Welcome to the forum! I suggest you download Gas Gun Design Tool as it's the best way to be able to calculate approximate performance figures and work out what parameters you need to tweak. I was thinking maybe a Quick Exhaust Valve that is actuated by a hammer valve? That's one possibility, it depe...

With a standard hammer valve the hammer has to open *and hold open* the entire valve that's going to be used to flow the air you're using for propulsion. As pressure and bore size go up, you'll need more and more hammer force to open the valve. A QEV + hammer valve reduces the hammer force required ...