The M16 rifle is one of the world’s most famous firearms, iconic as the American weapon of the Vietnam War—and, indeed, as the U.S. military’s standard service rifle until only a few years ago. But the story of the M16 in Vietnam is anything but a success story. In the early years of the war, the U.S. military had a problem: its primary infantry rifle, the M14, couldn’t stand up to the enemy’s AK-47s. The search was on for a replacement that was lighter weight, more durable, and more letha...