After years of rarely appearing in the hands of criminals, machine guns have been making a comeback thanks to tiny devices called auto sears, which turn semiautomatic pistols and rifles into full-auto weapons that spray multiple bullets with a single squeeze of the trigger.

In March, an investigation by VICE News and The Trace uncovered a staggering rise in the number of illegal auto sears and other so-called machine gun conversion devices recovered by law enforcement and linked to criminal cases, including mass shootings, gang violence, and domestic extremism.

Now, Democrats in the Senate are trying to do something about it. 

VICE News has learned that nine Democrats, led by Minnesota’s Amy Klobuchar and Michigan’s Gary Peters, plan to introduce legislation on Monday aimed at focusing federal efforts and increasing transparency around what has until recently been a relatively overlooked threat.

The Trace and VICE News’ reporting, cited by the senators in their unveiling of the bill, found that more than a thousand illegal auto sears have been recovered in connection with at least 260 federal prosecutions since 2017, with the number of cases filed annually climbing nearly 800 percent in five years. The government was not tracking this data previously, and part of the Senate proposal would require the attorney general to include “information about illegal gun modification devices in the Justice Department’s annual firearms trafficking report.”

Auto sears are already strictly regulated under federal firearm laws, and can be obtained legally with the right paperwork, fees, and an extensive background check. But they are cheap to manufacture and easy to install, which has allowed the black market to boom.

“As we work to tackle the gun violence epidemic that is plaguing our country, we must address the alarming prevalence of gun conversion devices that can turn ordinary hand guns into automatic weapons,” Klobuchar said in a draft press release. “These devices can be easily 3-D printed or cheaply purchased online, but can have devastating consequences for public safety.”

Peters said the proposal would “help improve federal law enforcement coordination, including with state and local partners,” but what that would actually mean in practice is unclear. 

There are already multiple federal agencies that investigate illicit auto sear trafficking, including the Department of Homeland Security, the Postal Inspection Service, and the Bureau of Alcohol Tobacco Firearms and Explosives. 

The real problem is that auto sears are so small — the size of a thimble — they can be hidden in packages and smuggled into the country with alarming ease, according to Rick Vasquez, former chief of the ATF’s Firearms Technology Division. Chinese manufacturers have been identified as primary suppliers, and the devices sometimes appear for sale online disguised as parts for Airsoft or paintball guns. They are also difficult for the untrained eye to identify.

“The fact of the matter is the ATF is already well aware of these,” Vasquez said. “They’re coming in from China by the ton, concealed in with other items. If you have an order of car parts, how can you tell if an auto sear is in the mix? They’re tiny.”

With what’s being dubbed the “Preventing Illegal Weapons Trafficking Act of 2022,” the senators want to mandate development of “a coordinated national strategy,” and “ensure that proceeds from the illegal trafficking of automatic gun devices are subject to forfeiture,” though prosecutors already have several tools at their disposal to seize illicit cash.

There are different types of auto sears, but they’re particularly common for Glock pistols. These Glock “switches” allow the user to toggle between semiauto (one bullet per pull of the trigger) and being able to empty an entire magazine of ammunition all at once in a split second. The rapid spray of bullets can be difficult for shooters to control, and shoddy installation can lead to a “runaway gun” which continues firing even after the trigger is no longer being pulled. 

Glock does not make or sell auto sears to the public, but it does offer a full-auto 9mm handgun “for the military and police market.” The devices popping up on the streets are unsanctioned by the company and just having one uninstalled without the proper registration can be punishable with a 10-year prison sentence. Glock did not respond to a request for comment.

Pistols modified with auto sears have already turned up in deadly mass shootings, including one that left six dead and 12 wounded in Sacramento, California, on April 3. In June, a former U.S. Air Force sergeant linked to the anti-government Boogaloo movement was sentenced to 41 years in prison after he pleaded guilty to using an AR-15-style rifle equipped with an auto sear to kill a security guard in a drive-by shooting at a federal courthouse in Oakland, California.

“It is critical that Congress take swift action,” said Adzi Vokhiwa, federal affairs director for the gun control advocacy group Giffords. “These small devices are capable of grave damage and destruction.”

Whether Klobuchar and Peters have the votes in the Senate to pass their auto sear bill is uncertain. Congress has already passed one bipartisan gun reform law this year, but that took a spree of horrific mass shootings to spur action and required major compromises to appease Republicans, who have fought past efforts to gather and research data on gun violence.

More reliable information is sorely needed about the number of illegal auto sears circulating in the United States and how often they are being used by criminals. ATF officials reported seizing 1,500 weapons modified with conversion devices in 2021 versus just 300 the previous year. There are likely far more auto sears that have gone unreported amid investigations by other federal agencies and state and local authorities. 

Vasquez, the former ATF official, said that while it’s good that auto sears are on the radar of federal lawmakers, the impact of the proposed law could be minimal. Auto sears are already restricted, he noted, and the feds have been busy enforcing the laws on the books.

“It sounds like you’re basically going to make a law to tell federal agencies to do what they’re already doing,” Vasquez said. “Who are you going to prosecute if they violate the law?”