In 1934, after a string of high-profile shootings, Congress passed the National Firearms Act (NFA) to address the proliferation of machine guns that enabled mass killings with a single pull of a trigger. Today, a “machinegun” is defined as “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 “also include[s] . . . any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun.”

In 2018, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) promulgated a regulation (the Rule) that classified devices known as bump stocks as “machineguns.” Bump stocks attach to semiautomatic firearms and enable them to harness the recoil energy of the firearm in a manner that allows the trigger to reset and continue firing without additional physical manipulation of the trigger by the shooter. As the Rule explained, bump stocks are properly classified as machine guns because they allow a shooter to produce automatic fire with a single pull of the trigger.

Respondent challenged the Rule, and both the district court and a unanimous Fifth Circuit panel rejected the challenge. While the en banc Fifth Circuit ultimately reversed, there was no majority view on the best interpretation of the statutory definition. Instead, a majority concluded that lenity principles require interpreting the statutory definition of “machinegun” not to encompass bump stocks.

CAC filed an amicus brief with the Supreme Court in support of the Rule. Because the text and history of the NFA make clear that bump stocks fall within Congress’s definition of “machinegun,” the Supreme Court should reverse.

The key terms in the current statutory definition of “machinegun”—“automatically” and “by a single function of the trigger”—have been in place since the NFA was passed in 1934. At that time, the term “function” meant “performance,” or some “activity,” “action,” or “doing.” And an object acted “automatically” if it could continue to act on its own after the initiation of the action by a person—that is, once set into motion, it could “produce results otherwise done by hand.” Thus, a weapon could shoot “automatically . . . by a single function of the trigger” if it could continue to fire after the shooter’s single initial pull of the trigger.

This understanding of the ordinary public meaning of the key language in the NFA is reflected in agency rulings and guidance discussing the statute shortly after its passage, as well as contemporaneous newspaper articles describing the new firearm registration requirements in layman’s terms to their readers.

The history of the NFA confirms what its text makes clear: the Act applies to any gun that fires continuously after the shooter initially pulls the trigger. Congress defined “machine gun” broadly with a focus on guns that could shoot continuously once set into motion by the shooter’s initial action. As the House Report put it, Congress was using the “usual definition” of a machine gun: “a weapon designed to shoot more than one shot without reloading and by a single pull of the trigger.”

Subsequent to the NFA’s passage, Congress remained concerned about gun violence and mass shootings and expanded the definition of “machine gun” to ensure that all guns capable of producing such mass casualties satisfied the definition. Most significantly, in 1968, with the passage of the Gun Control Act, Congress expanded the NFA’s definition of “machine gun” to include parts and components that could transform a weapon that was not a machine gun into a machine gun—just like bump stocks do.

The ATF Rule at issue here should be upheld because it simply implements Congress’s judgment about “machine guns,” as the text and history of the NFA make clear.

In June 2024, the Supreme Court ruled in favor of the respondent in a 6-3 decision divided along ideological lines. The Court held that ATF exceeded its authority by classifying bump stocks as machine guns. Echoing points made in our brief, Justice Sotomayor highlighted in her dissent that the majority opinion “flies in the face of this Court’s standard tools of interpretation . . . [b]y casting aside the statute’s ordinary meaning both at the time of enactment and today.”