You can hit things fine with a flex mounted .50, out to 800 yards even (actually done that). There isn't that much real vibration with a hard mount and controlled bursts (I actually found the M2 somewhat boring to shoot, compared to the M-60). A lot of the shaking you see on film is actually the barrel moving in a locked recoil action, and not a vibration of the gun and the mount.
As far as the effects of the slipstream, cold, heavy clothing and armor and oxygen system, fear, lack of deflection training, etc. these should be issues for the waist guns, but not the turret mounted or glass-enclosed weapons (except for gunnery training/lead issues etc.).
I generally think bombers are too easy to kill, according to the ancedotal evidence. They should, though, be relatively easy to wound, easy to kill gunners, easy to set on fire for an ultimate explosion etc. with the occasional 1-pass kill by an aircraft with the appropriate cannons. The bomber gunners should also be deadly for fighters attacking in a low-deflection/low energy manner. My$.02.
[edit: Firing while manuvering is another matter too. It would be very difficult to track and engage a target in a bumpy enviromnet with a flex-mounted gun. I know that in an M113 traveling over moderately bumpy ground you would be shooting everyting from the sky to the dirt if you tried to engage a target. Hell, half the time you had to concentrate on not getting beat up by the end of the gun

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Charon