But also the weight of 1 MG 151/20 plus 1 MK 103 are only a little heavier (~50%) than 2 MK 108 guns. Though 6 MG 151/20 would have been an interesting armament too, or for a pure dogfighting armament, 6 MG 151/15 guns would be totally superior to the P-80's M2 BMG's. 6x MG 151 guns (both 20 or 15mm weigh 42kg each) weigh about the same as 4x MK 108 cannons (60kg each) so weight wouldn't be a problem, though I'm not sure if there's enough room to be practical. (they would certainly fit as larger armaments like 6x MK 108 were made too, but these had problems, and I don't know if other problems would occur)
As for the MG-FF/M, it was inferior in performance every way to the MG 151/20, except for weight, and this did not make up for the poor performance elsewhere as the 151/20 still had a higher specific firepower (energy/time/gun's mass). Though the Mk II Hispano cannon was better in some respects (though heavier and a lower ROF), and the Mk V in all.(same gun weight and ROF but much higher velocity and shell mass) Though the USSR's B-20 was probably the best overall 20mm cannon of the war, with slightly higher performance than the 151/20 and weighing only 25kg, that's 3kg less than the MG-FF/M and 4kg less than the Browning M2!
Here's a good resource for gun and ballistics data:
The WWII Fighter Gun Debate: Gun Tables
The site is geocities based and up and down a lot, so I posted the cached version from google.
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