From the Syllabus. Not ridiculous, not extreme. Garden-variety statutory analysis:
The National Firearms Act of 1934 defines a “machinegun” 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.” 26 U. S. C. §5845(b). With a machinegun, a shooter can fire multiple times, or even continuously, by engaging the trigger only once. This capability distinguishes a machinegun from a semiautomatic firearm. With a semiautomatic firearm, the shooter can fire only one time by engaging the trigger. Using a technique called bump firing, shooters can fire semiautomatic firearms at rates approaching those of some machineguns. A shooter who bump fires a rifle uses the firearm’s recoil to help rapidly manipulate the trigger. Although bump firing does not require any additional equipment, a “bump stock” is an accessory designed to make the technique easier. A bump stock does not alter the basic mechanics of bump firing, and the trigger still must be released and reengaged to fire each additional shot.
For many years, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) consistently took the position that semiautomatic rifles equipped with bump stocks were not machineguns under §5845(b). ATF abruptly changed course when a gunman using semiautomatic rifles equipped with bump stocks fired hundreds of rounds into a crowd in Las Vegas, Nevada, killing 58 people and wounding over 500 more. ATF subsequently proposed a rule that would repudiate its previous guidance and amend its regulations to “clarify” that bump stocks are machineguns. 83 Fed. Reg. 13442. ATF’s Rule ordered owners of bump stocks either to destroy or surrender them to ATF to avoid criminal prosecution.
Held: ATF exceeded its statutory authority by issuing a Rule that classifies a bump stock as a “machinegun” under §5845(b). Pp. 6–19.
(a) A semiautomatic rifle equipped with a bump stock is not a “machinegun” as defined by §5845(b) because: (1) it cannot fire more than one shot “by a single function of the trigger” and (2) even if it could, it would not do so “automatically.” ATF therefore exceeded its statutory authority by issuing a Rule that classifies bump stocks as machineguns. P. 6.
(b) A semiautomatic rifle equipped with a bump stock does not fire more than one shot “by a single function of the trigger.” The phrase “function of the trigger” refers to the mode of action by which the trigger activates the firing mechanism. No one disputes that a semiautomatic rifle without a bump stock is not a machinegun because a shooter must release and reset the trigger between every shot. And, any subsequent shot fired after the trigger has been released and reset is the result of a separate and distinct “function of the trigger.” Nothing changes when a semiautomatic rifle is equipped with a bump stock. Between every shot, the shooter must release pressure from the trigger and allow it to reset before reengaging the trigger for another shot. A bump stock merely reduces the amount of time that elapses between separate “functions” of the trigger.
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