Which is a very easily recognized pattern, color, and size. The entire point of a dollar is that every single one looks identical.

Imagine if every single dollar bill was a different color, shape, size, printing pattern, etc… Now imagine trying to block that. Now consider that as soon as you figure out how to block all of the current versions, anyone in the world can just design a new version in 5 minutes.

Somewhat related, the US Gov provides play money that you can print for your kids, which I found helpful to teach my kids about how money works. https://www.uscurrency.gov/sites/default/files/download-materials/en/Printable-Play-Money.pdf

True, but nothing else looks like money. Lots of things have a similar shape as the barrel of a gun.

Money is also quite detailed, with a known list of configurations. Any counterfeit would need to match the details in those known configurations extremely well. Finding that match with a high degree of accuracy is a fairly well understood and common engineering task. This is not the same task as identifying anything that could possibly be used to represent money with a high degree of accuracy, which is essentially what would be needed in the gun printing problem.

Most currencies have a special pattern that printers are programmed to detect and refuse to print. Since illegal gun part designs can't be forced to include a marker declaring that they're gun parts, a 3d printer would have to 1) know what a gun is, 2) know how a gun works, 3) be able to tell whether any particular shape could be used as part of a gun, and 4) be able to tell whether any particular shape could be cut and reassembled into a shape that could be used as part of a gun

that's different, bank notes follow the same pattern/design, the components that could be printed for firearms vary so much in shape and size, even for the same components across different platforms.