A barrel (14)for a gas powered ball machine of the type used to simulate balls propelled whilst playing ball sports, especially those machines used to propel balls with a raised seam, such as a baseball or softball. The barrel (14) has one or more openings (160) along its cylindrical length that allow expanding gas to escape as a ball is fired from the machine. The openings (160) assist in predicting the pressure and volume of gas required to repeatedly propel a ball at a predetermined speed from the ball machine.