CAUTION: Anecdotal evidence about to be presented...
My dad, as I say repeatedly, was a nose gunner on B24s. During a training mission around Muroc the Navagator was required to shoot the nose guns as part of his quals (my dad's sentiment was "this guy couldn't find his bellybutton with both hands"). Anyway, dad tells the guy to fire short bursts, so he gets in the turret, jams the triggers down, and fires off about half the can of ammunition. When he was done the guns were so hot rounds were cooking off in the chamber. The Nav gets out of the turret (and by now they're starting to fly over populated areas) and tells my dad he'd "better do something" about the guns cooking off (dad just popped the top plate).
I've never heard of fighter guns getting hot enough to cook off ammo in the chamber, but it would seem to be just as likely after a prolonged burst. When I get home I'll look through dad's old gunnery manual and see if it says anything about hazards of prolonged fire.