I've read about the gunner who loaded 100% incendiary rounds, even loading his own belts to save the armourers the trouble. Some Canadian squadrons used 100% tracer, the extra barrel wear being considered worthwhile as the hail of fire from an alert gunner was enough to make most fighters break off. Especially when they were sneaking up beneath the bomber to use their upward-firing cannon, which caused most bomber casualties after their introduction in 1943. ~Again the Canadians of 5 Group recognised the danger but there was no ventral turret, just a 50 calibre machine gun on flexy mounting like the American side guns.
Can't remember the reference, but following statistical research it was proposed to delete the mid-upper turret which was little used and gain an extra 50 mph which would make life much more difficult for overtaking fighters. This research also led to the introduction of the bomber stream in which defences were swamped by hundreds of aircraft bombing in only 20/30 minutes. The furious crews refused to give up their mid-uppers and maybe they were right, for the one ton weight gain would swiftly have been negated by the order to carry an extra ton of bombs instead.
However, many a German airman could testify as to the effectiveness of the rear gunner, especially if he was trying to creep underneath his target. Even quadruple .303s were devastating at close range from above when all he had was 8mm Perspex above his head, and upper surfaces of fuel tanks had nothing to protect them. Ultimate version was the Village Inn gun-laying radar, google it for several accounts of how well it worked -- indeed so well that it wasn't used because so many friendly aircraft were close by in the bomber stream.