You seem to suggest here that somewhat limited ammunition for the 20mm cannon was unusual. That 60 round drum was not, in fact rare.
The Spitfire Mk VB, which was still flying and fighting in the Med in 1943, had this ammunition system for their 20mm cannon.
The Bf 109E had them too, though these were being phased out by the Germans in the Med in early 1942.
I believe the early Bf 110s had those 60 round drums too. As did the French D.520, many other planes.
7 seconds of cannon ammunition doesn't sound like a lot, but cannons do a lot more damage than machine guns as we have often discussed around here. How many seconds of cannon fire does it take to shoot down an enemy plane?
The time of fire varied a bit depending on the gun. Hispano 60 round drum lasted about 6 seconds because it fired faster.
Now a lot of what you mention are leftovers. Yes they were flying Spitfire Vbs in 1943. They stopped making them in 1941, sort of.
There are two meanings for the Vb,
one is the plane had the "B" type wing with one 20mm cannon with a 60 round drum and two .303 machine guns.
The other is the plane had the B type armament in a "C" type wing. The C wing could hold two 20mm cannon and two .303 machine guns in each wing, the 20mm cannon could be fed with 120 round belts. You could also install just eight machine guns, which is why it was called the "universal" wing. The eight machine gun option was never used? The four cannon and 4 machine gun may have been tested but never issued used
except for ferry flights to Malta where upon landing two guns were taken out to use as spares. Some Spitfire Vs lost the machine guns but kept the four cannon. At times these were called Vc's. to differentiate them from the two cannon aircraft. The Vc with the universal wing first showed up in Oct 1941. Now you had several factories making Spitfires and not all changed over at the same time. You also had a number of older Spitfires rebuilt to the MK V standard and I don't know what some of the rebuilds got. But at the end of 1941 you had Spitfire Vs showing up with 120rpg even if the numbers were small at that time. It British fashion they kept the newer planes at home and sent the 2nd line stuff east.
The French planes went of production in 1940 and nobody bothered to upgrade them in 1941-42-43,
109E had gone out of production in Mid 1941? Yes they used left overs.
But equating left overs to the new production planes is painting a bit of false picture at any given time. The planes with 60 round drums were decreasing with each week.
But I do agree it was a limitation, it was much better to have 100+ rounds for 20mm by early 1943. By later 1943 I think 200 rounds was more common.
Amount of ammo varied by gun and plane. Hurricanes held about 90 (in belts, not large drums) Spitfires stayed at about 120 until the later Griffon models. 20mm Hispano is heavy, especially for four guns. Typhoon had 140(?). P-38 held 150 rounds. By 1944-45 with even bigger engines the USN was putting in around 200.
The 109 used about 150 for the Mg 151, down from 200 rounds for the MG 151/15. because of weight. they had the volume. Soviets used around 120-140 on the Yaks and Lagg-3.
times of fire was closer to 12-15 seconds, you want more than 3 short bursts. but going past five 3 second bursts? enough 20mm Hispano ammo for four guns to last 6 seconds was 150lbs. Japanese army, once the Ho-5 gun finally showed up, used about 150 rounds per gun and that was the lightest 20mm round used.
The US planes in the Pacific did not have 20mm and had a fairly large amount of ammunition, but they did have another related problem in that their M2 .50 cal machine guns often jammed, especially if fired while pulling G. This was only gradually improved toward the end of 1943.
This also varied with the type of plane and the time. Yes they had quite a bit of trouble in early 1942, got better even if not totally solved. Some planes got better quicker than others.
The P-51B&C with the tilted guns had the problem come back with a vengeance.
I would say that the limited ammunition in the A6M2 was indeed a problem, but I would debate the implication or contention that it was an unusual, let alone unique problem.
The lack of ammo in the A6M2 was not unusual but it was old. The Germans didn't like it in the BoB, At times they ran out of ammo before they ran out of fuel. And the two 7.9s in the cowl was not enough. Germans carried double the amount of ammo as the A6M2 did for it's machineguns. 30 seconds of firing time was too much. You ran out of 20mm ammo 23 seconds earlier. or over 7 firing opportunities earlier.
Many air forces figured a pilot could only keep pointing at a target for 3 seconds. A plane flying at 300mph is covering 440fps so in 3 seconds the firing plane will cover about 1/4 of a mile while the target plane covers ????
I don't know what the ideal is but I figure that 6-7 seconds is too little and over 30 seconds is too much. Somewhere around 15-18 seconds might be close.
Now for bomber busting things might be adjusted upwards a bit.