I think the mustang carried about 1800+ rounds of 50 cal. ammo: but feeding 8 M2 machine guns how much trigger time would they have in a mission?
M-2 in the aircraft mode could do about 600 rounds a minute . . . P-51H carried 6 M-2s and 1880 rounds . . . 1880 divided by 6 = 313 and 313 divided by 600 = about 30 seconds of firing time. Doesn't sound like a lot until you think of it the other way . . . 10 rounds per second per gun . . . a one second burst is at least 60 rounds down range . . . really does not take more than 5 to 10 well placed rounds to bring down an adversary. I'm more familiar with USN fighters . . . so maybe someone knows more than I . . .
