In April 1938, an Oklahoma bank robber was arrested for carrying an unregistered sawed-off shotgun across state lines. The robber, Jack Miller, put forward a novel defense: that a law banning him from carrying that gun violated his Second Amendment rights.For most of U.S. history, the Second Amendment was one of the sleepier ones. It rarely showed up in court, and was almost never used to challenge laws. Jack Miller's case changed that. And it set off a chain of events that would fundamentally change how U.S. law deals with guns.