We all may know what makes a gun dangerous, but what makes it both dangerous and unusual?

Largely lost amid the Supreme Court’s decisions on birthright citizenship, asylum and campaign finance was its announcement last Tuesday that it would take up that enigmatic question, which is at the heart of the country’s struggle to balance the rights of gun owners against the specter of mass shootings.

The court’s decision could be a watershed moment. The conservative majority may double down on a string of rulings that greatly expanded Second Amendment protections, or the justices could reconsider legal tests they created to weigh the constitutionality of firearms — like the “dangerous and unusual” question — that have confounded lower courts.

The stakes are clear from the opening pages of briefs by the parties in two cases that the court said it will hear in the fall. Lawyers for Cook County, Ill., whose ban on assault weapons is being challenged by pro-gun groups, listed 25 mass shootings to show that AR-15-type rifles are unquestionably dangerous.

“Assault rifles are the weapon of choice for criminals and terrorists set on quickly massacring innocents,” they wrote, “but are rarely put to lawful public use.”

But are they unusual? Gun rights plaintiffs in the other case, seeking to overturn a similar ban in Connecticut, argued that the AR-15 is so popular it has been called America’s rifle: “If the Second Amendment does not protect it,” the plaintiffs wrote, “then it is unclear what that amendment does protect.”

The court’s ruling is likely to come down by next summer — 20 years after it agreed to take up District of Columbia v. Heller. Its landmark opinion in that case, issued a year later, declared that the Constitution safeguards an individual right to keep arms at home for self-defense. In 2022, in New York State Rifle & Pistol Association v. Bruen, the court went further, extending the right to also include carrying firearms in public.

The two new cases present the Supreme Court with its clearest opportunity yet to explain what kinds of guns are covered by those earlier rulings. And because the weapons involved are AR-15-type rifles, which engender strong emotions on both sides of the American gun debate, the fallout could be profound.

“This will, without question, be the most important case since Heller and Bruen,” said Luis Valdes, a national spokesman for the group Gun Owners of America.

The two earlier rulings created formulas for lower courts to determine if laws restricting possession and use of certain types of guns pass constitutional muster, but judges have repeatedly expressed confusion over them. A patchwork of rulings has sometimes upheld bans on semiautomatic rifles and large-capacity magazines, but other times not.

Courts must first decide if a weapon is both dangerous and unusual. Since any gun is dangerous, the emphasis has been on finding if it meets the Heller ruling’s metric of being in “common use.” If not, the firearm is deemed unusual and falls outside Second Amendment protections.

But if the gun is found to be in common use, it is presumed to be protected. Any modern restrictions on it must then pass the “history and tradition” test, created in the Bruen case, to determine if there is a record of analogous laws in the past.

Lower courts have had questions: How many of a certain kind of gun does it take to be considered common? What is the best way to measure common use? Is this really the right tool for striking a balance between protecting Second Amendment rights and curbing gun violence?

Federal appeals judges in Illinois, hearing a challenge to the state’s assault weapons ban in 2023, complained of a circularity to the test’s logic. Fully automatic machine guns, for example, are not protected by the Second Amendment only because early federal laws prevented them from coming into common use. But had those laws not been enacted, the guns would likely have become popular and therefore enjoy protection, they said.

“Constitutionality kind of winks on and off depending on how popular a weapon is at any given time,” Judge Diane Wood said during the hearing. “And that’s an unusual position to take.”

The appellate court rejected the test and upheld the state ban, declaring that “even the most important personal freedoms have their limits.”

The need to pass the common-use test has created a demand for surveys and polls that try to discern how popular different firearms and accessories are, sometimes leading to disputes over their validity.

A professor who prepared a frequently cited study purporting to show that AR-15-type rifles are commonly used for self-defense had previously unreported financial ties to the gun lobby, The New York Times reported in 2024. Other firearms researchers questioned the integrity and accuracy of the study, which was criticized by a federal judge for having “methodological errors and inherent biases.”

Undeterred, pro-gun plaintiffs in the new Supreme Court cases introduced the same study as evidence.

Beyond measuring usage, some legal scholars say the Supreme Court should revisit the foundational basis for the common-use test. The term first appeared in a narrow 1939 opinion that declared sawed-off shotguns were not protected by the Second Amendment because they would not be useful to a citizens’ militia. Writing for the majority in Heller, Justice Antonin Scalia seized upon that earlier reference, but offered few details about how or why it should be applied more broadly today.

“The question of common use, based on a random line from that earlier ruling, has no meaningful connection to the history or text or values of the Second Amendment,” said Joseph Blocher, co-founder of the Center for Firearms Law at Duke University. “It’s a really weird way to do constitutional law.”

Still, there is little to indicate that the Supreme Court’s conservative majority would be inclined to declare that semiautomatic rifles fall outside constitutional protections. Four justices — Samuel A. Alito Jr., Neil M. Gorsuch, Brett M. Kavanaugh and Clarence Thomas — have a solid record of supporting gun rights. Justice Amy Coney Barrett and Chief Justice John G. Roberts Jr. have done so less frequently.

Justice Kavanaugh has already signaled his thinking. When the justices turned away a case last year challenging Maryland’s assault weapons ban, his statement made clear that did not mean that the court agreed with the ban.

“Given that millions of Americans own AR-15s and that a significant majority of the States allow possession of those rifles, petitioners have a strong argument that AR-15s are in ‘common use’ by law-abiding citizens and therefore are protected by the Second Amendment,” Justice Kavanaugh wrote.