Respectfully, sir, I would suggest that it's going to be through the regulation of firearms parts.
Each 3-D printed receiver, the component they can 3-D print, requires specific parts to turn it into a functioning firearm, such as a trigger assembly, a barrel, a slide and rails, the metal pieces that go inside it. If we're able to regulate the sale, possession and importation of these specific parts, that would allow us to more effectively target people who are putting together the 3-D printed receivers into the functioning guns.