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The top turret or ball turret could cover the same area, but they'd probably be looking for fighters closer to the center of their coverage area, and a fighter slipping in from the side might not be seen in time.
A B-17 box under attack is spraying thousands of poorly aimed .50cal rounds per minute.
Why would they be necessarily be "poorly aimed"?
The gunners trained in the 345th Bomb Group were expected to break two clay targets with one shot as their paths intersected, thrown from opposite skeet houses.
Worth remembering, too, that one-on-one was fairly rare, so the fighter pilot could find himself the target of half a dozen rear turrets from a properly set up box formation. It's generally said that this was the main reason for the Luftwaffe switching to head-on attacks.
When the box formations were properly kept. Every aircraft had a clear field of fire directly above, below, behind, and ahead. So every turret gunner was pretty well clear to fire almost any fighter they could see. The waist gunners on some of the aircraft would have pretty restricted fields of fire, but since they were the less effective position anyway, so what.
I don't think many gunners, even under combat stress is going to continue firing at a target when they see that fire is going to hit one of the other aircraft in the formation. When you consider they've been in this formation for hours, and that formation mate has been in the same ralitive position the whole time.
Of course every formation wasn't kept perfect, aircraft did get out of position, and gunners did make mistakes.
Were they on aircraft flying at 20,000ft+ at 200mph when they did this?
Its worth remembering that AA gunners on ships often hit their own ships, let alone other ships that were in the area.
Me-410 bomber crew were protected by about 400kg of armor. I think the Ju-88 was similiar. That might stop more then just flak shards.
Were they on aircraft flying at 20,000ft+ at 200mph when they did this?
In June of 44 the LW tactic was to close in with Allied 4 engine bombers from the rear hoping just to take on the tail gunners position, this remained so till the last engagements during March of 45 with piston engine fighters, jets continued the rear mount as well till wars end. front attacks were eliminated almost altogether preferring the striking power en-masse from the rear and then when the first 1-2 attacks had occurred LW fighters then could come from any angle to attack - i.e. T/E 110G's and the Me 410's.