I'm guessing an ejector gun with 2 round "caddies" attached in a convenient location on your body. Right thumb flips the opening lever, grab two shells off one of the caddies with your free left hand them slap down the barrels with the same left hand thus opening the gun and ejecting the empties, reposition left hand and shove the 2 new shells in the tubes, close the action using the same left has it gets repositioned to the handguard, shoulder, aim, fire, repeat.
At least that's how I'd try doing it.