Bump stocks, machine guns, and the rule of lenity

The U.S. Supreme Court recently agreed to hear the case of Garland v. Cargill, addressing whether a "bump stock" qualifies as a machine gun and is therefore banned by federal law. Gun rights and gun control advocates will surely use the case to argue about the legality and wisdom of gun restrictions, but Cargill presents a more important underlying question: how should courts treat a federal agency's interpretation of an unclear or ambiguous law? The answer is simple: if Congress does not clearly make something a crime, then a federal agency can't do so under the guise of "interpreting" the statute.

A bump stock device can be attached to a semi-automatic rifle, enabling the user to fire more rapidly by harnessing the rifle's recoil energy. Rather than pulling the trigger for each shot, a shooter can simply keep constant pressure with his trigger finger while continuously pushing forward with his non-trigger hand, and the bump stock does the rest. An experienced shooter can use a bump stock to fire semi-automatic weapons at rates approaching those of automatic weapons. But because it concluded that a bump stock does not "automatically" fire multiple rounds with a single trigger pull, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) has consistently ruled that bump stocks are not machine guns.

That changed in 2017. A horrific mass shooting in Las Vegas resulted in the murder of 58 concertgoers and left hundreds more wounded. Investigators found that the shooter had used more than a dozen semi-automatic rifles outfitted with high-capacity magazines and bump stocks, enabling him to fire hundreds of bullets in just a few minutes. Various bills were soon introduced in Congress that would have banned bump stocks. But before the bills could be fully considered, President Trump directed ATF to reevaluate its position.

ATF issued a new nationwide rule concluding that bump stocks fall under the National Firearms Act definition of a machine gun, which is any weapon that can "automatically" shoot multiple rounds with a "single function of the trigger." ATF concluded that "single function" includes not only a "single pull" of the trigger but also anything "analogous" to a single pull. Contrary to its earlier rulings, ATF also concluded that a gun fires multiple rounds "automatically" if it does so "as the result of a self-acting or self-regulating mechanism," such as a bump stock. ATF's new rule required owners of bump stocks to destroy them, turn them over to ATF, or face criminal penalties.

Some bump-stock owners sued, and lower court rulings - including the Fifth Circuit decision in Cargill - have been all over the map. Some judges agreed with ATF that bump stocks are machine guns; some found the statute to be ambiguous; and some concluded that bump stocks are not machine guns. The Supreme Court granted review to resolve the confusion.

The government argues bump stocks are covered by the statute because "single function" is the same as "single pull," and once a shooter using a bump stock starts pulling, he doesn't have to release the trigger between shots - the bump stock keeps the gun firing repeatedly. It also points to Congress' larger goal of prohibiting more dangerous weapons because they can fire many rounds quickly. In contrast, the bump stock owner argues that bump stocks do not "automatically" allow multiple shots with a "single function of the trigger" because the trigger resets for each shot, and the shooter has to do more than pull the trigger once to fire multiple shots - he has to push forward on the barrel while simultaneously keeping a constant pressure with his trigger finger.

In deciding between these competing views, the Justices will rely on standard interpretive tools. They will review how dictionaries define the terms "single," "function," and "automatically." They will look at how those terms are used in other statutes and in common usage. Some Justices may consider Congress' purpose and whether banning bump stocks is consistent with that purpose. Some may consider what individual members of Congress said about the scope of the statute when they voted on it in 1986; other Justices view a resort to such "legislative history" to be improper. And the Court may apply other interpretive tools, all with the goal of deciding whether bump stocks fit within the statute's definition.

The Court may conclude that the statute is sufficiently clear to pick one interpretation over the other. But what if - even after using their interpretive tools - the Justices still think that the statute is ambiguous? My organization, Pacific Legal Foundation, filed an amicus brief in the Fifth Circuit addressing that very question. Our brief focused on the "rule of lenity," a well-established doctrine that requires resolving ambiguity about the scope of statutes carrying criminal penalties in favor of those facing the penalties. Here, if the National Firearms Act is ambiguous about whether bump stocks are machine guns, ATF's interpretation must be rejected.

There are two main reasons for the rule of lenity. First, Congress must provide fair warning about the scope of criminal statutes as a matter of fundamental due process. Ordinary readers should be able to understand whether the law prohibits an action they are considering. Second, the principle of separation of powers requires that the politically accountable members of Congress and the President be the ones to decide what behavior is prohibited. Unelected bureaucrats (like those at ATF) should not be allowed to determine whether something is a crime.

After considering the parties' arguments, if the Court concludes that the Act's definition of a machine gun is ambiguous or unclear, it must decide the case in favor of bump stock owners. Whether to criminalize bump stocks is a decision for Congress to make, and it must do so clearly enough for ordinary citizens to understand.