The heart of the air cannon I built was the valve. I used a solenoid operated sprinkler valve. Since it is a diaphragm valve it opens almost instantly when triggered. This is important as if it opened slowly (like a hand valve) it would let the air out slowly and not provide enough propellant force to the paintballs out of the barrel at a decent velocity. The valve I chose was rated up to 125 psi, but I always used just 100 psi, it did the trick, and I saw no need to go any higher. Another option I chose was a back check valve. This isn't really necessary, but since it hangs open normally, it makes muzzle loading easier. The real reason I chose the model with a back check valve was it meant I didn't have to mess with other adaptors and elbows, the valve outlet was pointing 180 degrees from the inlet, and spaced just right for an air cannon! You may notice the silver metal box in the above picture. I put that over the solenoid to protect it from getting banged around, which would strip out its threads.