Thats what I'm saying, the piston will not open fully. Maybe with a Longer area behind the piston with a long, progressive spring It will achieve the same flow. Still going to have a slower opening time VS no spring, and I can't see the multi-shot idea working.
Yes, the pressure remaining should equal the pressure prior to combustion (in a closed chamber). In the closed chamber testing I've done, the gases cool very quickly, all you see is the pressure gauge spike and drop back to original pressure, almost as if you just flicked the needle with your finger. The valve will open and vent some of the gases, so i would say the remaining pressure would actually be less than initial pressure.