Squadron Leader Ralph Sorley is widely unappreciated for his contribution to the Spitfire’s armament. The original specification listed four .303-caliber guns, which, considering the nature of combat, would have been woefully inadequate. But Sorley, who had flown with the Royal Navy in WW I, was the right man in the right place at the right time. Working in the operational requirements office, he recommended upgrading the new fighter’s armament to eight machine guns. As it was, eight .30 calibers proved marginal against bombers.
In anachronistic terms, the “throw weight” of the British and German fighters’ armament favored the latter. A three-second burst from eight .303s produced 13 pounds of projectiles downrange while the 109’s combined cannon/MG battery yielded 18 pounds. But undoubtedly far less than 10 percent of all rounds fired connected with a target, which is why so few pilots became aces. In one eye-opening episode, six Spitfires fired 7,000 rounds at a Do 17 without destroying it.
Sorley recommended sighting the eight machine guns into a ten-foot circle at 100 yards. That was a reasonable approach for an average pilot, but later Fighter Command adapted a 400-yard convergence that was beyond the abilities of most pilots and dispersed the pattern. In some squadrons, pilots could make their own arrangements, but if they had to fly another aircraft, their familiarity vanished. With 300 rounds per gun, the pilot had perhaps 20 seconds of trigger time.
Part way through the Battle, Spitfire IIB models arrived with two 20mms in place of four machine guns. Loading 60 rounds per cannon, the new mark was more lethal against bombers but the Hispano-Suiza design was unreliable, especially under G. Modifications only improved performance after the Battle when the mixed armament became standard for Spitfires. The “bump” on the wing of later models accommodated a motor to enhance feeding.
The RAF used a variety of ammunition, seeking an optimum combination to destroy enemy airframes. Ball, armor-piercing and tracer were commonly used in sequence, but incendiary rounds also were developed. In the Great War, Britain produced the Buckingham series, originally intended to ignite Zeppelins.
A more effective round came from the Belgium; the De Wilde was modified by the RAF with considerable success.
An unavoidable problem came with mixed armament. The .303 and 20mm left the muzzle at different velocities and possessed different ballistic properties. Consequently, they could only be harmonized for a specific range, and the cannon’s trajectory fell off fairly rapidly. As always in air combat, the closer a pilot could get to his target, the better.
Armorers carrying out the rapid re-arming of a Spitfire of No 19 Squadron at Duxford. The fabric patches covering the gun ports have been blown away, indicating that the guns were fired. The armorers under the near wing have removed two of the used ammunition boxes and are in the process of removing the other two. (Photo courtesy of Eddie Creek.)