Two decades before a secretive gambler lugged an arsenal that included 12 "bump stocks" into a Las Vegas hotel room and opened fire on a crowd of concertgoers, a 40-something ex-Marine was tinkering in his Florida garage, looking for a fresh take on an old idea. Inspired by footage of the heavy recoil of anti-aircraft guns, Bill Akins wondered if he could design a device to harness a semi-automatic rifle's recoil to fire bullets at a frequency near that of an automatic weapon. For decades, avid gun owners had been bracing guns against their hips to increase the rate of their trigger pulls and enjoy the thrill of shooting something akin to a machine gun, a technique known as "bump firing.