I forgot to add that for a 2" barrel you would need the large piston face to be about 3", this way the surface area of the piston surrounding the barrel is greater than the size of the barrel.

i got thinking about Boilingleadbath's earilier comments and came up with this as a possible way around it, let me know what you guy's think, as i am not to sure myself.

http://i26.photobucket.com/albums/c109/greihawk/piston.jpg

When the chamber is being filled the ball bearing moves forward and ...

yes it will be be via either of those, to start off with a blowgun, until i have tested it and seen if its any good.

sorry, should have put that it wasn't to scale, i drew the piston 1st and then pasted it in, the piston guide tube will be 3 1/2" and the piston itself will be machined out of 4" aluminium bar, i drew the pic the way i did so that it would be easier to understand the principles.

This is my idea for a piston cannon that seals on the back face when valve is operated, hopefully resulting in very little pressure loss from the main chamber. I am going to build this gun in the next couple of weeks and try it out, any comments or criticisms in the meantime are most welcome.
http ...

the coil increases the voltage produced by the camera, therefore making the spark hotter, and to answer a earlier post i would suggest a momentary switch, as the power has to be stopped for it to leave the coil, im not to sure, but if you leave it on, there is a chance that the camera could bleed ...