Federal law restricts the ownership of machine guns but not semiautomatic weapons. In 2018, under the Trump administration, the Bureau of Alcohol, Tobacco, Firearms and Explosives adopted a formal rule classifying a semiautomatic rifle equipped with a bump stock as a machine gun. Last week, the U.S. Supreme Court held that the ATF exceeded its statutory authority in adopting that rule in the case Garland v. Cargill, 602 U.S. __ (2024).

The court’s majority and dissenting opinions disagree on the main issue—whether a bump stock meets the statutory definition of “machine gun.” In particular, they disagree on the technical issue—whether a semiautomatic rifle equipped with a bump stock can “automatically” fire more than one shot “by a single function of the trigger.”