There are some interesting Youtube videos about bomber gunnery. One features Burgess Meredith who bagged PAULETTE GODDARD and went on to Batman fame as The Penguin.
One of my best friends and colleagues was a 15th AF B-17 waist gunner. He said the ball turret was the most accurate because it had a lead-computing gunsight before the others. IIRC the .50s were zeroed at 500 yds.