ATF's May 2026 final rule rewrites the federal machine gun definition following the Supreme Court's Cargill bump stock ruling, setting new enforcement lines.
The Bureau of Alcohol, Tobacco, Firearms and Explosives published a final rule on May 6, 2026, revising the federal regulatory definition of "machine gun" in direct response to the Supreme Court's 2024 decision in *Garland v. Cargill* [1]. The rule, which took effect the same day it was published in the Federal Register, updates ATF's codified definitions to conform with the Court's holding and withdraws the regulatory language that *Cargill* rendered invalid [1].
The *Cargill* decision held that bump stocks, devices that harness a rifle's recoil to enable rapid, repeated trigger engagement, do not convert semiautomatic firearms into machine guns within the meaning of the statutory definition under 26 U.S.C. § 5845(b) [1]. That ruling invalidated the ATF's 2018 bump stock ban, which had reclassified bump-stock-equipped rifles as machine guns subject to the National Firearms Act's strict registration and transfer requirements. The May 2026 final rule represents ATF's formal administrative effort to close the definitional gap the decision created, aligning the agency's regulatory text with the Court's statutory interpretation [1].
The significance of the rulemaking extends beyond bump stocks. The revised definition will govern how ATF identifies and enforces prohibitions on automatic weapons across a broad range of devices and configurations going forward [1]. The updated language sets the enforcement baseline that federal prosecutors, licensed dealers, and courts will use to evaluate whether a given firearm or accessory crosses the statutory threshold. Any imprecision in that language carries direct consequences for criminal charging decisions and civil forfeiture actions. The rule also opens a litigation front: challengers may argue that the revised definition is either too broad, implicating Second Amendment protections under the framework *New York State Rifle & Pistol Association v. Bruen* established, or too narrow, failing to capture devices that function identically to the bump stocks *Cargill* addressed [1].
ATF and DOJ did not identify pending litigation challenging the new rule at the time of publication [1]. Industry groups and Second Amendment advocacy organizations are expected to evaluate the final text for legal exposure. Courts in circuits that have active enforcement actions involving automatic-weapon classifications will likely be the first venues where the revised definition is tested. ATF's rulemaking record, including the administrative findings supporting the updated language, will be the central battleground if and when a challenge is filed.