ROSEBURG, Ore. — The 26-year-old gunman who opened fire on fellow students in his community college English class, killing nine people, was an Army boot camp dropout who studied mass shooters before becoming one himself. A day after the rampage in this Oregon timber town, authorities said Christopher Sean Harper-Mercer wore a flak jacket and brought at least six guns and five ammunition magazines to the school. Investigators found another seven guns at the apartment he shared with his mother.