I made one, it was a beast!!! I went to my local plumbing store and bought lots of drainage pipes (it ended up quite exspensive). this is how it ended up looking I had a scew top to the gas chamber, I used butane from a blowtorch as fuel so I had to fill it up and screw the top on quickley. I used the electronic ignition from a lighter to make a spark, I wired it up to two spokes that peireced eitherside of the gas chamber and stopped 1mm apart, it made a good spark. I used tennis balls as ammo as the barrel of the gun was pretty large, I put two spokes in an X patern near the gas chamber end to stop the ball from going too far. now then, the importnant bit! ITS VERY DANGEROUS!!! this one I made nearly killed Iolo, I didnt use strong enough glue on the joins and the whole thing blew up!!! It really winded Iolo (fired the ball bloody far though!!!) I recken a steel virsion of mine all welded up would be ace!!!