SpudFiles

I found the patent for the above mentioned cartridge - on firing, the bolt basically hits the cartridge inner body and moves it forward, thus opening the valve against the outer body, releasing the air and firing the projectile.

Altough the design is nice, it is not valveless. Its basically a sort of hammer valve where the barrel IS the hammer.

Note this gun would fire if you drop it on its back end or use it to poke something.
To overcome the problem of all that pressure pushing, you could get a rod to the other side of the chamber and make an piston in some sort of extra chamber part.

psycix: i think i understand your idea, would the piston have a pilot valve behind it? so that when you open the pilot the piston goes back and pulls the barrel with it? If thats how it would work, wouldn't it be more effective to just make a normal piston cannon?

The point of the "backforce piston" is to reduce the load on the valve to make it easier to open...

I know this topic is slightly old but I decided to go ahead and build a prototype of this design. I made it out of a 1.5" by 3/4" bushing, the neck of a 20oz pop bottle some 1/2" pipe and a 1/2" endcap, and a baloon neck for a seal. I imagine more professional materials could be use but this seems to work. I plan on lining this up with a bolt loading system so I charge the bolt then pull the trigger to push the bolt into the valve. The bottle neck was sawn off the bottle with about 1/8" left between the neck and the bottle, this little piece fits snugly inside the 3/4" socket of the bushing, I put an O-Ring between them and hotglued it in, it doesn't seem to leak at all.