Sometimes whether the Supreme Court decides a case is as important as what it decides. A case in point is the right to keep and bear arms. The justices decided that the Second Amendment protects an individual right in District of Columbia v. Heller (2008) and that states as well as the federal government may not infringe it in McDonald v. Chicago (2010). But it hasn’t agreed to hear a single case since to define the scope of the right.

The court is supposed to give priority to “circuit splits” among the lower courts, along...