In December 2005, five groups of Wall Street investors flew in private jets to Portland, Maine, where they took waiting limousines to a warren of metal buildings that resembled a midsize lumberyard. They had come to Bushmaster Firearms in pursuit of a highly profitable product whose market was growing faster than any other in America’s stagnant gun industry. The product was the AR-15, and red-hot Bushmaster, the nation’s leading manufacturer of the rifle, had decided to auction itself to the highest bidder. Bushmaster’s owner Dick Dyke had once feared that he could never sell the company because so many people...