This picture gives a good idea about the capacity of the Spitfire cannon armament. It is the Spitfire Mk. Vc (C-wing) cannon bay showing the clockwork drum containing 60 rounds, enough to feed the Cannon for 30 seconds. The .303 Machine Guns with 170 RPG were fired by the air compressor and also run out in 30 seconds on a MK Vc.
In aerial combat the average squirt the pilot could afford was 2 to 3 seconds.
The empty space to the left is for a second cannon bay orignially intended to house a second 20mm cannon for low level sweeps against trains, trucks, troops etc. As it turned out, the dded weight of four cannon armament made the Spitfire sluggish in performance and was rarely used operationally. In the event, the second bay found its use in the “E” type wing, where the space was utilized for a 12.7 millimetre American Browning machine gun.
[photo courtesy of Langdon Badger, via Spitfire Society]