The first Spitfires had the 'A' wing, with eight Browning  machine guns firing 1,200 .303" bullets a minute. Each gun had three hundred and fifty rounds, so the Spitfire could fire for about seventeen or eighteen seconds.

<i>The .303 was incapable of penetrating armour and did too little damage to be very effective against bombers (a high level of incendiary or 'tracer' mix was more effective, especially against self-sealing fuel tanks, against which the standard .303 ammunition was totally useless)</i>

The 'B' wing (Mk V) replaced four of the machine guns with two Hispano 20mm cannon, firing at about 600rpm (but throwing explosive shells, so much more lethal). Originally the cannon were fed from 60-round drums giving six seconds of firing time, but this was quickly changed to a 120-round belt feed - so you had ~12 seconds total with everything** firing, with a few seconds more of four machine guns alone. This is the configuration with which the RAF ended the Battle of Britain (and continued to use until the very end of 1942, by which time the FW190's had been knocking them out of the sky for about 6 months)

<i>**The 20mm cannon and .303 machine gun had totally different ballistics, so firing 'everything' together (at a moving target) was always just a waste of ammunition</i>

The 'C' wing had four cannon with 120rpg so twelve seconds of firing, but the weight of this installation meant it was rarely used until the very last Spitfires (the 22 and 24) where they also got a little more ammunition.

Finally, the 'E' wing had two cannon and two M2 Browning .50" machine guns with 250 rounds each: this gave them twelve seconds of everything firing, plus the same again with just the machine guns. This was a good installation (the 20mm and .50" had reasonably close ballistics and the .50" was still lethal against tougher targets, while the .303" had struggled with armour and self-sealing tanks) and was common on the Griffon-engined Spitfires.

<i>NOTE. The .50" machine gun was the 'standard' American mg (which, no doubt, is why the British refused to use it until the very end of the war).</i>