Why is it so difficult to stop mass shootings in the U.S.?

Following recent mass shootings, NewsHour begins a series called “Guns in America” where we will talk with people intimately involved in the gun control debate. Tonight we speak to author and advocate Mark Kelly, husband of congresswoman Gabrielle Giffords, who was shot in the head in 2011, and Meghan Hoyer, part of the team at USA Today producing “Behind the Bloodshed: The untold story of America’s mass killing.”