By most measures, the Sten was a forgettable product. This British submachine gun, thrown together for World War II, was wildly inaccurate and so unreliable that the only thing troops counted on from it was a jam at the worst possible moment. Yet the Sten was one of the true hit products of the mid-20th century, and with a run of some 4.5 million, it became one of the all-time bestsellers in the world of weaponry. By contrast, the U.S.-made Thompson M1 machine gun and its variants, used by American troops in World War II and a more accurate and reliable weapon, saw a run of only 1.7 million in its half-century lifetime. How could a gun as lousy as the Sten sell so well--and even outsell the M1? One big reason: It was easy to copy. The Sten's blueprint was widely circulated, and the gun was purposely designed to be easily manufacturable by anyone with modest metalworking skills and tools, making it the darling of resource-strapped Allied units.