Here it is in action at 110 psi:
I hooked up a 12 inch 6mm barrel and did some chrony testing at 120 psi. The firing chamber volume is around 3.5 cubic inches, a little wasteful but that's what I had
Using 0.25g airsoft BBs, I got an average velocity of 590 fps, more than enough to punch through a soup tin. With some adapted 1 gram 0.22" pellets I got an average of 355 feet per second, fairly decent.
In short, even at relatively low pressures with a relatively short barrel, this valve setup packs quite a punch. For say a field legal airsoft sniper, if you fit a longer barrel you could get away with much, much less pressure, a regulated main chamber would last a long time.
Conversely, increasing barrel length and/or calibre, and upping the pressure, you have enough power available to do some serious damage. I might do some testing with steel BBs and marbles down the line.
Some photos, my test setup with Combro CB-625 chrony at the end of the barrel, and a mangled soup tin.
The items needed are illustrated below.
Note the sticker describing the three ports:
"P" denotes the pilot port, where the slide valve should be attached. Your air supply/main chamber should then be attached to the other end of the slide valve. In this case I hooked it up directly to my compressor. This port is compatible with 1/4" BSP.
"A" is where the firing chamber should be attached. For each shot, all the air that is in this chamber will be emptied. In this case I made a small chamber from a section of PVC pipe with epoxy endcaps, with a 1/4" socket screwed into one end to attach to the QEV. This port is compatible with 1/4" BSP.
"R" is where the barrel or magazine should be attached. This port is compatible with 3/8" BSP.
A note on thread sizes:
While NPT threads are common in the United States, BSP threads are widely used in many other countries.
WARNING: Never, never try to mate a BSP fitting with an NPT or NPS fitting if the pressure holding capability is at all critical.
NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most diametrical sizes have a different pitch. NPT/NPS threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form); BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).
The threads are weirdly described on eBay but in my experience so far it's safe to assume that if the threads have a diameter similar close to 1/2" or 12.7mm, they are probably 1/4" BSP.
US members should be able to find adapters for BSP parts easily.
Some 3D rendering courtesy of dewey-1:
Note that moving the slide valve away from the QEV fires the valve, and also blocks off the air supply, making for a very efficient system.
Some modifications to make the valve more ergonomic are adding some sort of trigger to the collar to allow it to be pulled with a single finger as on a conventional gun, and a spring to return the collar to its "rest" postion when the trigger is released.