I don't think anyone really knows. Might be something like the slicer is checking the file hash against a (online) list, slices it, signs it and the printer checks the .gcode file if it's signed by a known slicer. A blacklist of very specific models. Make a small change and it will not be detected any more.

Maybe the slicer is also required to upload the 3d file and some kind of "looks similar" algorithm will block your nerf gun from being sliced.

The irony, is almost anyone with a machine shop can probably fabricate a more effective weapon than a 3D printer, where you might as a gun get one or two less than accurate shots in.