Eugene Stoner was an unassuming family man in postwar America. He wore glasses and had a fondness for bow ties. His figure was slightly round; his colleagues called him a teddy bear. He refused to swear or spank his children. “Boy, that frosts me,” he’d say when he was upset. He liked to tweak self-important people with a dry sense of humor. He hated attention. A lifelong tinkerer and a Marine veteran, he was also fascinated by the question of how to make guns shoot better. When an idea came to him, he scribbled it down on anything he could find—a pad of paper, a napkin, the tablecloth at a restaurant. He had no formal training in engineering or in firearms design. Yet it was inside Stoner’s detached garage in Los Angeles, during the 1950s, that the amateur gunsmith, surrounded by piles of sketches and prototypes, came up with the idea for a rifle that would change American history. Today, this weapon is the most popular rifle in America—and the most hated. The AR-15 is …