FRANKLIN, Tenn. — Devon Dixon believes guns serve a purpose. That is why she is licensed to carry and conceal firearms. She is also a mother living in the Nashville suburbs with three school-age children. She worries about their safety, especially after three 9-year-olds were among the six killed in a school shooting in the city last year. “It’s pretty heavy on my heart,” she said. But those concerns weren’t enough to convince Dixon that Tennessee lawmakers were right to pass a bill Tuesday that