The rush of guests entering the White House correspondents’ dinner was over, and now Helen Mabus, a volunteer ticket checker, had a moment to herself. “It was very quiet,” she recalled. But just then a man out in the corridor caught her attention: He was holding what she quickly realized was a rifle. Before she could react, he tore off toward the security checkpoint about 40 feet away. “He either unfolded a part of the gun or pieced it together,” Mabus said. “It became longer before my eyes.