The Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) did not exceed its authority when it reclassified unfinished gun part kits under the same rules as fully functional firearms.

That’s the conclusion of the Supreme Court of the United States (SCOTUS). On Wednesday, a 7-2 majority upheld the Biden-Era ATF rule that requires makers and dealers of unfinished frames and receivers to obtain federal licenses, serialize their products, and conduct background checks on customers. The Court found the rule, designed to prevent the proliferation of unserialized guns–often labeled “ghost guns” because they can’t be traced back to the original manufacturer, was valid because The Gun Control Act of 1968 (GCA) grants the ATF latitude in how it defines what is and isn’t a regulated gun part.

“The GCA embraces, and thus permits ATF to regulate, some weapon parts kits and unfinished frames or receivers, including those we have discussed,” Justice Neil Gorsuch wrote from the majority in Bondi v. VanDerStok. “Because the court of appeals held otherwise, its judgment is reversed, and the case is remanded for further proceedings consistent with this opinion.”

The decision is a setback for gun-rights advocates who’d successfully blocked implementation of the ATF rule in the lower courts. It allows the agency to effectively shut down unfinished firearms parts kit makers and dealers while making any future attempt by the Trump Administration, which made no effort to intervene in the case after taking over the Department of Justice in January, to undo the rule–something Donald Trump promised during his campaign. However, the ruling is limited and leaves open potential future challenges to ATF enforcement actions related to the rule.

In the majority opinion, Justice Gorsuch argued SCOTUS should judge the case as a broad challenge to the rule’s constitutionality. That meant the Court would only need to identify one viable application of the rule for it to withstand scrutiny.

“As presented to us, this case does not ask us to resolve whether ATF’s new regulations in §478.11 and §478.12 may be lawfully applied to particular weapon parts kits or unfinished frames or receivers,” Gorsuch, a Donald Trump appointee, wrote. “Instead, the plaintiffs have pursued what the lower courts called a ‘facial’ pre-enforcement challenge to the agency’s authority to regulate any weapon parts kits or unfinished frames or receivers. In a challenge like that, the government represents, ‘the possibility that [ATF’s regulation] may be invalid as applied’ in some cases ‘does not mean that the regulation is facially invalid.’ Instead, [the plaintiffs’] burden is to show that the Rule itself is inconsistent with the statute on its face.'”

Gorsuch said the GCA requires makers and dealers of “weapons” that can “expel a projectile by the action of an explosive, designed to do so, or susceptible of ready conversion to operate that way” to obtain licensing and follow other rules for selling those weapons. He noted the lower court found the ATF rule went beyond that power, but said the majority disagreed–at least to a point.

“As the Fifth Circuit saw it, § 478.11’s provisions addressing weapon parts kits are facially invalid because no weapon parts kit can ever satisfy the statute’s two requirements. We disagree because, to our eyes, at least some kits will satisfy both.”

While examining how the ATF rule could be applied in a way that is consistent with the GCA’s language, Justice Gorsuch focused primarily on the now-defunct Polymer80’s “Buy Build Shoot” kit. Those kits included an unfinished pistol frame alongside the tools and parts needed to quickly produce a working firearm. He said the kits can be viewed as firearms under the GCA because, similar to unassembled IKEA furniture, the context of the law likely included nearly-finished firearms alongside fully-functional ones.

“An author might invite your opinion on her latest novel, even if she sends you an unfinished manuscript,” he wrote. “A friend might speak of the table he just bought at IKEA, even though hours of assembly remain ahead of him. In both cases, the artifact noun fits because the intended function of the unfinished object is obvious to speaker and listener alike.”

