Just got a 20 foot barrel for this thing! I should be having some supersonic fun pretty soon.

The cement was cured, so I went out and shot it. I forgot to mention that last night I too out the "spark plug" and tinkered around with it until the spark gap was less than 1/16." It can now ignite 6x mixtures, and I'll try 7x in a little while. The first shot I did with the new 20 footer was a 6x shot (the first 6x mix). It managed to put a battery through a piece of 1" plywood and a 2 x 4 stacked against each other, then went another 30 feet and went through my fence, andother 1" of wood. The battery was not found (obviously, because you can't see the projectile, even with only a 3x mixture).
The second shot, the only other one in those 5 minutes, was also at a 6x mixture. This time, it was a ball bearing, to see if I could tell if it was supersonic or not. I'm pretty sure it was, because I was using 3 layers of soda can as a burst disc. I'm also pretty sure DDT occured. I saw the board 40 feet in front of the barrel get kicked up, then heard a CRACK that sounded like shooting a 9mm blank next to your ear a split second later.