I have just slipped into my anorak....
The P47 and P51 had six x 0.5 cal, giving a much heavier weight of metal in a burst than eight x 0.303.
The B17G certainly had 10 x 0.5 (2 chin, 2 upper, 2 ball, 2 waist, 2 rear). Some had cheek guns as well.
The awesome B25J(?) solid nose version had, I seem to recall, ten forward facing, two mid-upper and two rear 0.50. It must have rattled a bit when they fired them.
Early Spitfire cannon jammed through ingestion of dirt through the muzzles - thee solution was standard-issue condoms, rubber, aircraftmen for the use of.