The US Senate has rejected calls to update the law in response to the advent of plastic guns made with 3D printers.
The Democrat-led body passed a bill last night that extends the Undetectable Firearms Act of 1988, which bans firearms that can pass undetected through metal detectors, for another 10 years. The Republican-led House of Representatives approved the bill last week.
President Barack Obama signed the legislation into law before the ban was to expire at midnight, but some Democrats have criticised a failure to close what are seen as loopholes created by new technologies, such as 3D printing, an aide said.
In May a group called Defense Distributed produced and tested the world's first 3D-printed gun, The Liberator, though it was only capable of firing a single bullet.
Senate Democrats led by Charles Schumer of New York yesterday failed to win quick approval of a measure to require all firearms to include at least 3.7 ounces (105g) of non-removable metal essential for its operation.
Without the provision, backers argue, the metal could be taken off the gun, permitting a functional weapon to pass undetected through metal detectors and X-ray machines and carried into supposedly secure areas.
The Center for American Progress, a liberal advocacy group, called it "crucial" to extend the ban, but denounced the measure passed by Congress as "deeply flawed."
"It does nothing to address new technologies like 3D printing that could allow terrorists and other dangerous people to easily make fully functional, undetectable guns," said Winnie Stachelberg, an executive vice president at the centre.
The ban was first signed into law in 1988 by Republican President Ronald Reagan shortly after the introduction of the Austrian-made Glock firearm, made largely of synthetic material, which created a fear of undetectable weapons.
Democrats say that danger has increased thanks to the underground production of plastic guns with 3D printers.
Democratic Senator Chris Murphy of Connecticut called for an update of the law, saying, "This isn't science fiction anymore .... Someone can make a gun in their basement."
Senate Republicans refused to provide the unanimous consent needed to approve the change on a legislative fast-track and Democrats then joined Republicans in voting to extend the ban.
Top Republican on the Judiciary Committee Senator Charles Grassley of Iowa did agree that changes may be needed in the ban, but complained that Schumer sought a vote before many lawmakers understood his provision and what it sought to fix.
"Congress needs to gain an understanding of printed gun manufacturing technology and its relation to permanent metal parts," Grassley said, adding that hearings are needed.
Schumer said he was encouraged by Grassley's comments, a key Republican voice in the Senate, telling reporters, "I'm hopeful that we can come to a compromise."