By Noah Feldman / Bloomberg Opinion

As the endless parade of mass shootings continues, you might find yourself wondering: Just how far does the Second Amendment extend? If we each have the right to bear arms, is there a constitutional right to a military-style semiautomatic rifle like an AR-15? What about a rocket-propelled grenade launcher? A small tank?

Under current law, the Second Amendment extends only to weapons that are not “unusual” and are “in common use” by law-abiding citizens. Whether that includes AR-15s is a question the Supreme Court has not yet resolved, although the justices have recently been asked to weigh in. A key question today — though not when the Bill of Rights was ratified — is whether a weapon is ordinarily used for self-defense. That could be what allows legislators to limit the sale of so-called assault rifles, as they did in the 1990s.

The background to the current doctrine is the first modern treatment of the issue by the Supreme Court, a 1939 decision called U.S. v. Miller. At issue was whether Congress had the legal authority to make it a crime to bring a short-barreled shotgun across state lines.

To give you a sense of how different things were with respect to gun issues 84 years ago, the court held unanimously that the Second Amendment didn’t protect the weapon. The opinion was written by Justice James McReynolds, a libertarian whom liberals hated so much that they called him one of the four horsemen of the apocalypse. In other words, he was the person on the court who would’ve had the most reason to extend the reach of the Second Amendment. And yet he considered it obvious that the right shouldn’t extend that far.

In the opinion, McReynolds went right back to the words of the Second Amendment which says: “A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed.”

McReynolds offered a straightforward explanation of the “obvious purpose” of the amendment: “to assure the continuation and render possible the effectiveness of such forces,” namely the militia. Consequently, the Second Amendment “must be interpreted and applied with that end in view.”

As McReynolds accurately noted, the founders, inspired by classical republicanism, distrusted standing armies, which they thought would be tempted to take over the government like the Roman armies of ancient times. They preferred “well regulated” state militias.

Those militias, in turn, were made up of ordinary citizens. And the ordinary citizens in the state militia were expected to provide their own weapons. McReynolds even cited a Massachusetts law that went all the way back to 1649 and specified what sorts of weapons militia members were obligated to own and maintain. Musketeers were expected to keep muskets, bandoliers, bullets and powder. Then there were “pikemen,” who had to maintain a “pike [a kind of spear], corselet, head-piece, sword, and knapsack.”

From this it followed, for McReynolds, that the Second Amendment protected only weapons that had “some reasonable relationship to the preservation or efficiency of a well regulated militia.” That did not include the short-barreled shotgun, he added. Such a weapon was not “any part of the ordinary military equipment” nor would its use “contribute to the common defense.”

The virtue of the Supreme Court’s 1939 test is that it was at least true to the original meaning of the Second Amendment. Its practical disadvantage, of course, is that if updated to the present, the rule would protect military-style weapons; not only semiautomatic rifles but machine guns, RPG launchers, predator drones and the like.

Modern gun rights jurisprudence began in 2008, when Justice Antonin Scalia wrote a Supreme Court opinion called District of Columbia v. Heller. That opinion featured the astonishing act (astonishing for an originalist, at least) of reinterpreting the original meaning of the Second Amendment. This took some jurisprudential jujitsu. Scalia discounted the introductory clause that explains the purpose of amendment as ensuring a well-regulated militia. He shifted the meaning of the right to bear arms to personal self-defense.

Along the way, Scalia made up a new limitation for the newly created right. He represented the Miller precedent as saying the amendment was restricted to “the sorts of weapons … in common use at the time.” To this he added that there existed a separate English common law tradition of prohibiting people from carrying “dangerous and unusual weapons.” The court doubled down in 2022 in a landmark decision striking down “localized restrictions” on carrying concealed guns, with Justice Thomas citing an “enduring American tradition permitting public carry.”

Effectively, the court remade the principle of the 1939 Miller case into an ad hoc rule. Today, the Second Amendment applies to all weapons that ordinary people carry on an ordinary basis for self-defense.

The one good thing — maybe the only good thing — that can be said about the modern rule is that it excludes grenades and tanks. Logically, it should also exclude AR-15s, which are not commonly carried for self-defense.

As with so many other issues, the decisive vote may belong to Justice Brett Kavanaugh. In 2011, when he was a judge on the U.S. Court of Appeals for the D.C. Circuit, Kavanaugh wrote a dissent arguing that semiautomatic rifles were constitutionally protected, just like semiautomatic handguns. That was at an earlier stage in the history of mass shootings, though, and perhaps his views have evolved.

Here’s hoping that, when the Supreme Court ultimately addresses the issue, the justices can recognize that that AR-15s do not belong in the same category as handguns. They are weapons of war — for, in fact, a well-regulated militia — not for ordinary people to carry for ordinary use.

Noah Feldman is a Bloomberg Opinion columnist. A professor of law at Harvard University, he is author, most recently, of “The Broken Constitution: Lincoln, Slavery and the Refounding of America.”