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I did hear the MG131 (13mm) was designed to be able to replace any 7.92mm weapon, used on aircraft, so maybe the 13mm offers the best bang for the limited space?This one is a can of worms.
Yes the Germans used an exploding bullet, but not every bullet in the belt was explosive.
The German ammunition was an HEI-T and the tracer sucked up a fair amount of space leaving about 1.2 grams of HE ( A 20mm Hispano held about 10 grams), the German incendiary bullet held 2.2 grams of incendiary material ( The US .50 cal M8 AP incendiary held about 1 gram). The Germans also mixed an AP-tracer into the belts.
The US bullets were about 8-9 grams heavier in addition to moving faster so they had 75% more kinetic energy. If they hit wing or fuselage skin and went right on through this extra energy was wasted. If they hit something like wing spars, engine mounts, flange plates, etc they could do more damage than the German AP round could. This occasionally including breaking open fuel tanks.
If the German HE shell explodes on the wing/fuselage skin or just inside it won't do much, if any, damage to "stuff" several feet further inside. This was a major problem for both the Germans and the British 20mm ammo in the first few years of the war, the fuses were too sensitive and exploded on contact making big holes in the aircraft skin but not causing crtical damage inside the aircraft.
So a lot depends on the location of the hit and the type of projectile that scored that hit. Some projectiles will cause more damage in certain situations than others. The Projectile that causes less damage in situation A might cause more damage in situation B.
Could be the tracer needs to be visible, so it burns bright and fast.IF the 13mm makes it to the fuel tank without exploding first. Depends on how far from the aircraft skin the tank is.
13mm AP will put a hole in the fuel tank. Many self sealing tanks could seal up a 7.5-8mm hole. They had a lot more trouble sealing up 12.7-13.2mm holes although they could reduce them in size. Depending on the size of the tank, it's construction and how much fuel was in it (and where the bullet hit ) the large caliber machine guns did split the seams on fuel tanks on occasion using ball or AP ammo. a small to medium tank full or nearly full that takes a .50 cal round in the liquid and not in the airspace is going to suffer quite a strain. There are number of videos on you tube of peoples shooting various containers full of water with .50 cal rifles, Air absorbs some of the shock, Plastic barrels will stretch/deform more than a metal one will and absorb some of the force. Large aircraft tanks (like bomber fuel tanks) are built stronger than small tanks so there are a lot of variables.
The .50 cal used a number of different rounds. The British did use an incendiary based off their .303 round but carrying 5 times as much incendiary material. About 2.5 grams worth.
By 1943 the US was using the M8 API which used about 1 gram of incendiary material in the nose in front of the AP core.
Russians used a similar bullet in their 12.7mm guns.
Tracers had trouble setting fire to things. They often used a different chemical compound than the incendiary rounds and since they started burning either right after leaving the barrel or a short distance away they had often consumed a fair amount of the material before hitting the enemy aircraft. They could set fuel tanks on fire, it just needed roughly twice the number of tracers, on average, to do it.
Could be the tracer needs to be visible, so it burns bright and fast.
Where as the incendiary can be a slower burn, which would last longer (best guess).
IF the 13mm makes it to the fuel tank without exploding first. Depends on how far from the aircraft skin the tank is.
Gentlemen
You may want to check:
WORLD WAR 2 FIGHTER GUN EFFECTIVENESS
and
AMMUNITION DATA TABLES
FWIW
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