The M61A2 20mm is just the latest generation of the Civil War-era “Gatling Gun,” developed by inventor John Gatling, who wasn’t looking for a more effective killing machine.

It’s not quite the GAU-8 Avenger 30mm Gatling gun mounted on the A-10, but it can still go “Brrrt.” It’s been mounted on air defense systems and in AC-130 gunships . And, in this case, in ...