You are shooting with 2 barrels only, which happens if you don't 'fire all' but use primary or secondary fire only. The selected guns run out of ammo eventually, leaving the remaining two with their full ammo complement (=1600 rounds)
In reality, the Wirbel gunner had two foot pedals to fire each set of two guns either seperately for continous fire (giving the crew the chance to reload the other set meanwhile) or combined for maximum short time rate of fire.
