consider a marksman with a rifle, to hit, at 600 yards, a man on a horse (man or horse) with a rifle without a telescopic sight would be a fantastic shot. To hit the same man on a galloping horse would be unbelievable. Now put the marksman on a vibrating platform moving in all directions by small amounts and change the man on the horse for an aircraft which relative to the marksman can be doing zero MPH or 600 MPH (200 MPH bomber speed and 400 mph fighter speed in opposite directions) and also it can move in any direction. Additionally in a dorsal or ventral turret the wind speed varies from approximately 200mph head wind, tail wind, side wind and the effect of gravity varies from maximum in a horizontal shot to zero on a vertical shot. The use of a machine gun instead of a rifle is to compensate for inaccuracy, a sniper always uses a single shot rifle.
A man using a computing gun sight in a turret is not having to make all those complicated calculations and compensations himself. The analogy to a marksman using an iron or optical sight is not relevant. This is also why I said above that I'd be amazed if the gunners in a US bomber who were using iron sights ever hit anything.
The use of an automatic weapon rather than a single shot in the context of aerial combat is not to compensate for inaccuracy. It is to try to ensure enough hits to destroy the target. One hit will knock a man of a horse, it almost certainly won't knock an attacking fighter out of the sky.
The British leapt through all sorts of hoops in the 1930s to work out how many rifle calibre machine guns were required in their fighters to ensure enough hits on a bomber (their intended target) to destroy it with a two second burst. It's why the Spitfire and Hurricane carried eight machine guns.
Cheers
Steve