[FrankJ]

We are using a 2-1/2" copper tube. Roughly 2-1/2" ID. About 24" long. Insertion depth past a minimum doesn't seem to have a big impact on distance fired. 16 psi won't push the tee-shirt out of the tube. Our shoot pressure ranges from 30 to 60 psi for normal shots. Air storage pressure is 110-120 psi with a separate shooting accumulator that is fully discharged on every shot. We adjust the shooting accumulator pressure to control the shot distance. Cannons with 3" barrels will shoot with lower pressure. How tightly the tee shirt is wrapped will greatly affect distances.

A couple of factors hard to calculate would air drag on the tee shirt and force required to move the tee shirt in the barrel. (Barrel drag)