If you have access to a 3-d printer, you can now manufacture a working gun. Whether it can be used more than once is not known. Once used, presumably, it could be melted down and simply disappear.
No serial number. No ballistics. No fingerprints. No weapon.
Barring an eye witness, how could you possibly identify the shooter? “We believe this puddle of plastic might have been the gun.” Really?
To me, this is the ultimate license to kill.