Auto sears are devices that convert semi-automatic firearms, which are designed to fire one shot per trigger pull, into fully automatic machine guns, allowing a shooter to continue firing as long as the trigger is depressed and the gun has ammunition. In other words, with an auto sear (also known as a “machine gun conversion device,” or MCD), someone can fire an entire magazine’s worth of ammunition in mere seconds. Worse yet, auto sears are very easy to manufacture and can even be 3D printed, which is why these components — especially those designed by third parties for Glock-style pistols, known as “Glock switches” — are showing up at more and more crime scenes around the country.
Are Auto Sears Legal to Own?
Generally, no. Machine guns have been highly regulated since the passage of the National Firearms Act (NFA) of 1934, which imposed strict registration requirements on fully automatic firearms — such as the Tommy guns used by gangsters of that era — and other dangerous weapons. With the Gun Control Act of 1968, Congress amended the federal “machinegun” definition to include the parts used to create them.1According to 26 U.S. Code § 5845(b), “The term ‘machinegun’ means any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.” Then, in 1986, Congress prohibited civilians from owning machine guns manufactured after May 19, 1986, as part of the Firearms Owners’ Protection Act (FOPA).
Thus, auto sears, which are relatively recent inventions, are generally illegal to own. Only federally licensed gun manufacturers and dealers who have paid a Special Occupational Tax (SOT) can produce and sell new machine guns to military and law enforcement personnel. Under federal law, possession of an unregistered machine gun can lead to up to 10 years in prison and a $250,000 fine, and many states impose their own penalties.
How Glock Switches Work
Glock switches are MCDs designed for use on Glock-style pistols. They attach to the back of the pistol’s slide and interfere with the internal trigger mechanisms to allow for fully automatic firing. It’s also important to note that these devices are not manufactured by Glock, even if they are branded or referred to as such.
In essence, a switch has a small protrusion called a “disconnector” or “leg” that pushes the gun’s trigger bar down and causes the gun to fire every time the slide returns forward. Modified in this way, a Glock-style pistol can fire at a rate of up to 1,200 rounds per minute, or 20 rounds per second.
While many switch designs feature a selector that allows a shooter to toggle between semi-automatic and fully automatic modes, some omit the selector entirely and are designed for fully automatic firing alone. These “backplate” switches do not extend from the rear of the gun’s slide, making them harder to identify and intercept.
AUTO SEARS FOR AR-15s
Many semi-automatic AR-platform weapons feature the same components as the U.S. military’s M16 and M4 rifles, which are capable of fully automatic and burst firing, including bolt carriers. This is why it is so easy to install a drop-in auto sear (DIAS) and convert a semi-automatic AR-15 into a fully automatic weapon. Though there are a variety of shapes and styles, every DIAS essentially allows the gun’s hammer to contact the firing pin every time the bolt returns forward for repeated firing.
Other auto sears for AR-15s are simpler designs, including “lightning links” and “swift links,” which are illegal for civilians to own. But even a coat hanger can be used to convert an AR-15 into a fully automatic machine gun.
THE RISING TIDE OF GLOCK SWITCHES AND OTHER MCDs
Many MCDs, including those designed for Glocks and AR-15s, are imported from China. In May 2019, a Topeka, Kansas, man was indicted for importing an undisclosed number of Glock switches from China for “$19 a piece.” According to one news report, the ATF “identified 4,000 converter switches smuggled into the U.S.” from China and have recovered more than 3,000 of them. “But hundreds from that shipment are still out there somewhere and being made by other manufacturers.”
Retailers have sold MCDs through websites like Wish.com, Ali Express, and even Amazon. But these devices are also being made here in the U.S. According to the ATF’s assistant special agent in charge for Los Angeles, “We have seen cases here in L.A. where these are manufactured at home or within a machine shop,” and 3D printing files for Glock switches and AR-15 MCDs are easy to find online. Searches using the terms “auto sear,” “swift link,” “lightning link,” and “Yankee Boogle” — a reference to the extremist boogaloo movement — yield several results on some of the largest and most popular 3D printing sites.
Popular 3D gun printers also use YouTube and Discord to showcase fully automatic weapons equipped with their MCD designs, and some demonstrate how to 3D-print the devices.
In September 2020, two boogaloo supporters were charged with attempting to provide material support to Hamas, by delivering, in part, a 3D-printed MCD. A month later, a West Virginia man was indicted for selling more than 600 MCDs for AR-15s that had been 3D printed. One of those ended up in the hands of a boogaloo supporter who was later arrested for murder in California.
According to the ATF, law enforcement across the country recovered 814 MCDs, including “Glock switches,” from 2012 to 2016, but that figure jumped to 5,454 from 2017 to 2021, reflecting a 570-percent increase. A more recent ATF report shows that police recovered 11,088 MCDs between 2019 and 2023, including “Glock switches.” In that period, MCD recoveries increased by 784 percent, and 5,816 were recovered in 2023 alone.
MCD recoveries
Other agencies have offered similar reports. Washington, D.C.’s Metropolitan Police Department has recovered more crime guns with MCDs, seizing “20 auto sears in the District during the first half of 2021, double the number in the same time last year.” The rise in Glock switches has left Houston police feeling “terrified,” and law enforcement in Baton Rouge have seen “more than 80 cases over the past two years involving Glock switches or auto sears attached to weapons involved in crimes.”
Glock switches have been used in several mass shootings, including those identified in the table below.
| Date | City | State | Reporting |
|---|---|---|---|
| 11/17/19 | Fresno | CA | Source |
| 5/22/21 | Minneapolis | MN | Source |
| 4/3/22 | Sacramento | CA | Source |
| 5/19/22 | Chicago | IL | Source |
| 12/31/22 | Mobile | AL | Source |
| 4/15/23 | Dadeville | AL | Source |
| 8/26/23 | Boston | MA | Source |
| 10/8/23 | Chicago | IL | Source |
| 1/28/24 | Memphis | TN | Source |
| 3/6/24 | Philadelphia | PA | Source |
| 4/12/24 | Memphis | TN | Source |
| 6/1/24 | Detroit | MI | Source |
| 6/6/24 | Memphis | TN | Source |
| 6/15/24 | Round Rock | TX | Source |
| 7/7/24 | Detroit | MI | Source |
| 7/28/24 | Auburn | WA | Source |
| 8/25/24 | Lubbock | TX | Source |
| 9/21/24 | Birmingham | AL | Source |
| 10/19/24 | Lexington | MI | Source |
| 11/10/24 | Tuskegee | AL | Source |
| 5/26/25 | Philadelphia | PA | Source |
| 7/26/26 | Seattle | WA | Source |
Glock has stated that it has “collaborated with law enforcement officials to target illegal sellers and users of switches” but has determined that, according to The New York Times, its pistols “cannot be altered” to “make it harder to attach switches.” Gun makers also continue to produce easily modified AR-15s and similar assault weapons, leaving law enforcement agencies — and the public at large — to face this growing problem on their own.