I have read the entire thread and you have made some interesting new solutions and I think you are getting close to perfect. The new valves have almost maximum flow for a drop in upgrade. And I think you would be able to drop the operating pressure quite a bit when you have tuned them correctly.
I've thought about what someone said (I think it was russc) that we needed a valve that can close more quickly and not waste CO2. I'm not an engineer, so this is may be very stupid. And I'm afraid most of you doesn't want to use a 9V in your nelson gun.
But why not use an electromagnet to close the valve instead of springs? springs never "stay" at their maximum open position, they immediately go to closed position at the same speed as they open. If we use an electromagnet, the valve opens quickly with no resistance and we can control how long we want the valve open, with the use of a simple programmable board. The board and the 9V could be hidden in the grip. The cupseal would have to be made of a ferrous metal, as steel.
Here's a picture to explain.
You could use a microswitch (2) to tell the board when the hammer is hitting the powertube. The board would the start a timer and when the timer hits zero it would activate the electromagnet (1). In this example I used copper-wire in coils with a iron core as the electormagnet. Maybe there is a better alternative?
Keep in mind this is only an idea, I dont know if this has already proven to not work. And maybe you dont want to add electronic components to a nelson
And please, before you bash me for my grammar mistake, read my signature