The 9×23mm Largo (9mm Largo, 9mm Bergmann-Bayard, 9mm Bayard Long) centerfire pistol cartridge was developed in 1901 for the Bergmann Mars pistol. The round was considered powerful for the day, producing a muzzle energy of between 330 and 430 ft·lbf (450 and 580 J) depending on the loading. A number of small changes to the Mars and the cartridge were made and the pistol that eventuated was called the Bergmann-Bayard 1903.
This pistol was adopted by the Spanish army in 1905 as the Pistola Bergmann de 9 mm. modelo 1903. Unable to find a German manufacturer to complete the Spanish order for 3000 pistols, Theodor Bergmann turned to a Belgian manufacturer, Anciens Etablissements Pieper (who used the trademark "Bayard") to complete the order. The final pistol, modified by AEP, was known as the Bergmann Bayard 1908, or in Spain as the Pistola Bergmann de 9 mm. modelo 1908. Although adopted in 1908 first deliveries did not take place until two years later. Meanwhile other manufacturers such as Campo-Giro had adopted the 9mm Bergmann-Bayard round and, due to its long history of use in Spanish submachine guns, carbines and pistols, today it is most commonly known as the 9mm Largo.
At the same time the Bergmann-Bayard model 1910 semi-automatic pistol was adopted by the Danish military and remained in production until 1935.
Left to right: 9×23mm Largo, 9×19mm Parabellum, 9x23mm Winchester, and 9×23mm Steyr.
The cartridge headspaces on the mouth of the case. With 125 gr (8.1 g; 0.29 oz) jacketed projectiles, the muzzle energy is 336 foot pounds, slightly lower than a standard-pressure 9mm Luger. Compared to the 9mm Luger +P, performance is also lower but at lower pressure in the 9×23mm Largo. While external dimensions are almost identical, the 9×23mm Largo is a very different cartridge from the modern, high-performance 9×23mm Winchester. Firing the thicker-walled 9×23mm Winchester round in a 9×23mm Largo pistol is dangerous, as old 9mm Largo pistols cannot handle the pressure generated by the 9×23mm Winchester. Performance is similar to the contemporary 9×23mm Steyr, but the cartridges were developed independently and their dimensions are just different enough to render them non-interchangeable.