Ethan Thornton’s vision for Mach Industries wooed blue-chip investors Sequoia and Bedrock, but he has struggled to execute, hamstrung by technical challenges, safety hazards and a cavalier approach to leadership. By David Jeans, Forbes Staff At19 years old, Ethan Thornton had a grand vision for a new company: He’d do away with the U.S. military’s centuries-long reliance on gunpowder munitions by...