At least 12 of the weapons found in the hotel suite used by the gunman who killed 58 people in Las Vegas on Sunday had been modified. He used a device called a bump stock, which enables a semiautomatic rifle to fire faster, and can be purchased online. Guest: C. J. Chivers, a New York Times investigative reporter and Marine veteran. For more information on today’s episode, visit nytimes.com/thedaily.