High Velocity: Some of the worst massacres in recent memory have had something in common: the AR-15 style rifle. Scott Pelley reports on why the high-velocity rounds used in the gun make it so deadly.The Last Slave Ship: The Clotilda was burned and sunk in an Alabama River after bringing 110 imprisoned people across the Atlantic in 1860. Three years ago, its remains were found. Anderson Cooper reports on the discovery of the wreck and the nearby community with descendants of the enslaved aboard the ship.Simone Biles: Sharyn Alfonsi speaks with the gold medal-winning gymnast about the year-long delay before her final Olympic Games, USA Gymnastics' response to Larry Nassar and why she thinks the training she went through in previous years wasn't right.