LAST autumn Cody Wilson, a law student at the University of Texas, leased a Stratasys 3D printer. He wanted to print a gun, and more—he and the group he founded, Defense Distributed, wanted to develop blueprints for 3D printing of guns and gun parts, and distribute those blueprints online. Mr Wilson’s motives are overtly political; he wants to “[expand] a free sphere of action…in contradistinction to a planned regulatory scheme…The file is the message. Anyone can have it, anyone can print it, anyone can use it.” Stratasys was not amused. Mr Wilson says they reclaimed their printer before he had even set it up.

Undeterred, Defense Distributed raised enough money first to lease time on 3D printers around Austin, and then to buy two of their own. Earlier this month they successfully tested a printed, plastic 30-round magazine for an AR-15, one of the most popular rifles in America. They called their magazine “Cuomo”, after New York’s governor, who championed legislation banning magazines that hold more than seven rounds. Others have successfully printed stocks, grips and triggers, though not the chamber or the barrel of a weapon. That is much harder; but all this tinkering makes many people nervous.

Some of that fear may be overblown. Making a gun for personal use is usually not illegal, and home-made guns are nothing new. Ginger Colburn, a spokeswoman for the Bureau of Alcohol, Tobacco and Firearms (ATF), says her agency has seen guns made from “pens, books, belts, clubs. You name it, people have turned it into firearms.” And it may lead to bad law. Michael Weinberg, a staff lawyer at Public Knowledge, an open-source advocacy group, fears clumsy regulation of 3D printing, rather than of the weapons themselves.

To that end, Steve Israel, a Democratic congressman from Long Island, plans to introduce legislation renewing and expanding the Undetectable Firearms Act. That bill outlaws guns undetectable to common X-ray machines. Mr Israel wants to make plastic magazines illegal too. Easier said than done. Banning plastic gun parts when none existed was one thing. Enforcing a ban when anyone with an internet connection and a 3D printer can make them is entirely another.