But monitoring whether people make their own guns on a 3-D printer is going to be impossible, barring sticking an A.T.F. agent in every home. It's also hopeless to try to build a technology into these printers that prevents people from printing a gun. One project mentioned in Mr. Wilson's video, called the RepRap printer, will be capable of replicating itself by printing other 3-D printers.
After committing a crime with a printed weapon, a person could simply melt down the plastic and reprint it as something as mundane as a statue of Buddha. And guns made of plastic might not be spotted by metal detectors in airports, courthouses or other government facilities.
"This becomes scary when you consider the fact that it could be yet another opportunity for people to evade background checks and get a gun," said Daniel Gross, president of the Brady Campaign to Prevent Gun Violence and the Brady Center to Prevent Gun Violence.