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5x MG 151 - 239 rpg - 428 kg - 97% firepower - firepower per weight: 102%
10x MG 131 - 311 rpg - 413 kg - 93% firepower - firepower per weight: 102%
8x .50 Browning M2 - 250 rpg - 452 kg - 100% firepower - firepower per weight: 100%
25x Browning .303 - 399 rpg - 549 kg - 96% firepower - firepower per weight: 79%
25x MG 17 - 406 rpg - 596 kg - 95% firepower - firepower per weight: 72%
Personally, I`d very much doubt that in terms of damage done, there would be much difference between any of the 12.7mm calibers, regardless of kinetic energy.
The more powerful rounds simply made a neater hole in the thin alumiunium skin...
The MG 131 had OTOH much higher Rate of Fire than the M2 Brownings, and it was much lighter as well.
However the higher rate of fire was negated (in the single-engine fighters) by synchronisation for firing through the prop
Was the the extra weight and drag worth the extra hitting power gained?
The reason i ask - would a lot of fighter pilots not simply have RTB'd as soon as their cannon ammunition was exhausted?
The Germans used electric priming so synchronisation had less effect on the rate of fire. The rate of fire went form 900 RPM to approx. 810, still higher than the 750 RPM of the M2 .50.
The two MG 131's together make up less than 30% of the firepower of a 109 (with Mg 151/20 motorcannon). As long as you have 20mm ammo the 131's provide some extra power. When this is depleted you better get home. But if somehow you are forced to engage, two 131's is still a lot better than two MG 17's.