The pistol cartridge that was never meant to be
During trials to adopt a new pistol for the German army, the Germans were very impressed by Georg Luger's toggle-locked design, which the Swiss had already adopted just after the turn of the century. They had one problem with it though: the calibre was a bottlenecked cartridge called 7.65 Parabellum, which was not considered to be enough gun. To avoid having to do any major retooling or redesign to his pistols, Georgy-boy simply produced a 7.65 Parabellum cartridge without the neck. This thus became 9mm Parabellum, and the only major modification necessary to the pistol was a different barrel - all the magazines etc. were unaltered.
That was in 1908.
The problem with taking an existing, well-designed necked cartridge then simply not necking for economy of design is that the end result is not necessarily well designed. The case capacity is not really large enough for a powerful 9mm cartridge. Thus, many military loadings used compressed charges, just to get enough powder in. When handloading 9mm Para, seating depth and consistent crimp are even more important than in other cartridges, due to the minimal or non-existant airspace over the powder. Inconsistency in the compression of the charge or the size of the airspace can lead to wildly varying velocities, and subsequent loss of accuracy.
And yet, despite these shortcomings, it has been consistently used in pistols and submachine guns ever since, and is indeed the most popular centrefire handgun cartridge of all time. It is available in loadings from 90gn to 150gn, and can just about make major in IPSC competition when loaded hot with a 124gn bullet.
In British and Commonwealth serivce it has been used in the Browning Hi-Power, L2A3 Sterling, F1 SMG, and THEY use it in the H&K MP5 & SIG pistols. In the French Foreign Legion, it was used in the Mle.1950 and MAT 49.