Complex sentence: Ammunition for the tail turret was 2,500 rounds-per-gun, due to the weight the ammunition being stored in tanks situated near the mid-upper turret's position and fed rearward in runways down the back of the fuselage to the turret.
Split into simpler sentences: Ammunition for the tail turret was 2,500 rounds-per-gun. Due to the weight, the ammunition was stored in tanks situated near the mid-upper turret's position and fed rearward in runways down the back of the fuselage to the turret.