Guns, drugs and cars contribute substantially to the life-expectancy gap between the United States and other developed nations, a study found.Deaths from old-age ailments sometimes get more attention in longevity research, but deaths from these three causes tend to happen at younger ages, contributing to many decades of life lost, the researchers said.Data from the Centers for Disease Control and Prevention show injuries including violence, car crashes and drug poisonings and overdoses are the leading cause of deaths for …