I've mentioned this in discussions around here before, but hadn't shown anything. Until I uploaded the animation the other day, only a small handful of people had seen it. There's an attempt going on at MCB for and open source project, so I decided to finally share it.
It is a closed bolt spool valve (more like pressure controlled poppet, but debatable and largely semantics) that I designed a few years ago. The current working version is built around using an Ion body for ease and cheapness. I've got plans to build one from the ground up at some point, just haven't gotten around to it yet.
It works in a manner nearly opposite that of the prototype Legion Shocker and the Nova. Air enters through the side of the rear of the can (just like in an Ion). A normally open valve supplies pressure behind the valve piston, and when vented, the change in pressure bias pushes the piston back both sealing the dump chamber and opening the valve.
Focus in the design was reducing pre-expansion volume for the firing air, and a smooth flow path. I would say it was successful as I get almost 2,300 shots per 20 ounce CO2 tank from it, albeit at an unverified velocity, but more than fast enough to break paint in game just as reliably as any other gun shooting normal velocity and at a good speed for trajectory (so probably in the normally accepted velocity range). Testing over a chrono should happen shortly.
Current versions have the step that stops the firing valve in its forward travel on the piston portion, not in the power tube for ease of manufacture and eliminating the further restriction it caused (currently at .241" across). Getting that step in there was a pain. I've since made the cut out section between the 2 tail o-rings go further to the front (reducing the length behind the middle o-ring) in an attempt to let the chamber start filling faster and reducing shoot down. I haven't tested the functionality of this modification yet.
The rear plug can be moved in and out of the gun, and it very effectively alters the velocity. While backing it out as much as possible while maintaining functionality seems the obvious choice for efficiency, I'd like to experiment with devolumizing the dump chamber and jacking the pressures, where the adjustable restriction might prove useful in testing. My gauge, which is a standard Ion gauge, registers the pressure at just about 100psi while shooting 300FPS.
EDIT ---Video of the actual prototype shooting