Possibly because even before the P-51B was introduced strafing aircraft on the ground was part of many mission profiles and in that only the number of bullets per second matters because a ground target is only in the gunsight for a fraction of a second.As ever good points, but the P51 development went from 4 x 0.5 to 6 x 0.5 they would have done it for a reason, presumably because 4 x 0.5 was lacking. I am not aware of any significant complaints about a lack of 'trigger time' (I do like that phrase) of the P51