[FrankJ]

We use a 2-1/2" nominal (2.465" ID) copper pipe as our barrel. Tee shirts have to be carefully rolled and are still tight. It takes about 30 PSI to pop them out. Normal demos we shoot at 40-60 psi. We go up to 110 psi on the football field. We are a little unique in that we use a shooting accumulator that discharges completely on every shot rather than feeding the cannon directly off the storage tank.

You want your hose between the barrel and the air accumulator as big and short as possible. That will limit the cannon's range of motion. I would mount the fire valve on the cannon to keep the flow in the hose as high as possible. Tapping pipe threads in a block of steel is doable. Especially with working pressures less than 120 psi. 3/4 pipe is getting on the big side for hand taps though.