There is some confusion about high base and low base shotgun shells. Low base have more room for powder, wads and shot. High base (it refers originally to the fiber insert at the base of the shell) is the designation for lighter loads. The confusion comes from high brass and low brass, which once differentiated between lighter and heavier loads. The bases in modern plastic shells are molded as part of the shell, and I don't think there is a difference between light field/target loads and express loads. In the old days, of paper shells, the brass was supporting the body of the shell; nowadays, the brass is to prevent tearing the plastic during extraction. The plastic is plenty strong to accommodate the pressure, but extraction, particularly with semi-automatics can be rather violent.