It's 10mm (about 0.39") calibre, and works like an exhaust valve piston, in the sense that it's a barrel sealer, which is actuated by the air in the chamber pushing a piston back to allow air to escape through the barrel. The different is that instead of being held against the barrel by air pressure in a pilot chamber, the piston is held in place by a latch that doubles as a trigger. The piston in this case was made by cutting down and reinforcing the plunger of a 20mL syringe.
The advantage of this system is that actuation is instant when compared to having a pilot chamber, though of course you're limited to the amount of pressure you can use before it begins to leak, I rarely went above 100 psi. also, the piston acceleration is very violent and needs a bumper to keep it from flying out and hitting you in the face.
You might note the amount features I added, like an integral suppressor that is very effective, and the very ergonomic handle (that came off a toy shotgun) as well as the detachable shoulder stock. I also fitted open sights to assist with aiming, I made the rear sight adjustable using a bolt and three nuts - basically, the "notch" in the sight is the space between the two central nuts, that's adjustable for windage (left and right) by simply turning the bolt clockwise or anticlockwise.
For more detailed information on using epoxy, check out the how-tos here and here.
