The U.S. Supreme Court on Tuesday will hear oral arguments in Garland v. VanDerStok, a case that promises to settle the question of whether weapon parts kits for “ghost guns” require the same regulation as complete firearms bought and sold at gun stores. The case hinges on a key question: Is a bundle of parts that can be readily assembled into a working firearm the same thing as a gun? As nine of the most powerful legal minds prepared to consider whether ghost guns qualify as firearms, Chris...