I'm building a airsoft type weapon, and I need something that releases a quick powerful burst of gas (Co2) to power the bb. This is going to be a semi auto only model. What do need? The Co2 will be an external source similar to paintball and attached via hose line.
... and from the cross section of the piston. To fill the chamber fast enough the valve must also have an adequate cross section to let the pressure build up rapidly. Otherwise you'll get a strong push but not a shot.