Hi Bob....It would be only maybe under a minute depending on ..Pilot or observer and position in aircraft.......Once the gun is accessed (ie by pulling down on Foster mount or whatever ..for pilot )..To remove old drum..Press in the thumb latch on RH side atop the drum whilest holding the leather handle , and simply lift it upwards off its spiggot.....Store...Take new one from rack( or wherever )..With the afore mentioned thumb latch pressed in ,and holding it as before by the strap , place over the spiggot and allow to drop down fully and release thumb catch.....Job done.....and back to flying the 'plane from whatever attitude it got into when the previous opperation was taking place.......
..Regards..John.