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.