Jack: did you mean .50 (i.e. .5 inch), not 50mm? 50mm - 2" - is approching light field artillery size. The largest caliber on aircraft (other than experimental oddballs) that I'm aware of is 40mm, used for tank-busting. E.g., Hurricane IID in Libya. The guns are in underwing gondolas, something the Germans did with 30mm cannon on their Me109G and Fw190:
Bill: true, as far as it goes. However, one or two hits by a 20mm explosive shell is often enough to bring down a fighter, where you need to drill lots of holes with a m/g to get the same effect. And that is even truer against bombers, especially when the Germans started armoring vital spots like engines and pilot seat backs. In the Pacific, against Jap aircraft with no armor or self-sealing tanks I can see the rationale, but not in Europe. The extension of the theory, that multiple m/gs are more logical because of the limited time in combat that a target will be in a fighter's sights, can be reversed, using the same logic. How long can you "hose" another fighter with m/g fire in a dogfight, vs what are the chances of making one fatal 20mm hit.
The other argument is that the average fighter pilot was not a marksman in WWII, especially in the desperate days of 1939-1940 when thousands of young men were hastily trained and shoved into battle (it wasn't uncommon for them to have fewer than 10 hours on a Spitfire or Hurricane) and had all their work cut out to fly and keep in formation. There was absolutley no time for aerial target practice, so virtually none of these pilots could accurately estimate the "lead," or angle to aim ahead of an enemy plane in a turn. Thus the "spray and pray" 8 X .303 m/gs of the Spit and Hurri (and later 12 on the Hurri).
By 1941 the Spitfire armament had been changed to 4 X .303 m/g and 2 X 20mm cannon. Aerial practice against towed targets was a part of fighter pilot training by now, and the theory was that the combination gave the best chance of a kill. The only fly in the ointment was that the two guns had different trajectories, making aiming tougher.
Speaking of different trajectories; the Bell P39 Airacobra had possibly the most bizarre armament -
three different guns - .30 and .50 m/g and a 37mm cannon!
By 1944 the Germans were arming some
bomber zerstoerer (bomber destroyer) fighters with
30mm cannon, each having a round four times the weight and explosive power of the 20mm. The Me262 had four of these massive guns. The US finally woke up midway through the Korean War and started arming fighters with up to 6 X 20mm. And any advantage of multiple m/gs disappeared with the advent of the multi-barrel rotating cannon.
It should be mentioned that the art of aerial combat marksmanship was also revolutionized by the German
Revi and the British K-14 gyro computing gunsights. In his book
I Flew for the Fuhrer, German multiple ace Heinz Knoke says gleefully: "With the new Revi sight it was virtually impossible to miss!"
With the British K-14, the pilot only had to feed the wingspan of the enemy plane into the sight (they were pre-set for the ones likely to be met, such as Me109 and Fw190) and the lead angle, the hardest factor for the average pilot to estimate in a turning battle, was automatically computed by the sight. The wingspan was set by a rotating grip on the throttle, eliminating the need for the pilot to take his hands off this crucial control. All the pilot had to do was keep the target within a projected circle formed by six diamond shaped spots of light, the K-14 did the rest.
Aerial combat was revolutionized overnight. Average fighter pilots achieved five times as many hits with the K-14 compared with conventional optical and reflector sights. Although you will find it hard to discover any reference to this in USAF histories, the Brits turned over the engineering drawings and specs to the US in June 1944. Typical Yankee "know-how" had them manufacturing the K-14 and getting them to USAF squadrons by late the same summer. The computing gunsights of modern jets are essentially improvements of the K-14.
P.S. The answer to my trivia question: The night-fighter version of the Corsair, the F4U-5, was armed with 4 X 20mm cannon.
