In a compressed air gun for use by police and firefighting forces for breaking open doors, including a compressed air cannon with an ammunition ball, wherein the compressed air cannon has a compressed air chamber, a barrel extending from the compressed air chamber and a discharge valve for releasing the pressurized air from the compressed air chamber into the barrel the ammunition ball consists of a hollow plastic ball which is filled with a heavy material providing, upon being shot out of the barrel by the compressed air, an impact force sufficient to break down a door to be opened without rebounding or generating shrapnel that could endanger a person using the gun.