“The term weapon can work this way, too,” Gorsuch continued. “Imagine a rifle disassembled for storage, transport, or cleaning. It may take time to render the rifle useful for combat, but its intended function is clear. And, as a matter of every day speech, that rifle is a weapon, whether disassembled or combat ready. In the same way and for the same reason, an ordinary speaker might well describe the ‘Buy Build Shoot’ kit as a ‘weapon.’ Yes, perhaps a half hour of work is required before anyone can fire a shot. But even as sold, the kit comes with all necessary components, and its intended function as instrument of combat is obvious. Really, the kit’s name says it all: ‘Buy Build Shoot.'”

Gorsuch pointed to the GCA’s inclusion of “starter guns” in its definition of “weapon” to bolster his case. Starter guns are firearms with blocked barrels that are designed for use with blank rounds. However, as the majority noted, they can be modified to fire live rounds with projectiles in a short amount of time with common tools.

“[N]otably, the statute teaches that a starter gun is a ‘weapon’ before anyone invests that work,” Gorsuch wrote. “All of which indicates that Congress used that term, as an ordinary speaker might, to embrace some unfinished instruments of combat like Polymer80’s product.”

The majority also argued that Congress could have regulated only finished firearms if that was what it intended. Instead, Gorsuch wrote, legislators went beyond that.

“If Congress had wanted to regulate only operable firearms, it could have simply addressed ‘weapons’ that can ‘expel a projectile by the action of an explosive,'” he wrote. “But Congress didn’t stop there. Instead, Congress explained that a ‘weapon’ also qualifies for regulation if it is either ‘designed’ to accomplish that function or ‘capable of being ‘readily . . . converted’ to do so. Those latter provisions necessarily contemplate that some things short of fully operable firearms will qualify as ‘weapons.’ And if that is true, it is difficult to see how the easy-to-assemble ‘Buy Build Shoot’ kit might not be among them.”

Justices Clarence Thomas, a George H. W. Bush appointee, and Samuel Alito, a George W. Bush appointee, were the only ones to dissent. Thomas argued the majority turned the GCA’s language on its head. He argued the GCA only regulates “frames” and “receivers,” the key unfinished part included in the “Buy Build Shoot” kits, in their completed form.

“Congress could have authorized ATF to regulate any part of a firearm or any object readily convertible into one,” Thomas wrote. “But, it did not. I would adhere to the words Congress enacted.”

He accused the majority of rewriting the statute–something he said SCOTUS had avoided doing in last year’s bumpstock ban case.

“The Government asked this Court just last Term to ‘rewrite’ statutory text so that it could regulate semiautomatic weapons as machineguns,” Thomas wrote. “We declined to do so. The Government now asks us to rewrite statutory text so that it can regulate weapon-parts kits. This time, the Court obliges. I would not.”

He took particular issue with the ATF’s contention that the mere inclusion of tools or jigs alongside unfinished parts automatically transformed the regulatory status of those parts. He included pictures of two unfinished AR-15 receivers and noted the ATF considered only one of them a firearm under its rule because of what it was sold with.

“Both images in Figure 1 depict unfinished receivers. Both lack machining and indexing in the key areas,” Thomas wrote. “The only difference is the presence of a jig and drill bits. Yet, according to the Rule, these extraneous items make an unfinished receiver a regulable firearm. The presence of items such as jigs or ‘marketing materials’—that are not, and never could be, part of a frame or receiver—has no bearing on whether a piece of metal or plastic constitutes ‘the basic structure and principal component of a firearm.'”

He argued that the most straightforward reading of the GCA would be that anything that can’t function as a frame or receiver isn’t regulated as one. He noted the ATF agreed with that assertion under its previous interpretation of the law, which stood for decades.

Thomas and Alito both also took issue with the standard the majority employed to uphold the ATF rule.

“[A] regulatory definition that is accurate in only a single valid application cannot possibly ‘explain explicitly,’ ‘fix or establish,’ or ‘set forth the meaning’ of a statutory term,” Thomas wrote.

Instead of pointing to IKEA furniture, Thomas pointed to motorcycles.

