Rogelio Barajas got off his flight from central Mexico to O’Hare with empty luggage that smelled of gunpowder.It smelled that way because Barajas had returned from one of his regular journeys transporting guns: More than 100 in all, smuggled from Illinois to Mexico over the course of four years. His scheme eventually drew the attention of federal investigators in September 2008, when Mexican police recovered a rifle in an abandoned car. Using the weapon’s serial number, the Bureau of Alcohol, Tobacco, Firearms and Explosives linked traced it back to Barajas, who had . . .