Sugest you read "BOMBER OFFENSIVE "by Sir Arthur Harris, he goes into great detail on the problems of getting the 50call gun into service, during my time as a sprog on various RCAF outfits I was privaliged to listen to acounts from various ex Bomber Comand crews on the subject of the British "pea shotter" 303 guns, including the only Air Gunner "Ace" in the RCAF.