From Shooting Times:

In simplistic terms, the 9×23 Winchester is a stretched out 9mm Luger (pictured). Both are tapered cartridges with nearly identical neck, head and rim dimensions. The 9×23 case is 4mm longer, but more importantly the 9×23 operates at much higher pressure than the 9mm Luger. Winchester’s 9×23 brass has an extra-thick case wall that allows this cartridge to run at high pressure without concern of a case blowout in the unsupported region of a conventional, non-ramped barrel.