Also if people are worried about the constant pinging you could just play a ricochet noise every couple of shots.
Rather than "tatatatatatatatatatatatata" against the hull
A "ping...pong...toing...ting...ping-pong...pong" etc.
How about a bit of both?
At long range there could be the sound of random ricochets, but when the enemy is right in your face and scoring hit after hit, it could be the more prolonged volley sound.
Obviously the sound level would have to be fine tuned so as to not drown out all other sounds completely, but it could making facing gatling guns that little bit more intense.
It would also be nice if there was a difference between the sound of the rounds hitting hull and hitting the balloon.. for example, the hull would sound like metal, but the balloon would make ripping and puncturing 'thock thock thock' sounds when bullets hit it (I'm imagining 'thock' something like the sound of a tennis ball being hit, but louder and more violent).