You'd just end up shooting something you didn't even know you were shooting at. If you're flying AMA it's against your rules. Finally, it's just damn silly.

Now, there is at least one sonic system produced for electric models that turn off the motor of the guy you are shooting at. That could be fun on a park flyer other than the ones that it is hooked to. Check the electric forums for them I think they're by hobbyzone. But projectiles would just be dangerous for everybody.