[MrForbes]

Let's assume that I'm trying to figure out what the actual maximum pressure is, and that T shirts don't "jam" in the barrel...at most it takes about five pounds of force to move the shirt thru the barrel. Since the barrel bore has about 5 or so square inches of cross sectional area, the shirt will begin to move when the pressure in the barrel exceeds about one psi. The actual pressure probably depends on the mass of the shirt, and how fast it accelerates, how fast the valve opens, and all kinds of other interesting dynamic things that would be pretty hard to calculate.