Shortround6
Major General
The point is that the USN much preferred the four gun with more than 400 rounds per gun installation of the F4F3 over the six gun with 240 rounds of the F4F4 to the point that the late F4F4s switched back to the four guns as well as the FMs. The Hellcats and Corsairs carried the six guns with 400 rounds without much performance penalty. It is apparent that firing time was important to US pilots. Perhaps it did not matter as much to British and German pilots and the reason might have been that their AC could not stay in the air as long as the US models. Shorter duration flights don't need as long firing times. If you are going to be in the air for four hours or more it is nice to know that your ammo supply may be adequate. A fighter with empty guns is useless. If you read "The Shattered Sword" the Zekes having to land continually to replenish the 20 mms during the attacks from the AC based on Midway had a major impact on the eventual outcome of the battle. I have read that in the Pacific some Corsair pilots would switch off two guns until the other four ran out in order to stretch out firing times. Does not seem logical to me to simply ignore the huge difference in firing times for the P51 with 20mms and the F4F3 with four fifties.
A .50 cal with 400 rounds weighs about what 20mm Hispano does with 120 round (mounts, cocking units, heaters and such can throw this off) for a plane that can carry only the weight of four .50s with 400rpg each maybe it does make sense to go for the longer firing time. When you go to Six .50s with 400 rounds each vs four 20mm Hispanos and use the FULL weight of the .50 cal guns and ammo then the firing time differences could drop to only 20-25% greater. With the Navy figuring that one 20mm was equal to three .50 cal guns the four 20mms have a target effect of twelve .50cal guns. They may not need as long a burst per target and equal up some of the firing time that way.
As far as the Zeros go, the early Japanese drum fed 20mm cannon actually weighed less (3-6kg) than the American .50 cal Browning (or the later Japanese navy 13.2mm copy) and the weight of the ammo was such that 60 rounds of early Japanese 20mm ammo weighed about as much as 105-110 round of .50 cal ammo. Granted the Japanese cannon used a heavy drum but how far would they have gotten at Midway defend their fleet using Zeros armed with a pair of 7.7 mm machine guns and a pair of 12.7/13.2mm Brownings with just 120 rounds (help make up for the drum) per gun? 9.2 seconds firing time vs the 7.5 seconds of the 20mm cannon.