Bomber gunner effectiveness must be measured by MUCH more than shoot-down tally. The presence of gunners drastically reduced the attack opportunities available to enemy fighters.
-- Early in the air war, the Germans analyzed the sweeps of B-17 machine guns using shot-down bombers. They concluded that a stern or side approach, while maximizing attack shooting TIME, also caused maximum fighter EXPOSURE to defensive fire. They determined the bomber was weakest from the classic "12 o'clock high" vector; only the top turret could bear (on the B-17 E and early F models). This became the prescribed attack method. It had the WEAKNESS of reducing the actual time a fighter could attack; after a pass, the fighter had to make the long circle around the moving bomber formation to make another pass. Thus, waist, tail, and belly gunners DID contribute to bomber survival, even if they were at a poor angle for good shots.
-- Bomber crews, of course, realized what was happening. Even before the chin-turret G model came along, the crews/mechanics were stuffing forward firing 50s into the nose compartment. A training issue arose: the navigators & bombardiers (both officers) in the nose positions had never taken the formal gunnery training course (for enlisted-rank gunners), so the LEAST proficient shooters were in the MOST important gunnery positions! There was LOTS of rapid remedial gunnery training at bases in England.