On the evening of Nov. 30, 2015, Adarius Montrells Sims of Birmingham, Alabama, was driving his gold Grand Am home. Tired after hanging out all night, he fell asleep at the wheel. The car crashed, and medics found Sims unconscious. After the crash, a police officer peering into the wreck spotted a .40 caliber pistol on the passenger-side floorboard. Police ran Sims’s name and saw he had two prior marijuana possession convictions, a record that made that gun illegal in Alabama. But Sims hadn’t hurt anyone and his record was nonviolent. So he was sent to a gun court rehabilitation program,...