original. This makes it very easy to detect - you know exactly what to look for and you can block it with high confidence, because anything that is not sufficiently similar to the expected image is also too low-quality to be of use. There's also a fairly limited number of banknote designs in the world, and they are only updated rarely.

Not so with guns. Literally hundreds of existing designs, thousands if you account for all the variations. But more importantly, a working firearm is fundamentally an engineering problem with too many solutions. Real world designs tend to be as simple as they can be while still functioning (which is why most modern guns today are very similar internally - the field has converged, what difference remains is mostly legacy path dependent quirks). But if the goal is to defeat a filter, they can be easily made into something more complicated, looking very different but working largely the same.

And if you make the filter fuzzy enough to catch such things, it will also trigger on many completely unrelated things.