The last part is called the “residual clause.” With admirable restraint, Justice Antonin Scalia wrote a decade ago that the clause “is, to put it mildly, not a model of clarity.” In fact, it has tied the federal courts in knots. In the past decade, the Supreme Court has had to settle disputes over whether “violent felony” applies to attempted burglary (no), driving under the influence (no), failure to report for incarceration (no), and intentional flight from law enforcement in a motor vehicle (yes). But confusion persists, with different standards prevailing in different appellate-court jurisdictions. For example, in the Fifth Circuit, reckless assault is “violent,” while in the Sixth, reckless homicide is not. In the Fourth Circuit, battery of a police officer is not “violent,” in the Tenth it is. In the Fifth, Sixth, Seventh, and Tenth Circuits, fleeing law enforcement on foot is “violent”; in the Eighth, Ninth, and Eleventh it is not.

As these crazy results piled up, the Court’s cries for help have grown louder. In 2006, Scalia, dissenting in the attempted burglary case, argued that the Act “violates ... the constitutional prohibition against vague criminal laws.” In 2008, Justice Alito wrote that “only Congress can rescue the federal courts from the mire into which ACCA’s draftsmanship” has thrust it. In 2011, Scalia again urged the Court to admit that ACCA “is a drafting failure and declare it void for vagueness.”

Against this backdrop, Johnson v. United States reached the Court last November. As the head of something called the Aryan Liberation Movement, Johnson boasted to FBI informants that he had, and planned to use, napalm, explosives, an AK-47, 1,100 rounds of ammunition, and silencers. He was convicted of being a “felon in possession” of firearms and ammunition; the district court promoted him to career status because of two previous convictions of robbery and one of possession of a short-barrel (“sawed-off”) shotgun. Before the Court, his federal defender argued that mere “possession” of an illegal weapon was not “purposeful, violent, and aggressive” enough to qualify as a “violent felony.” She asked the Court to add short-barrel possession to the list of felonies that aren’t “violent”; if it did so, she said, “this Court need not get into whether [ACCA] is unconstitutionally vague and the baby should go out with the bath water.”

Two months later, however, the justices, without explanation, threw up their hands. They set the case for reargument on the sole issue of whether ACCA is “unconstitutionally vague.”

Unconstitutional vagueness is a three-legged doctrine. Courts throw out statutes they can’t understand because, first, if a judge can’t understand it then God help the citizen who is trying not to break it; second, if the language is loose then cops and prosecutors can pick and choose cases on arbitrary, and possibly discriminatory, grounds; and, third and probably most important, if courts have to struggle with the language then the legislature has shifted its work—writing the law—to the judiciary, in violation of the separation of powers. The last leg is probably the most important to the Court; the reargument is a sign that, since Congress has done nothing despite repeated urgings, at least four members of the Court are ready to strike down the clause altogether.