Fifty-eight people were killed in the Las Vegas massacre last week, the deadliest mass shooting in modern US history. Seventeen guns were found in gunman Stephen Paddock's hotel room, including a dozen that had been modified with a "bump stock," an added part that allowed the guns to fire more rapidly. And up until today, anyone with access to YouTube could watch a tutorial on how to modify their guns in the exact same way. Now those videos are not allowed.