I'm not quite sure but I've never had any luck with pistons.

I want to make a underbarrel paintball shotgun. The basic parts will be a bernzomatic propane or mapp gas cylinder and a 3/4" nipple.
http://lh4.ggpht.com/_SsAaC-6OMKY/SzsMdyDN2LI/AAAAAAAAAHc/b0T0DFvuw3s/s800/newvalve.gif
I will use a solidly attached rod to the valve to save space.
Just a few ...