you could make a compressed air cannon from conduit and a bicycle tire pump or air compressor, when my friends and I were younger we would make these and mould projectiles from concrete sometimes would use a patched lug nut . we would compress air to about 250 psi in the tank put a concrete projectile down the bore, just release the valve and it would shoot the approx. 2 once chunk of concrete at about 800 fps we chornographed it. its not very accurate we never bothered putting a sight on it, but at 30 yards it would penetrate through a 2 inch thick board like a hot knife in butter. when it goes off its sounds like a big paintball gun maybe just a bit louder Id imagine at close range it would knock a deer on its can.