Looks fun! I agree A is oversize. The reservoir will lose pressure every shot, and if A is not very much smaller than the reservoir, that loss of pressure will be quite noticeable every shot. The hammer valve can have a self regulating effect but that is totally different technology. I have a slide ...

Those little 3/2 normally open lever valves make good triggers. You should get good results with a barrel down to even 400mm
I got 860fps with 0.12g BBs @ 150psi with this, chamber is big, barell is 400mm

I think that the pressure drop is for the chamber. When you see the warning about barrel choking flow it's a sign your chamber is too large for the barrel... should be nice and loud but maybe not the balanced performance you want. Increasing barrel diameter nearly always increases energy, and veloci...