FWIW, the German pilots understood quite well that the gunners on the ground would fire with wild abandon at bomber formations, irregardless of whether a stray Luftwaffe plane happened to be mixed in amoung them. Standard practice was for the LW fighters to break off their attacks as the bomber stream entered the flak envelope, and go around to the other side and wait for them to emerge. Heavy ack is sort of like naval mines...once it leaves the barrel, it's an equal opportunity killer. Even over your own airfields you weren't completely safe from your own ack. More than one pilot was fragged by an over eager gunner on the ground.
As for the barrage question, that's a good one. The Germans mixed it up, with some batteries firing simultainious barrages (a "flak box") while others simple fired independently within a battery. I'd like to see a random mix of both.
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Sabre, a.k.a. Rojo
(S-2, The Buccaneers)