I just can't get past the niki's rate of fire on the cannon... seems planes fly thru the stream between bursts.
I think the guns fire two by two. Try firing one round from your primary gun. If you do it correctly, when you use the fire all trigger, the four guns will be out of sync. You end up with a steadier stream, and less room for the dastardly cons to fly through.
At least the sound makes it seem to be the case -- I assume the rounds follow the sound. (Then again, I don't think the default sound for the 37mm on the P-39 is in sync with the actual rate of fire, so I could be all wet.)