"But this once-easy relationship between city and gunmaker has been rattled by the discovery that the firearm used to kill 17 people at a Parkland, Fla., high school last month was made here. The gun was a Smith & Wesson M&P15, a version of the controversial AR-15 military-style rifle. And that weapon had been used in mass shootings before, including in Aurora, Colo., and San Bernardino, Calif."