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Colorized photo of ground crew loading .50 cal ammunition into a P-47 Thunderbolt. 1944.
(lemmy.world)
Ground crew members carefully load .50 caliber M2 ammunition into the wings of a P-47 Thunderbolt, the legendary U.S. fighter of World War II. Each of its eight wing-mounted guns could hold up to 425 rounds, making the “Jug” a fearsome force in air-to-air combat and ground attack missions, built for durability and intense aerial battles in 1944.
In these single engine WW2 era fighters, generally between the engine and the cockpit. So, basically right up against the pilot's knees and above their feet.
The Spitfire specifically had two tanks, a smaller lower one to make room for the rudder pedals.