The big red STOP button was called the 'Jesus Christ switch'. Our guns certainly fired more that 4 rpm. Are you sure that you don't mean 40? I did an air gunners course at Manorbier in the very early 70's, firing at drone sleeves towed by old Meteor aircraft, hosing them down with GPMG's. The drones I mean but we did try to hit the Meteors.

The biggest 'bangers' for noise were the recoilless types. The Carl Gustav 84mm and the BAT family. Mind you, the 3.5 was a bit of a pu55y-cat