> commercial printers will refuse to print anything that looks like counterfeit currency

They will refuse to print images that contain the EURion constellation, a cluster of points with specific distances between them. (https://en.wikipedia.org/wiki/EURion_constellation)

More generally, "Does this image represent US currency?" and "Does this 3D mesh represent a dangerous gun part?" are different kinds of questions.

Checking if an image is US currency is simple. Not only can an image similarity hash be used, but the design of the currency can be changed to make checking easier. The checking program just has to run a textbook algorithm and compare the result with a handful of hashes (we only have a few denominations of bills).

We can be confident that the checking program will always see substantial portions of the bill, so it can always see the pattern. If someone chopped the bill into tiny pieces, it would look fake when glued together. Looking like a real bill is the thing we want to control, and that's something we can reliably measure with simple algorithms.

Checking if a 3D shape is a gun part is an unsolvable problem. (It would be equivalent to making a program that can detect hidden "subversive" meanings in text.) The checker would have to understand how the piece could operate in the hands of a skilled person. Parts could be printed in multiple pieces, or have material removed after printing, so it'd have to anticipate all possible post-processing steps.

Even if the problem is limited to "match against these known gun part hashes", gun parts are not defined by their overall appearance. The files can be mutated until they no longer trip the detector, yet can be easily post-processed into functional components. (Also, a similarity hash for 3D toolpaths may not exist.)

(Such a hashing regime would be useless to stop "3D-printed guns" as a concept, because the design could be changed to not match the hash at all, without post-processing.)

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.

Because you don't really have to do any additional steps once the ink is on the paper like you do with a 3D printed firearm.

Looking at just the silhouette or shape, you couldn't really tell the difference between the body of a firearm that was later going to receive a metal tube from the hardware store or a plastic bracket that I'm going to use to support a shelf.

And even if you could accurately tell the difference, how do you know I'm not helping my nephew build a Nerf gun?

One is a specific pattern, with specific distances, colors, etc.

The other is a human-invented category of objects that lacks consistency in size, shape and design.

“Don’t print this specific pattern” is a much more closed loop than “figure out if this could be part of a firearm, bearing in mind that guns share a ton of parts with perfectly legal things”. Especially since the things that are most indicative of being a firearm are usually bought instead of printed.

Eg a “3D printed AR-15” usually means a 3D printed lower and the rest of it is bought off the shelf of a gun store (the rest isn’t regulated, you don’t need a background check to buy a barrel or upper or etc). So it’s really just a grip, the slot for the magazine, and some holes for pins in the right spot.

That’s super hard to identify. Grips come in a variety of shapes and sizes and have a bunch of legal uses (nerf guns, slingshots, Wii remote holders) and are hard to detect. The mag well is just a rectangular hole? Probably some slots for springs, but you have to assemble it afterwards so the printer doesn’t see that.

The pin holes are probably the best indicator, although they’d be trivial to drill or melt afterwards, or print them as a separate piece and screw it on top of the lower.

It’s just an almost impossible problem. I don’t even think humans could do it reliably with the info the machine gets. Whether it’s an AR lower or a prop/nerf gun/airsoft/cosplay/accessibility attachment (pistol grips have some nice ergonomics) gets into the nitty gritty of the interior and whether they support real linkages or not. I just don’t think we can do it with any degree of accuracy.

That’s without getting into the more avant-garde 3D printed guns that are made either out of hardware store parts rather than gun store parts (common in Europe, barrels and what not are regulated there), or are made out of an amalgamation of parts from other guns (trigger from a Glock, barrel from an AR-15, mags from something else, etc).

Generalizing an algorithm that can detect “things that aren’t a gun I recognize but could be used as a gun” is going to be fraught with false positives. You don’t 3D print the linkages, so it’s just a frame, and there an awful lot of things that could conceivably be part of a gun. Anything vaguely semi-circular could be a trigger, anything with a tube could be a barrel, anything hollow and rectangular could be a magazine.

I don’t see a way that either a) it only blocks current designs and the community quickly adapts around it and continues, or b) it tries to block anything gun-like and false positives spiral out of control.

It feels sort of moot anyways. Phillip Luty wrote a whole book about assembling a fully-automatic submachine gun out of parts from the hardware store all the way back in the 90s. It does require familiarity with hand tools and access to more tools than most people have at home (drill press comes to mind), but you don’t have to be an expert machinist to do it even without 3D printers.

This is all besides the fact that it’s so easy to obtain a real gun in the US that 3D printing guns is a niche hobby. I’ve looked at them and watched some of their competitions, they’re still pretty crap. The fully 3D printed ones still jam constantly and I believe still use off the shelf magazines because those are surprisingly hard to make well. The partially 3D printed ones (eg lower only) tend to be substantially less durable than market alternatives. Unsurprisingly, a plastic 3D printed lower is more likely to shatter while firing or dropping it than a milled steel/aluminum one.