Propane cannon , I have built four of them the largest is made out of the large oxygen cylinder . I tried to use smaller pipe sizes but would not produce the desired effect . A very loud boom ! The other thing is the mixture or the propane to air ratio. A very critical mix and all most impossible to do. There are to many variables to deal with the biggest is out side wind conditions. This will get the mixture off . So I went to compressed oxygen. This made the cannon very loud and somewhat scary! But it works very well, I have bean told that they could hear and feel it twenty miles away sounded like rolling thunder coming. Some other friends called me to ask if that was me making that noise. They though some one was shooting a shot gun in the next pasture, they were five miles away.So earmuffs are a must you will get a headache after about ten shots.I will post pix later and instructions if needed.