The 9x18mm caliber was the standard pistol caliber for Eastern European countries, many of which still use this caliber today. 9x18mm caliber uses a larger diameter bullet than other 9mm rounds. 9 mm Parabellum is 9.017 mm (0.355 inches), however 9x18mm rounds are loaded with bullets measuring 9.220 mm (0.363 inches). As a result, different bullets must be used to load 9x18mm cartridges.
9x18mm is ballistically inferior to the 9x19mm Parabellum cartridge. While there are no official SAAMI pressure specs for the 9x18mm cartridge, tests indicate that surplus ammunition develop pressures in the mid 20,000 psi, significantly less than the 35,000 psi or more developed by 9mm Parabellum loads. As such it is designed to be used in low powered blow back semiautomatics, much like the .380 ACP cartridge, rather than locked breech designs required for higher pressure cartridges like the 9 mm Parabellum.
The 9x18mm cartridge is not interchangeable with the more powerful 9 mm Parabellum, 9 mm Largo, and its ballistic performance is much closer to its little brother .380 ACP.