Firing an aircraft's fixed forward firing guns at a target at 12 o'clock aren't that complex. Calculate range and drop of shot, forget windage and humidity, and Bob's your uncle. Firing those same guns at a deflection target transiting your sight requires much more calculation, increasing the chance of a pure miss. Firing guns which are at deflection from your line of flight at a deflection target transiting your sight is intensely difficult. It's the stuff that complex algorithms are made of. That's why bombers couldn't defend themselves. Aircraft carry limited ammunition loads and they don't have the liberty of spraying it like a Chicago gangster with a Tommy gun. An exception, one American waist gunner, an ex football player and aerial gunnery instructor, shot down 7 fighters on a single mission, but he had to borrow ammunition from other gunners, who were also cuing him on targets. That's how you get to Carnegie Hall. If they could put his brain into the nose of an F9F2, they'd have something. Technology's a bitch.
Sincerely,
Al Gorithm.