The .38 S&W Special was originally designed as a blackpowder round for revolvers with a large capacity case. As a result, all early .38 Special revolvers were designed for the relatively low pressures generated by the blackpowder loads. In contrast, the 9mm round was designed for use in semiautomatic pistols (and later submachine guns). It has a much smaller case designed for smokeless powders and generally is loaded to higher velocities for military use - hence the higher pressures. SAAMI spec for 38s is 17,000 psi max pressure. SAAMI spec for the 9mm is 35,000 psi max pressure.