There must be big differences between firing a .50 on the ground at 0 speed (BTW I'm not referring in any way to the experiences of the above posters who I'm assuming know their stuff) and firing one at 20,000 feet at 350 mph with regards to heating and cooling times. For realism, I'd have thought that these factors would need to be addressed.
I seem to remember early Hurricanes (as well as others) having problems with their guns freezing up at altitude and this being fixed by diverting hot air from the engine. So it seems that heat related problems were a two way street.
From the point of view of playing AH, I don't mind at all if my opponent rattles off all his ammo in one continuous spray at 1k. I'm much more afraid of the guy who manages to get close in, firing several short bursts.
Some of the great aces of WWII averaged something like 15 cannon shells expended per kill. This being because they didn't open fire until the enemy aircraft filled the screen.
The only thing that might be partially solved (in my own opinion, of course) would be the continuous and deadly accurate streams of fire from the arse end of American bomber vics.
KD