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.