The U.S. Supreme Court will begin hearing oral arguments in the case involving so-called “ghost guns” on Tuesday.

The case, Garland v. VanDerStok, challenges the Department of Justice’s (DOJ) 2022 Final Rule that redefined important legal terms dealing with guns, including “firearm,” “receiver” and “frame,” making the longstanding American tradition of building personal firearms pretty much a thing of the past. Back in April, the Supreme Court voted 4-3 to consider the challenge.

At issue is whether the DOJ and Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) overstepped their bounds in promulgating the Final Rule. Plaintiffs in the case argue that the rule is just another example of the bureaucrat-run agencies ignoring the Administrative Procedures Act (APA) and overstepping their bounds by making laws instead of enforcing them.

The Fifth Circuit Court of Appeals unanimously decided just that last November, upholding an earlier district court decision on the matter. In the ruling, Judge Kurt Engelhardt, who wrote the majority opinion, agreed in no uncertain terms that ATF overstepped its bounds in making the Final Rule.

“ATF, in promulgating its Final Rule, attempted to take on the mantle of Congress to ‘do something’ with respect to gun control,” Judge Engelhardt, a Donald Trump nominee, wrote in the opinion. “But it is not the province of an executive agency to write laws for our nation. That vital duty, for better or for worse, lies solely with the legislature.”

Judge Engelhardt further wrote that the Final Rule “flouts clear statutory text” and “exceeds the legislatively imposed limits” on agency authority.

“Because Congress has neither authorized the expansion of firearm regulation nor permitted the criminalization of previously lawful conduct, the proposed rule constitutes unlawful agency action, in direct contravention of the legislature’s will,” the judge wrote. “Unless and until Congress acts to expand or alter the language of the Gun Control Act, ATF must operate within the statutory text’s existing limits.”

The lawsuit was brought by the Firearms Policy Coalition (FPC) on behalf of itself, two individual FPC members and Tactical Machining LLC. In a brief filed by the FPC in June, the organization spelled out exactly what the court needs to consider.

“The questions presented are: 1. Whether ‘a weapon parts kit that is designed to or may readily be completed, assembled, restored, or otherwise converted to expel a projectile by the action of an explosive,’ is a ‘firearm’ regulated by the Act,” the brief stated. “2. Whether ‘a partially complete, disassembled, or nonfunctional frame or receiver’ that is ‘designed to or may readily be completed, assembled, restored, or otherwise converted to function as a frame or receiver,’ is a ‘frame or receiver’ regulated by the Act.”