The molded piece of plastic or metal — a "bump stock," as it has become known to assault-rifle enthusiasts — harnesses a gun's natural recoil, allowing it to bounce back and forth off a shooter's trigger finger and unleash up to 100 rounds in seven seconds, according to an ad for one of the devices.
"It's a goofy little doodad," said Rick Vasquez, the former firearms official who first signed off on a recommendation that the Bureau of Alcohol, Tobacco, Firearms and Explosives need not regulate the devices.
As Vasquez reasoned, the invention did not technically alter a gun's trigger mechanism, as earlier attempts had, with springs, hydraulics or electric current. So it did not infringe on a law that bans the sale of machine guns manufactured after 1986 and restricts the sale of those made before then.
Functionally, however, weapons experts say, the increasingly popular bump stock has allowed even novice gun-owners to easily modify a legal semiautomatic rifle into one that resembles a battlefield machine gun.
"It's for those guys who want to look like super ninja when they're out on the range — they're the people my peer group makes fun of," Vasquez, a former Marine, said Wednesday, returning from a firearm instruction course he conducted in South Carolina. "If you want a machine gun, join the Marines."
Federal law enforcement officials said that gunman Stephen Paddock fired weapons outfitted with bump stocks, a dozen of which were found in his hotel suite. Audio of the attack, experts said, makes clear that the shooter unleashed a torrent of bullets faster than he could have fired manually.
The devices have quickly become a new political flash point in the nation's gun debate. On Tuesday, Sen. Dianne Feinstein (D-Calif.) introduced a bill specifically targeting bump stocks and similar inventions, including trigger cranks that accelerate a semiautomatic rifle's rate of fire.
"The only reason to fire so many rounds so fast is to kill large numbers of people," Feinstein said.
Bump stocks would have been banned under legislation Feinstein introduced after the 2012 shooting of elementary students in Newtown, Conn. That effort collapsed.
The National Rifle Association declined to comment on the use of the devices in the Las Vegas attack, or on Feinstein's proposed legislation. Two attorneys for the most prominent bump stock manufacturer, Slide Fire, which received the favorable 2010 ATF decision, also declined to comment.
The Slide Fire website on Wednesday said the device was out of stock "due to extreme high demands."
Federal campaign data shows Slide Fire and the handful of other manufacturers of bump stocks have succeeded so far with little to no contributions to federal lawmakers.
"These are small companies, with no involvement in lobbying or pushing anything politically, because all they need to operate is this one-page letter from ATF," said Mark Terry, a Miami patent attorney who represented a competitor called Bump Fire. "They are young entrepreneurial guys who want to sell a product that people like and want to buy."
Asked if he had any regrets given the shooting this week, Vasquez said: "It's not my place to judge. We followed the law, and everything was evaluated fairly and honestly with the regulations."
Magda Jean-Louis contributed to this report.