Nearly all turrets, I think I am right in saying, were self contained units and the cans were stored within the confines of the turret. Some turrets had other cans stored in the fuselage alongside the turrets. As for the ball turrets - I ain't got a clue!
I HAVE NOW!
Just gave Google a whirl. Here is a link that will tell you EVERYTHING you want to know.
http://www.liberatorcrew.com/AircraftGunnery.htm
Have fun.
PPP