The 9mm Steyr is a centerfire pistol cartridge originally developed for the Steyr M1912

Steyr M1912

The Steyr M1912 was developed in 1911 by the Austrian firm Steyr Mannlicher by Karl Krnka, based on the basic operating system of the Roth-Steyr M1907. It was developed for the Austro-Hungarian Army and adopted in 1912 as the M1912...

 military pistol. The cartridge headspaces on the mouth of the case. This was the service ammunition for Austria-Hungary

Austria-Hungary

Austria-Hungary , more formally known as the Kingdoms and Lands Represented in the Imperial Council and the Lands of the Holy Hungarian Crown of Saint Stephen, was a constitutional monarchic union between the crowns of the Austrian Empire and the Kingdom of Hungary in...

 during World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

 and remained the service ammunition for Austria

Austria

Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...

 and Romania

Romania

Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...

 between the World Wars. It is dimensionally similar to, but not interchangeable with the 9×23mm Largo. The cartridge performance is close to that of the .38 ACP

.38 ACP

The .38 ACP also known as the .38 Auto was introduced at the turn of the 20th century for the Browning designed Colt M1900. The cartridge headspaces on the rim. It had first been used in his Model 1897 prototype, which Colt did not produce...

. It is unrelated the modern 9×23mm Winchester

9x23mm Winchester

The 9x23 Winchester is a pistol cartridge developed by Winchester repeating arms. 9x23mm has a long and convoluted development history, but was commercially introduced by Winchester in 1996...

.