Key Takeaways
- In Garland v. Cargill, the Supreme Court held 6-3 that a semiautomatic rifle with a bump stock is not a “machinegun” under federal law, so the ATF's 2018 ban was invalid.
- The case turned on the meaning of “a single function of the trigger” in the statute, not on the Second Amendment.
- Congress remains free to ban bump stocks by statute, and a number of states already prohibit them under their own laws.
If a device lets a semiautomatic rifle fire hundreds of rounds a minute, why isn't it a machine gun under federal law?
In this TalksOnLaw Brief, Joel Cohen talks with Joseph Blocher, the Lanty L. Smith '67 Distinguished Professor of Law at Duke Law School and Faculty Director of the Duke Center for Firearms Law, about Garland v. Cargill, the Supreme Court decision that struck down the federal ban on bump stocks. Professor Blocher explains why a ruling widely reported as a gun rights victory is, as he puts it, technically not a Second Amendment case at all.
What a Bump Stock Does
A semiautomatic weapon fires one round each time the trigger is pulled. A bump stock harnesses the rifle's recoil so the weapon bumps forward and the trigger is pushed repeatedly against the shooter's stationary finger. Professor Blocher explains that, with the finger held in place and steady forward pressure, a well-functioning bump stock can fire roughly 400 to 800 rounds per minute. Most people first heard of the devices after the 2017 Las Vegas shooting, and in 2018, during the first Trump administration and with broad bipartisan support at the time, the ATF adopted a rule treating bump stocks as machine guns.
“A Single Function of the Trigger”
Federal law defines a machine gun as a weapon that automatically fires more than one shot “by a single function of the trigger.” The question was whether the ATF's reading of that phrase was permissible. Writing for a 6-3 majority, Justice Thomas, using diagrams drawn from an amicus brief, focused on the mechanics of the gun: the trigger must reset after every shot, so each shot involves a separate function of the trigger. Justice Sotomayor's dissent looked at the question from the shooter's perspective: one pull, maintained with forward pressure, starts a firing sequence that continues on its own. Professor Blocher describes the split as the gun's perspective versus the shooter's.
Related Definitional Fights
Professor Blocher connects Cargill to another case then pending, VanDerStok, which asked whether the ATF could regulate gun parts kits and unfinished receivers, sometimes called ghost guns, the same way it regulates finished firearms, including background-check requirements. Like Cargill, it raised a basic definitional question: when does a product become a regulated weapon?
What to Know Now
Professor Blocher stresses a point often missed in the headlines: the Court decided only what existing law covers, and the majority made clear that Congress could ban bump stocks by statute. He doubted such a bill would pass, and none has been enacted. No federal court had yet decided whether bump stocks are protected by the Second Amendment, and state bans, which he puts at almost 20 states, could become the setting for that question. The parts-kit case has since been resolved: in 2025, in Bondi v. VanDerStok, the Court upheld the ATF rule 7-2 against a facial challenge, holding that the Gun Control Act reaches some weapon parts kits and unfinished frames or receivers. Read together, the two decisions show the Court parsing firearms statutes closely, sometimes for the government and sometimes against it. Supporters of Cargill see a check on agencies expanding criminal laws by rule; critics worry it leaves rapid-fire devices to state-by-state regulation unless Congress acts.
