The 9mm Steyr is a centerfire pistol cartridge originally developed for the Steyr M1912 military pistol. Adopted in 1912, this was the service ammunition for most branches of the military in Austria-Hungary during World War I and remained the service ammunition for Austria, Romania and Chile between the World Wars.^[2] Some MP 34 submachine guns were also issued in this caliber in addition to 9mm Mauser. When the Austrian Army was incorporated in the Wehrmacht in 1938 following the Anschluss, many M1912 pistols and MP 34 submachine guns were rebarrelled to 9×19mm Parabellum for standardization purposes.

It is similar to the 9×23mm Largo cartridge in performance, but their dimensions are just different enough to make them non-interchangeable.^[3] The cartridge headspaces on the mouth of the case. Its performance is close to that of the .38 ACP. It is unrelated to the modern 9×23mm Winchester.

Reloadable, Boxer-primed cartridge cases can be made from 5.56x45mm NATO brass. This requires inside neck-reaming, as such a conversion would otherwise leave unacceptably thick mid-to-rear case walls from the original cartridges to form the mouths of the new, shortened cases. RCBS apparently can produce such a forming die set, complete with reamer. Loading data would be much like .38" ACP.