This is turning out to be quite an interesting build. I was going through your parts list and I noticed that both valves you listed are 1/4" npt, I believe all of the air cannons I have seen have used 1/2" npt for volume. Did your design end up requiring less air volume, or was it for ease of plumbing with the rest of your build?
I may have to move the valve from the tank to the launcher. The fitting for all the air lines is 1/4" so we will see if there is enough volume when I get this all together. I may have to get a bigger valve.
Today I got all the wiring done, minus any sound cards, and IT WORKS! The delay timer is set as low as it can go so, one second blast of air is released from the secondary air tank that refills as soon as the trigger button is released. I'm working on the cannon part now. It is a 12 " long 1/2" iron pipe with one set of threats cut off. There is a brass fitting that goes down to 1/4" so it can be connected to the air hose to the tank. Finally making progress.