I use a quick exhaust valve with a 3 way valve attached to the QEV to fill and fire it in one smooth motion.
If you want me to, I'll draw you a diagram tomorrow, but quick explanation is this:
Barrel of launcher attached to Quick Exhaust valve (Exhaust port)
1" galvanized pipe, roughly 14" long attached to QEV (Out port) (this is the expansion chamber)
1/4" 3 way ball valve attached to QEV ("IN" port on QEV, Common port on ball valve)
1/4" quick coupler (air pressure source) attached to ball valve (valved port)
2nd valved port on ball valve is left open to atmosphere
Operation:
Pressure from air tank to ball valve, Ball valve is turned to allow pressure to flow into QEV. When QEV is pressurized from IN port, it sends pressure to Out port (expansion chamber). When ball valve is turned 180 degrees, it allows pressure from In port of QEV to de-pressurize to atmosphere, which causes QEV to switch modes and "exhaust" all the pressure from the expansion chamber to the exhaust port (which is attached to barrel of launcher). All this happens near instantaneously, and happens exactly the same without having to try to open the ball valve really fast every time. which causes better range with lower pressures, and consistent projectile velocities.
This way you can fill and fire the whole thing with 1 valve, makes things so simple.
The QEV valve is rated for 150psi, but Humphreys (manufacture) says that it should handle 200 psi just fine, and if it does fail, the exhaust membrane (rubber) will fail first, causing the QEV to exhaust (launcher fires). So as long as you don't stick your face in front of the barrel when the expansion chamber is pressurized, it's very safe.
I am using propane as my pressure source, since it is about 100 psi in the winter, 200 psi in the summer, (roughly) and is in liquid form, so much higher capacity than just a pressurized air tank.
At 200 psi, I can shoot a nerf pocket vortex roughly 400 feet in a mortar trajectory. A flatter "rpg" trajectory goes about the same distance since it isn't fighting gravity as much. And a JCS foam rocket about 1.5 times that distance.
I am looking for an affordable and compact CO2 regulator that I can use with a paintball tank though, that way I could get a consistent 200 psi all year round.
QEV:
http://airinc.thomasnet.com/item/specialty-valves-acc-/super-quick-exhaust-valves/qe5?Ball Valve:
http://www.grainger.com/Grainger/Two-Piece-Ball-Valve-1WMW9Anyways, if you want me to go into further detail about my mortar, let me know. For now, time for me to go to bed, work early tomorrow. I'll take some pics of it tomorrow and point out the different parts as well if you like.