The AR-15 rifle used in the shooting that left at least 17 people dead at a high school in Parkland, Fla., was purchased legally, according to a federal law enforcement official. How did a semiautomatic weapon originally designed for warfare become easier to buy than a handgun? Guests: C. J. Chivers, a New York Times investigative reporter and Marine Corps veteran; Richard A. Oppel Jr., a Times reporter specializing in coverage of domestic terrorism and the military. For more information on today’s episode, visit nytimes.com/thedaily.