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