Seized plastic handguns that were created using 3-D printing technology are displayed at Kanagawa police station in Yokohama, south of Tokyo, in 2014. (KYODO/REUTERS)

IN THE 1993 thriller “In the Line of Fire,” a would-be assassin painstakingly builds a plastic gun that he could slip past magnetometers — and into a room with the president. Nowadays, all he would have to do is press “print.”

Manufacturing plastic guns is no longer the stuff of Hollywood; it’s an easy and inexpensive process. The ever-wider availability of 3-D printers allows all sorts of people to create all sorts of objects, including firearms, given the right schematics. Defense Distributed, an early innovator in this field, created and distributed blueprints for a plastic handgun in 2013, before the federal government ordered the group to take them down. By that time, the information had already been copied, and it remains accessible. Since 2013, enthusiasts have created plastic revolvers and plastic rifles, and they have used 3-D printers to fashion gun parts that turn semiautomatic guns into fully automatic weapons.

The 1988 Undetectable Firearms Act banned the production, transfer and possession of guns that magnetometers can’t detect, requiring at least 3.7 ounces of steel to be in every firearm created or carried in the United States. Congress reauthorized the law in 2013. That’s better than nothing, but it’s not enough. Legislators should have made the law stronger, as Rep. Steve Israel (D-N.Y.) proposed doing last week.

Mr. Israel argues that the act contains a large loophole. A professional or amateur gun-maker can legally produce a plastic weapon with a removable chunk of steel in it and legally sell or give it to someone else, who could easily discard the metal element and carry a deadly weapon anywhere. Mr. Israel wants to require producers and sellers of plastic guns to include a metal piece that can’t be removed. If outfits such as Defense Distributed wanted to enable people to build legal firearms, they would have to create schematics for weapons that contain critical, functional elements made of steel.

If, that is, the State Department lets them. State has moved to restrict online publication of “technical data” that can be used to construct weapons, demanding that federal officials give their approval first. The government claims that distributing, say, plastic gun schematics online is essentially an export of sensitive weapons information to foreign countries, and State has the authority to regulate arms exports under a Cold War-era law aimed at preventing weapons information from falling into Soviet hands. Keeping plastic gun schematics away from foreign terrorist organizations is a worthy national security goal now.

Worthy, but near-impossibly hard. Plastic gun blueprints are already on Web sites over which the U.S. government has no control. The State Department might also run into legal trouble for imposing a possibly unconstitutional prior restraint on the speech of plastic gun designers eager to share their work. We can’t blame State for trying, but we put more hope in Mr. Israel’s initiative to restrict the production of undetectable plastic weapons. His policy wouldn’t end the threat; the determined would still be able to work around it. But it would help reduce the number of undetectable guns in circulation.