Not sure how much they sound like guns, but there's a whole field of scaring off birds from farms, see e.g. http://www.birdcontrolpro.com/propane_cannons.htm ht

Not sure how much they sound like guns, but there's a whole field of scaring off birds from farms, see e.g. http://www.birdcontrolpro.com/propane_cannons.htm http://www.birdcontrolpro.com/propane_cannons.htm