Thats kind of questionable. If an Il-2 is strafing, then I'd rather have the quad .50's, simply because theres a greater chance I'm going to kill the pilot, or take out his guns through simple volume of fire, even if the overall damage the two weapons can do in, say, 30 seconds is the same (which its not).
If simply doing damage, no matter how insignificant, is the be-all and end-all of your sortie, yes, the 20mm would probably be a better choice. But the purpose of AA vehicles is to kill aircraft, or damage them to the point where they are no longer combat-effecive, which a single 20mm round is HIGHLY unlikely to do.
Krusty is correct on this, a single 20mm is worth something like 3x 50 cal hits, hitting someone with 1x 20mm means you need 3x 50 cal rounds hitting to get the same damage, volume of fire is the only thing the M-16 has to an advanage, where the single 20mm has the hitting power of actually bringing down the aircraft.
You might be able to score some hits with the volume of fire, but to actually bring an aircraft like the Il-2 down - then you would rather want that single 20mm. Same idea went towards the British early War aircraft putting 8 then 12 .303s on the Spit/hurris, the idea was volume of fire at close range would do the damage, where the germans opted for Accurate aiming with a pair of 20mms instead for damage.
Case point:
"Comparisons of the effectiveness of the British and German armament are not unlike the comparisons of the aircraft – each had its strengths and weaknesses. The British armament had a very high rate of fire (160 v. 50 rounds per second for the Bf 109), increasing the chance of scoring a hit. In weight of fire the German guns had a slight advantage (2.0 v. 1.8 kg per second), whereas in total muzzle energy there was nothing to choose between them. The key difference was of course the high explosive in the 20mm shells, which was enough to give the Bf 109E-4 almost double the destructive power of the British fighters. The M-Geschoss were not good at penetrating armour but this was considered a reasonable price to pay for the increased HE blast effect. The Luftwaffe discovered that the most reliable way of bringing down aircraft was the general destructive effect of HE blast within the structure, rather than relying on being able to hit vital but small areas (e.g. the pilot!), which could be, and quickly were, protected by armour. For this reason, AP projectiles eventually disappeared from German cannon ammunition belts except for special purposes."
To sum up, the British armament was highly effective against unarmoured fighters and the high firing rate improved the chances of hitting. At the start of the war it was probably the best armament available against fighters. However, the rapid introduction of armour protection during 1940 greatly reduced the effectiveness of all rifle-calibre machine guns and gave the cannon a clear advantage, especially against bombers which were easier to hit but more difficult to destroy. Many German bombers made it back to base despite being riddled by up to 200 .303 bullets, protected by crew seat armour and self-sealing fuel tanks. This compensated to some extent for their poor defensive armament, which consisted of hand-aimed 7.92mm MG 15 guns in single mountings.
Yes I understand this is the 303 vs 20mm test, however it gives a broad view of what you are asking.
