If you can fully slide the priming bolt forward and it comes back, then it's a vacuum problem.
I can slide it all the way forward, it just slides back very easily, and I can then just make sure it is pushed forward to fire, which is all good mucking around at home, but in a war situation where I am firing quite quickly, I really don't think it is at all practical to be pushing the bolt forward, checking to see if has stayed forward, pushing forward again and then firing. An no, no breech modification, just AR removal, air release hole plugged, spring addition and lock removal.
I really like that idea, except the stock tank burst. Split right down the middle. I attempted to replace it with bike inner tube but the rubber was too flexible and did not compress quickly enough. That is why I am doing this, to make use of the parts I have lying around.
Really the only thing i am having trouble with now is finding a one way valve for the tank.
Yes I am double posting but I wanted to bump it anyway, and I have a more complete design. It is all in Metric cos I am an Aussie, and it is all Aussie materials, it is just the design I would like scrutinized and flamed. Yes, the PVC will be pressure rated, inside the shell, and its hand pumped so it wont be reaching high pressures.