I would like to add that the balls I have checked are 8.560, The 3-2400 Argon tank I have has a ID of 8.750, The fit is nice, DD and the fellows here told me the specs. on making the powder chamber, I fired it last week in and after a few shots I was very glad I had the 6 in. X 5 in. steel chamber, When these go off and you see that big ball high in the air and the noise your be very glade to have made a safe mortar. I got pics. from sawing the tank,machinceing, to making the sled and firing, I just got to get someone in here to help me get pictures going. Joe