Seventeen years ago, Stevie Moore’s son was shot and killed with an assault rifle. On Saturday, Moore stood up in front of a crowd estimated at 420 people, holding a picture of Prentice Moore, his son. “This is my child,” he told the crowd filling the parking lot between Shelby County Juvenile Court and the Forensic Center. “This is not a number.”The crowd had gathered for a two-mile “Unity Walk Against Gun Violence.”