I think most of the gun accidents are people trying to shoot birds off the line rather than trying to shoot the line itself: stupidity and carelessness rather than malice. Makes it harder to convict them.
Shooting a gun arbitrarily in the air, even at an angle is criminal because what goes up, must come down somewhere and there's no way you can be certain there won't be people there.
Shooting at a bird or a wire isn't shooting arbitrarily. If it was illegal they wouldn't sell licenses for bird hunting. You do have a duty to be aware of what's in your shot's potential flight path, so one could be charged with that.
People shooting at birds ought to be using shotguns, and birdshot pellets have much lower terminal velocities than rifle rounds, since they're much smaller.
http://en.wikipedia.org/wiki/Celebratory_gunfire#Falling-bul...