Early on a Sunday morning in June, a tractor-trailer backed into the rear parking lot of the Navy Arms warehouse north of Martinsburg, W.V. Inside the truck’s shipping container were four huge wooden crates containing a long-forgotten batch of British No. 4 Lee-Enfield rifles with a unique history. Two green, military-style chests, each brimming with plastic-wrapped bolts, accompanied the wooden containers. Inside the warehouse sat cardboard boxes filled with newly made No. 4 rifle magazines, waiting for their recipients to be offloaded. Valmore Forgett, III, president and CEO of Navy Arms, had shepherded these guns from their storage spot in...