The bump stock ban is too ambiguous to stand, according to a new court order.

On Tuesday, a three-judge panel for the Sixth Circuit Court of Appeals ruled in favor of a Kentucky man who challenged the legality of the ATF’s administrative ban on bump stocks. The unanimous panel ruled that it is unclear whether bump stocks fit within the current statutory definition of “machinegun” because of the agency’s flip-flopping on the issue. Therefore, it found that the definition should be interpreted to exclude them.

“An Act of Congress could clear up the ambiguities, but so far Congress has failed to act,” Judge Ronald Lee Gilman, a Bill Clinton appointee, wrote on behalf of the court. “Because the relevant statutory scheme does not clearly and unambiguously prohibit bump stocks, we are bound to construe the statute in Hardin’s favor.”

The ruling notches another win for gun-rights advocates who have fought the ban in court since it was unilaterally implemented by the Trump Administration in the wake of the 2018 Las Vegas shooting. It may also cast a shadow over the rules adopted through executive action under President Joe Biden (D.), which feature similar flip-flopping on how unfinished firearms parts or pistol braces should be regulated.

The decision may add motivation for the Supreme Court to weigh in on the matter too since multiple appeals courts have now come down on either side of the ban’s legality. Before Tuesday’s decision, the Tenth Circuit in 2020, the Sixth Circuit in 2021, and the D.C. Circuit in August 2022 all ruled to uphold the ATF’s interpretation of bump stocks as machine guns. The Fifth Circuit, meanwhile, ruled against the ban after finding that the accessories did not meet the definition of machinegun earlier this year.

Much of the controversy comes down to what constitutes a machinegun, which federal law defines as any firearm that can fire more than one round “by a single function of the trigger.” Since bump stocks use the recoil of a gun to help the shooter to pull the trigger more quickly, but still require a distinct function of the trigger for each shot, gun-rights advocates have challenged the ATF’s reclassification of the devices as unregistered machineguns under the National Firearm Act—possession of which could be punished with up to a decade in federal prison.

Unlike the Fifth Circuit’s ruling, which found the ban clearly fell outside the ATF’s administrative authority, Judge Gilman said it was unclear whether or not bump stocks met the definition of machinegun spelled out by the federal Gun Control Act of 1968. But he pointed out that the ATF had been inconsistent over the years while interpreting the legality of the devices, and judges have been unable to do much better.

“There can be no doubt that a significant number of reasonable jurists have reached diametrically opposed conclusions as to whether the definition of a machinegun includes a bump stock,” he wrote. “The viability of competing interpretations is exemplified not only by the myriad and conflicting judicial opinions on this issue, but also by the ATF’s own flip-flop in its position. And because the statute is ‘subject to more than one reasonable interpretation,’ it is ambiguous.”

Instead, his opinion hinged on the fact that because such ambiguity exists in a statute that involves criminal penalties for violators, the party at risk of being penalized under an unclear law is entitled to a favorable ruling under the rule of lenity.

“When Chevron deference is not warranted and standard principles of statutory interpretation ‘fail to establish that the Government’s position is unambiguously correct[,] we apply the rule of lenity and resolve the ambiguity in [the criminal defendant’s] favor,’” he said.

In a separate concurring opinion, Judge John Bush, a Trump appointee, said he would go further than the majority opinion. He argued there isn’t any ambiguity in the federal definition of machinegun as it relates to bump stocks.

“The best reading of the statute is that Congress never gave the ATF ‘the power to expand the law banning machine guns through [the] legislative shortcut’ of the ATF’s rule at issue in this appeal,” he said. “Simply put, under the statute as it currently reads, the addition of a bump stock to a rifle clearly does not make it a machinegun.”

He also echoed the point made by Gilman that it is Congress’ job to decide if bump stocks should be regulated or not.

“Though my reasoning differs somewhat from the majority opinion, all judges on this panel agree on this point: it is up to Congress, not the ATF, to change the law if bump stocks are to be made illegal,” Judge Bush said.

The Sixth Circuit remanded the case back down to the district court. It ordered the lower court to rule in favor of Hardin and determine the proper scope of relief.

The Department of Justice did not respond to a request for comment on whether it planned to appeal the decision.