“Consider a hypothetical statute that defines ‘motorcycle’ as ‘a motor-powered, two-wheeled vehicle with pedals.’ If a regulatory definition copied the same language, and then added that ‘the term shall include any motorized vehicle,’ the regulatory definition obviously would be wrong,” he wrote. “Not every motorized vehicle is a motorcycle, and the fact that some motorized vehicles happen to be motor-powered, two-wheeled vehicles with pedals does not suggest otherwise. It is difficult to see how an overbroad regulatory definition becomes defensible simply because some set of circumstances exists in which the regulatory definition overlaps with the statutory definition.”

Thomas warned that the majority’s “novel ‘artifact noun’ methodology” “invites unforeseeable consequences and offers no limiting principle.” He argued it could even imperil millions of lawful AR-15 owners since their semiautomatic variants share very similar receivers with highly-regulated fully-automatic variants that are unlawful to own unless registered under the National Firearms Act of 1934.

“If an object already is what it may be converted into, then semiautomatic AR–15s would seem to be partially complete, automatic machineguns,” Thomas wrote. “This reasoning exposes the manufacturers, sellers, and owners of AR–15s to criminal liability under the NFA. But, Congress does not ‘hide elephants in mouseholes.’ An interpretive approach that would allow ATF to regulate the most popular semiautomatic rifle in America under a statute addressing automatic machineguns should give us pause.”

Gorsuch dismissed that concern.

“[T]he plaintiffs suggest, [the government] might next attempt to classify the receiver of an AR–15 rifle as a ‘machinegun’ because ‘it is possible to convert’ those receivers’ to function as machinegun receivers.’ That result, the plaintiffs warn, could leave many Americans facing new and unforeseen criminal liability for possession of a ‘machinegun’ simply because they own a ‘popular’ and ‘commonly available’ rifle. The plaintiffs’ fears are misplaced,” he wrote for the majority. “The government represents that AR–15 receivers do not ‘qualify as the receiver of a machinegun.’ Nor, the government emphasizes, has ATF ever ‘suggested otherwise.’ Much the same can be said of our reasoning today. As we have stressed, a statute’s text and context are critical to determining whether (and to what extent) Congress used an artifact noun to reach unfinished objects. And, without doubt, the NFA and the GCA are different statutes passed at different times to address different problems using different language.”

“Our analysis of the GCA thus does not begin to suggest that ATF possesses authority to regulate AR–15 receivers as machineguns under the NFA,” Gorsuch said.

Still, Gorsuch also noted the majority’s opinion was limited to the facts of the case.

“Admittedly, our reasoning here has its limits. Just because some kits, like Polymer80’s, qualify as ‘weapons’ that ‘can readily be converted’ into working firearms does not mean all do,” he wrote. “Think of the problem of the heap: Start with a heap of sand and begin removing grains; at some point, a heap no longer exists. That problem attends many artifact nouns. Even when used to capture unfinished products, artifact nouns generally reach only so far. It would be extravagant to speak of a novel when the author has dashed off only a few lines. Few would call a pile of unfinished logs a table. Subsection (A) may present a similar problem. Weapon parts kits vary widely. Not all come as complete as the ‘Buy Build Shoot’ kit. Some, too, may require more time, expertise, or specialized tools to finish. And at some point a kit may be so incomplete or cumbersome to assemble that it can no longer fairly be described as a ‘weapon’ capable of ‘read[y] . . . conver[sion]’ into a working firearm. While we recognize the problem, this case does not require us to untangle exactly how far subsection (A) reaches.”

He noted the same concept applied to the majority’s determination that the unfinished frames included in Polymer90’s kits were close enough to functional frames to be regulated as such. He left open the possibility that more litigation may be required to feel out the contours of the ATF’s regulatory power over unfinished gun parts.

“Future cases may present other and more difficult questions about ATF’s regulations,” Gorsuch wrote. “But we take cases as they come and today resolve only the question posed to us.”

UPDATE 3-26-2025 8:21 PM EASTERN: This story has been updated with more details from the Supreme Court’s ruling.