Weeks after the shooting in Tucson, sellers at an Arizona gun show allowed undercover investigators hired by New York City to buy semiautomatic pistols even after they said they probably couldn't pass a background check, Mayor Michael Bloomberg said Monday. "After Tucson, you would think that people, particularly at a gun show in Arizona, would have been much more careful in enforcing the law," he said. "That unfortunately in some cases wasn't the case." Bloomberg has authorized similar sting operations around the country as part of a push for tougher federal laws to help keep guns off the streets of...