Dubbing her "Heartless Hillary" Clinton, Trump and the NRA’s leaders cast the 2016 election in stark terms as a referendum on the likely Democratic nominee that carries long-lasting implications for gun enthusiasts nationwide. “The Second Amendment is on the ballot in November,” Trump told the more than 7,000 NRA members who paid to attend his speech, roughly 10 percent of the 70,000 activists in town for the group’s annual meeting. “I will never let you down. I will protect our Second Amendment. I will protect our country.” Showcasing his knack for nicknames and speaking in his characteristic vernacular, Trump drew...