One thing in going to airshows and VFW gatherings and speaking to real buff gunners during the war, most agreed that if the germans got in close enough and "the fighters guns lite up like a christmas tree" its too late for you, them bullets and cannon rounds do a number on the crew, one client of mine had 37 missions with the Hells Angels gunning said that alot of planes make it back..but alot of the crew wouldnt'.
He was quick to input that he never refused to crew a ship that the ("short handed")gunners were injured or killed on , but he sure as hell would never volunteer for it either! I asked him at what range he would start firing his 50s, "as soon as you know its a german!, them fighter pilots are just as scared as we were and if they think your not paying attention or dead at the gun they will go for your ship first " another thing he spoke of was that biggest reason they kept the gun barrels moving around is so that the germans knew the gun was manned and not knocked out or out of ammo, "give em reason to think twice !"
His joke was that the "tail gunner is for the pilot...if he bails out too early ! "
If you ever get to ride in or see up close a b17, there isnt much metal between you and the bullets flying thru it.