All the parts for this cannon are totally off the shelf starting with the re-purposed 50 caliber muzzleloader barrel. I am presently firing it with 250 to 300 psi air from a good quality 250 psi stirrup pump or my re-purposed 30# propane tank with pod pop-off protection at 315 psi. The 300 psi air is obtained by piggybacking an R-22 compressor onto a 120 psi shop air compressor. The lowest pressure rated component in the system is the Alpha QEV rated at max 250 psi with a burst pressure of 2 to 3 times that. With a properly rated homemade piston valve the operating pressure could easily be quadrupled. The energy is obtained by using 0.7 liter (from the gun stock aluminum tank) of 300 psi air for each shot. This means that the initial pressure available at the breech is 300 psi and the final pressure at the muzzle before the bullet exits is 268 psi for a uniformly accelerated bullet in a 24" barrel with very little decrease in initial firing pressure. No chrony data yet, just bullet splats of 1.25" diameter from a 182 gr. .495" round ball using a steel plate 3' from the muzzle. I can disconnect from the 30# tank for a remote shot or remain tethered for target practice. I also have a 1.5 liter HPA portable tank. It takes about 10 seconds to reload this gun. It's more fun than b***k powder. This gun is less than 8#'s has only 3 moving parts and is very sturdy. The barrel never needs cleaning. The tank size was chosen for it's comfort as a stock and the tank volumn was reduced with epoxy resin to 42 cubic inches to minimize air usage while still providing high mass air flow. The gun is loaded just like any muzzleloader. A 0.495" pre-lubed ball is just large enough to slide down the barrel with only the weight of the ramrod. A tap on the ramrod lodges the ball into the opening of the 3/8" breech nipple threads, (great for down hill shots). The QEV is an Alpha 5UPH4. The trigger is the 1/8" push button vent valve used as a pilot valve. This gun is also a fine arrow launcher and makes a pretty decent shotgun at 10 to 15 yards with a 180 to 200 grain shot load, (about the size of a 410 load). Videos of the gun can be viewed on my Youtube channel at: I am also shooting it with CO2 (stabilizer set at 400 psi).
http://www.youtube.com/user/mpmrla?feature=mhee
(Click on each pic to enlarge)
This is a pic of the finished cannon with cord wrap epoxied in place
and a red dot scope added. The Trigger is the 1/8" push button valve
( "M" in the diagram) that is plumbed toward the muzzle. It works great as a trigger and requires an increase in pressure to trigger the gun if you increase the operating pressure.
This is a pic of the cannon compared to a Gamo viper and a Beeman 0035
In this sketch of the gun all parts on the gun are laid out in the same
orientation as the sketch. The trigger is "M" and the fill valve is "k" (which can also be a foster nipple). Also, I have since added a 2" extension between parts "G" and "F" to place the trigger at a more comfortable distance. This 2" extension can be a tee so that by adding another vent valve you have a way to depressure the gun without shooting it.
182 grain bullet splats compared to a .22 cal and a .177 cal pellet. The
bullet splats indicate about 400 to 500 ft/sec muzzle velocity.
In this pic a 364 grain homemade slug is compared to a 182 grain ball
and the slug splat on the right.
I need some pointers 
