piston valve

[b-nice]

does it need to be air tight in the chamber and does it have to have and spring? if so should it be a compression or recoil. (push aginst barrel seal of pull away from barrel seal.)

[Copperboy]

That depends on a number of factors, it could be all pneumatic operated or all mechanical.
I suggest you try the "theories" buttom above and then look at "flow in piston valves" or something similar.

[boilingleadbath]

The piston should fit pretty well in the chamber, and this is often done with disks of slightly oversize rubber glued to a washer and put on a bolt. There is enough leakage in this system to allow you to pressurize the gun from behind the piston, but if you machein it with o-rings, you'll have to drill a small (like 1/16") hole.

The spring is to help seat the valve (make it seal) - you want a compression spring. It should be noted that the effect of a spring during firing is ignorable; even a relitivly strong 1 lb spring in a small 1" chambered launcher is only 1.3% of the force.