Anti-gun/gun control lobby would be my first guess. You can basically print all the serialized parts (the part required for registration) for most any gun then get the rest of the parts and assemble it yourself. The gun parts don't necessarily even need to be based on an actual manufactured gun, there are designs for completely homemade guns down to the barrel using parts you can easily pick up at any hardware store. Then there are also people who are printing parts that can turn some semi-automatic guns into selectable fully automatic. Problem is the plans are already in the wild for printing gun parts and for open source printers. I don't know what good would accomplish to deter people from printing when the person targeted is already motivated enough to print one to begin with.