For the cost of an Internet connection and some relatively cheap materials, anyone with access to a high-end 3-D printer can make a lethal handgun that can be slipped past security checkpoints — into schools, sporting events and government offices. That’s fine with some Republicans in Congress.

A blueprint for such a weapon was posted online last spring and downloaded more than 100,000 times before federal officials demanded that the Web site be taken down. Needless to say, anyone with a yen to find the design can do so.

Homemade plastic firearms fashioned from 3-D printers are not yet common household items, but the technology to manufacture them is becoming more available and less expensive. Will Congress really not act to contain the threat until a criminal or terrorist proves how easy it is to defeat detection by carrying out an attack with a gun that can be made in a garage in less than a day?

It’s starting to look that way. The Undetectable Firearms Act, a 1988 law twice renewed, is scheduled to expire Dec. 9. With some sensible tweaks, the legislation could be updated to ban the manufacture, sale and use of 3-D plastic guns by requiring that they contain minimum amounts of non-removable — and detectable — metal parts. An amendment that would achieve that, and render the weapons more detectable, has been drafted by House and Senate Democrats.

That would seem a no-brainer. But the National Rifle Association and its mostly Republican allies in Congress are balking, insisting that the existing law need not be modified and minimizing the threat despite warnings from security officials and the Bureau of Alcohol, Tobacco, Firearms and Explosives.

In The New York Times, Sen. Jeff Sessions, an Alabama Republican, suggested it would be “problematic” to add language to the existing legislation that would address the threat. Larry Pratt, executive director of Gun Owners of America, dismissed the warnings of federal officials and congressional Democrats as exaggerated, since 3-D printers are not yet in wide circulation. “They’re not going to be in Kinkos,” he told The Times. “And at the moment, ‘plastic guns made from 3-D printers’ can’t fire that many rounds.”

Well, that’s reassuring. Experience suggests that technology spreads more quickly and to more places than many people expect. Sen. Charles E. Schumer, a New York Democrat, is correct that the advent of undetectable weapons forged by 3-D printers is already turning “a hypothetical threat into a terrifying reality.”

— The Washington Post