One of the downsides of 30mm cannons is the low ammo supply. Any chance you just ran out? You should've gotten more than three bursts (I think 8 or so full bursts can be expected) but if you fired more than once at each of 2-3 targets, you may have gone Winchester. When master arm is on, and the gun selected, the lower third of the HUD will show two numbers. If it's 125 and 125, you're at full ammo. If zero, we've solved the mystery.