A three-judge panel on the D.C. Circuit Court of Appeals has ruled bump stocks are illegal machine guns.

The Court issued a new opinion on Tuesday upholding a lower court decision that found the ATF acted properly when it reclassified bump stocks as machine guns under existing federal firearms law.

“The central question on appeal is whether the Bureau had the statutory authority to interpret ‘machine gun’ to include bump stocks,” Circuit Judge Robert L. Wilkins wrote in his opinion on behalf of the court. “Employing the traditional tools of statutory interpretation, we find that the disputed rule is consistent with the best interpretation of ‘machine gun’ under the governing statutes.”

The ruling deals another blow to gun rights advocates who have long questioned the rationale behind the ATF’s reclassification of the device. Tuesday’s ruling marks the fourth time a Federal Appeals Court has upheld the ban, though a decision out of the Fifth Circuit is currently awaiting an en banc review.

A bump stock is an aftermarket device that replaces a rifle’s traditional stock with one capable of sliding. The sliding stock helps shift a shooter’s trigger finger back and forth through a combination of recoil and forward pressure provided by their support hand. That enables the shooter to actuate the trigger faster than is typically possible for a semi-automatic weapon without the device. However, unlike fully-automatic weapons, guns equipped with a bump stock still require a unique trigger pull to fire each round. The bump-firing technique can also be accomplished without using a bump stock.

Both the National Firearms Act of 1934 and the Gun Control Act of 1968 regulate machine guns, defined as any weapon that shoots “automatically more than one shot, without manual reloading, by a single function of the trigger.” At issue in the challenge before the D.C. Circuit was whether “a single function of the trigger” could be interpreted to include both a “‘single pull of the trigger’ and analogous motions,” as the ATF had chosen to do to include bump stocks under the statutory definition of machine guns. In reaching the decision, Judge Wilkins was persuaded that a bump stock in operation only requires one distinct pull of the trigger, while simple inertia would be responsible for follow-up shots.

“After the first pull of the trigger, the stock moves repeatedly back-and-forth, causing the trigger to ‘bump’ against the stationary finger,” Judge Wilkins wrote. “Thus, only a single ‘function’ or ‘pull’ of the trigger by the shooter activates the multiple-shot sequence; no further pulls are needed. Accordingly, under the best interpretation of the statute, a bump stock is a self-regulating mechanism that allows a shooter to shoot more than one shot through a single pull of the trigger. As such, it is a machine gun under the National Firearms Act and Gun Control Act.”

The Firearms Policy Coalition (FPC) first sued the ATF over the ban in 2018. The group initially filed a motion to stop the ban from taking effect but was denied by a D.C. District Court Judge in 2019, who found that the ATF was entitled to deference in its interpretation of federal law. FPC appealed that decision to the Supreme Court, which ultimately declined to hear the case in 2020.

With the ban allowed to take effect, a lawsuit on the merits of the ban resulted in D.C.C. District Court Judge upholding it in 2021. Tuesday’s ruling affirms that decision.

Bump stocks existed in relative obscurity until 2017 when a gunman used several rifles equipped with the accessory to murder 58 people and wound an additional 500 at a music festival in Las Vegas. It remains the deadliest mass shooting in American history.

Then-President Trump subsequently ordered the ATF to ban the device, and the agency finalized a rule to do so in December of 2018. The rule change left lawful owners of the device with the choice of either turning in their property without compensation or risk facing a federal felony punishable by up to 10 years in prison.

Representatives with the Firearms Policy Coalition, the plaintiff in the case, did not respond to a request for comment.