Haha this could be really cool if it's well balanced, I like the idea :D Also this gun with incendiary ammo :Q would be awesome. Although I don't see why I'd have piercing damage, I mean perhaps to everyone's ears xD And maybe it'd make sense if using it would cause damage to the gun itself but I don't know much but about this sciency stuff... :P
"OMG HARMONIC RESONANCE... STARTREK LOLZ." -No one ever.
Anyways, I'm pretty sure you'd want it to do fletchette and shatter damage, as the sound would make the balloon burst due to pressure, and then the guns would go soon after.