As with today, during the WW 2 period different nations had different standards for construction of their military aircraft. For example, the US standards often resulted in a heavier airframe than most European countries used.
An example of this is North American's XP-51F and G fighters that were built to British load factors rather than US ones. This resulted in an aircraft that was almost 2,000 pounds lighter than a standard P-51D! This resulted in a more maneuverable aircraft that also had a top speed somewhere between 40 and 50 mph faster than the D model.
The US also called for larger cockpits to accomidate larger pilots easily where say, the Germans often had ones that were more akin to racing cars and tight fits.
I suspect that if one played around with this idea one would find many of the US designs would have greatly improved using European standards (British and German) while the British and German designs would have turned into true dogs with US engineering factors applied.
The Japanese would surely have been hard hit using anyone else's design criteria; their aircraft being far lighter than other nation's in terms of airframe and skin requirements.

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why didn't they (americans) ever consider explosive cannon shells for single-engined fighters?

The British did the same with the Hawker Sea Fury & got a extra 50 mph
What I found shocking when reading Max Hastings book Armageddon is that RAF bomber command loss twice as many crews than the US 8 Air force because of the lack of armour around the fuel tanks

A great book on how the RAF went about making fighters in WW2 is Interceptor Fighters 1935 -45

The US did. The weapon of choice was frequently one of the 37mm cannon variants they had in production. These were generally not used due to their propensity to jam and their weight. The other issue was their small ammunition supply which was regarded as inadequite. The .50 cal on the other hand was perfectly adequite for the types of aircraft the US was encountering so it was retained in service.
There were a few USN/USMC aircraft that were fitted with 20mm cannon instead of .50 machineguns. These were primarily nightfighter variants of the F6F and F4U. Here the greater deadliness of the 20mm was considered necessary due to the likelihood of scoring fewer hits at night using radar to target an enemy aircraft.

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and how come american designers never mounted weapons on the cowling for single-engines? the germans obviously regarded a line-of-sight -firing weapon to be so essential they were willing to make sacrifices for their me-109G series and the ta-152. when they interviewed saburo sakai, his favorite feature in the zero was not the maneuverablity (he missed the claude fighter,) not its rate of climb, but rather the twin 7.9mm cowling-mounted MGs. it seems he liked to take a snap shots right at the enemy's cockpit within 50 meters if possible --without cannon fire. sort of like hunting wild geese with an air gun.

They did; it was the standard installation early in the war (see P-36, P-40, A-36) and even continued in the P-39 and P-63. Only when they realised that they needed a battery of .50 cal guns to be effective (too many to fit in a cowling) did they decide to follow the RAF route and put all of the guns in the wings, as this was simpler and more reliable.

The big Browning also lost a lot of its rate of fire when synchronised - around 40%.

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aha.. the slower the rate of fire, the higher the retardation upon synchronization, right?

No, it isn't as simple as that (in fact, other things being equal, the faster the "free" RoF, the bigger the percentage loss on synchronisation). The loss of RoF depends on the nature of the synchronisation system, the propeller type and the mechanism of the gun.

The best systems were WW2 German MG 131 and MG 151 guns which used electrical priming; the loss was only around 10%. The typical figure for most other systems was probably around 25%. Constant-speed propellers were easier to synch for than fixed-pitch single-gear types whose speed changed with the engine revs.

Why the big Browning (and its Japanese derivatives) was so bad I'm not sure; I suspect a heavy firing pin mechanism with considerable inertia, which built in a longer than usual delay between pressing the firing trigger and the gun actually firing. For best results, that delay had to be as short as possible.

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American standards were high but it was not seen as wast. The stronger the plane the harder to bring down.

Also by moving the guns away from the engine you make maintenance easier and free up space for other things or narrower bodies.

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"What might work in New York City is certainly not going to work in Montana. So, for the federal government to be having any kind of, you know, blanket rules that they're going to try to impose, I think doesn't make sense." Hillary Clinton

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ok, that being settled (thanks by the way,) i have a new set of questions, if the TS will let me hog for a while. i felt that the key to defensive air power rested on destruction of enemy bombers (both in flight and their facilities on the ground.) this, assuming i was an axis power faced with the reality of a long, drawn out war with say, the US or russia.

with that, one could have narrowed the performance requirement for pursuit planes. the all-around requirements were:

level speed
dive
climb
maneuverability
ceiling
range
firepower

i arranged the above qualities, the first four being largely defensive features in a fighter, the bottom three being largely offensive. maneuverability i believe, is a purely defensive feature and hardly helps one in shooting someone down (though you can keep him from shooting you down.)

so a pursuit plane with enough ceiling and offensive punch to chase bombers and shoot them from the sky, i believe, should have been the focus of development. the fighter simply had to have enough level and dive speed to run away from enemy fighters.

the story of the raiden fighter was a sad one in that it basically followed what i wrote. it had great success in attacking b-29s because of its speed, armor, and four cannon. but it fared badly when the p-51 were sent to escort the fortresses. it had enough level speed to stay apace with the mustang in several. but it couldn't turn or dive away when the american fighter got on its tail. but even then the main reason for the failure was inferior numbers and low pilot quality by the time the shooting over japanese airspace happened. there were japanese pilots who actually managed to down mustangs using the raiden (most notable was this guy akamatsu who shot down 15 p-51s.) but those guys were exceptional. heavily armed and armored fighters like the shiden and raiden were more difficult to fly (and so it took longer training.) veterans used to the zero complained those two planes flew like trucks and the feature they appreciated most was the added armor protection.

Actually, if you look at how aircraft have evolved today the characteristics are far more subtitle and involved than the simplistic list given above.

I would list the following as critical features of a good fighter aircraft:

Acceleration
Energy climb / zoom climb
Transient response
Roll rate
Visibility (own outwards and other inwards)
Survivability
Firepower

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Originally Posted by mac_bolan00
"and how come american designers never mounted weapons on the cowling for single-engines?"

And quite a few fighters up to the beginning of the war had cowl mounted guns.Including the P-26,P-35,P-43,Brewster Buffalo,CW-21 Demon. All used during the war.

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The single most important stat for a fighter is its thrust to weight ratio. Now shooting down WW2 bombers was not as simple as run in and shoot. They tried that with the Me-262 and other jet/rocket planes towards the end with mixed results.

If you are planning on avoiding the fighters, you would need at minimum a 100 mph more then the fighters can do, anything less and you will get shot up before you are done, assuming that the enemy is somewhat efficient. This leads to the problem with run and gun. Time, you need time to put rounds on target. Your effective range is around 600 yards (1800 ft or .34 miles) given a speed of 550 mph you cover that distance in 2.2 seconds. This is enough time to get on target and shoot a couple of rounds at most. Assuming you are a good shot and hit 75% of the time you have a you might hit with one round. This is why the Germans mounted the bigger cannons on their planes that one round does more. So you would heave to make multiple runs to get a bomber and you still have to avoid the fighters.

What would make more sense would be to have a fighter to engage the fighters and interceptors to shoot the bombers. This was the inent but life is never as easy as we plan.

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"What might work in New York City is certainly not going to work in Montana. So, for the federal government to be having any kind of, you know, blanket rules that they're going to try to impose, I think doesn't make sense." Hillary Clinton

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I disagree. I think that transient response is the critical feature. That is, the ability to transition from one maneuver or vector to another.

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I could buy into that.

__________________
"What might work in New York City is certainly not going to work in Montana. So, for the federal government to be having any kind of, you know, blanket rules that they're going to try to impose, I think doesn't make sense." Hillary Clinton

(Photo removed for humanitarian reasons.)