So, here she is:
As you can see I used an under/over design similar to that of Desert Storm. This allowed me to have the barrel and chamber much closer together for a more compact gun. I also just really liked the design. I know the pilot volume is big, it's that far back to give the ball valve handle some room, and to make aiming easier.
The ammunition:
My main ammo is 13.5mm ball bearings that fit perfectly in the barrel.
I also use a crude dart made from a bolt, some nuts and electrical tape which fits surprising snugly in the barrel.
Now for the all important damage shots
Yellow pages shot from five meters with a ball bearing at about 70PSI.
Over 600 pages of penetration.
The table seen in other pictures shot with a ball bearing (left) and bolt (right). The BB passed right through like it was butter leaving a huge exit wound. The bolt didn't completely exit, stopped by it's large back. These were also both at 70PSI.
All for now, I'll get more damage shots when I crank the pressure up.
That should be as soon as I can find a decent backstop. The destructive power at a mere 70PSI has shocked me somewhat.
you can get away with a much smaller chamber, typically a ratio of 1:1 chamber:barrel volume is a sufficient balance between power, noise and efficiency, and as was pointed out above, the smaller the volume behind the QEV the faster it will open.
... Good job on a first cannon thought and please do some high pressure tests on this cannon 
Why such a long pilot tail piece at the rear?