For many Americans, the tragic killing of 58 Las Vegas concert-goers in October 2017 made all too vivid the dangers posed by bump stocks, attachments that convert semiautomatic firearms into automatic weapons. Although the Las Vegas shooter’s motive remains unclear, there’s no doubt that these devices in the hands of someone seeking to cause harm—whether based on personal grievances or domestic or international extremism—present a grave threat to security here in the homeland. U.S. terrorism laws rightly cover crimes committed using weapons of mass destruction, such as bombs and nuclear devices, but automatic weapons, better known as machine guns, can be just as lethal.

In the wake of the Las Vegas attack, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) began moving down a path of protecting the public from this threat, a path that, in December 2018, yielded a new regulation prohibiting the sale and possession of bump stocks. Before that regulation takes effect on March 26, gun rights groups have filed several suits against the federal government, asking judges to block the prohibition. The first court hearings took place earlier this month. Because the new regulation is based on a reasonable interpretation of long-standing law, the courts should reject these challenges.

The key text at issue in these lawsuits comes from a law passed by Congress in 1934. Responding to the dangers increasingly evident from particularly powerful weapons, Congress imposed extensive restrictions on machine guns. And Congress identified a machine gun as a weapon that “shoots ... automatically more than one shot, without manual reloading, by a single function of the trigger.” Congress later expanded that definition to cover a part or parts “designed and intended ... for use in converting a weapon into a machine gun.”

Here’s the central statutory question now at issue: Does a bump stock, as a mechanism attached to a semiautomatic rifle, convert that rifle into something described by those words? If so, the text of the statute brings bump stocks under the prohibition set out in the 1934 law—and ATF appropriately should ensure they are prohibited by regulation.

The answer is as clear as it seems: yes. As ATF explained in a Dec. 26’s Federal Register announcement of the new regulation, a bump stock makes a firearm into a weapon that fires multiple shots automatically—by a single trigger pull. That’s what Congress described in its 1934 statutory language.

But gun rights challengers lean heavily on the fact that ATF previously determined that bump stocks do not convert rifles into machine guns. The plaintiffs essentially argue that ATF cannot change its legal interpretation of the underlying statute—and that, now that ATF has changed its interpretation, courts must change it back.

That’s a mistaken view of the law. Courts have grappled with various instances in which administrative agencies like ATF have altered their views, and one overriding principle emerges from that case law: An agency can, in the right circumstances, modify its position. That’s especially true when—as here—the previous view was poorly considered, not just in its result but also in its process.

When ATF reached its earlier conclusions, it showed little serious engagement with the key legal and factual questions of whether a bump stock transforms a rifle into a machine gun according to the 1934 statutory language. As ATF has now explained, those earlier determinations did not analyze the meaning of the term “automatically.” Instead, ATF appeared to assume that bump stocks do not cause a semiautomatic rifle to fire “automatically” because bump stocks do not include automatically functioning parts or springs that bring about rapid fire, but instead rely on constant pressure from the shooter on the trigger. When ATF reviewed its earlier determinations and examined more closely how bump stocks work, it concluded that, by harnessing the recoil energy of the firearm discharge through the bump stock, a single pull of the trigger allows the trigger to fire continuously. A “single pull” of the trigger, ATF now correctly reasons, is equivalent to what the law expresses as “a single function of the trigger.” The result, of course, is automatic fire.

What an agency generally cannot do is change its position on a whim—but ATF has not done so here. When a previous decision was poorly reasoned, when a new decision is in good faith and well explained, and when world events underscore the need for a fresh look, an agency is entitled to—indeed, should be urged to—update its view. That’s a point the Supreme Court has made clear in cases like 2005’s Brand X decision, affirming that agency interpretations of governing statutes are not “carved in stone” and that an agency should consider “varying interpretations and the wisdom of its policy on a continuing basis.” Indeed, ATF reconsidered a similar failure to regulate machine guns in 2006—and a federal appeals court correctly affirmed the agency’s decision three years later.

Even assuming the courts allow ATF to correct its earlier error, that will not solve all of the public safety and national security problems in this area. ATF’s new rule does not prohibit a host of other devices such as binary triggers, competition triggers and hellfire triggers that, like bump stocks, make it easier to rapidly fire multiple rounds of ammunition. The injury and death these devices are capable of inflicting rivals that of weapons of mass destruction that few would quarrel with prohibiting. But this step is important progress: For ATF to correct this interpretive error and prohibit bump stocks honors the text of a law written almost a century ago and takes the most obviously sensible step to prevent future tragedies like the shooting in Las Vegas. The correction is something to be praised, not something to be blocked by the courts, which should reject the challenges now before them.