Rambling Thoughts on the 9x19
by Greg Ritchie

The 9x19 Parabellum is the offspring of the 7.65x21 Parabellum, which is the offspring of the 7.65x25 Borchardt, which is basically the 7.92 x57 M1888 Commission Rifle round scaled down for handgun use.

[7.65x21 Parabellum - Wikipedia]

The 7.65x25 Borchardt, which was jointly designed by Hugo Borchardt and George Luger, and chambered in the C96 was meant to be a military sidearm but was rejected because it was too unwieldy The 7.62x21 Parabellum was developed out of the Borchardt and in 1898 the pistol we know as the 1908 Luger was patented. The Swiss adopted this handgun in 1900.

Still, Germany was not happy with the 7.65 caliber and asked for a 9 millimeter. Georg Luger, accustomed to bottlenecked cartridges to adjust headspace worked with a bottle necked version and was not sucessful. George came upon the notion of headspacing off the case mouth and simply tapered the case down to accept a 9mm bullet and found success! This is the real reason that the 9x19 Parabellum is tapered, not for easier chambering or extraction, even though they may be happy side effects, but just because that was the final dimensions of the 7.65 x 21 shortened to 19mm and tapered down to 9mm!

I also find it interesting to note that John M Browning used the rim to head space his cartridges, the 25acp, 32acp, and 38acp all used a straight simi-rimmed casees and not until 1904 when he patented the 45acp did he use a tapered (albeit only a .003 taper,)rimless case that headspaced on the case mouth. Some 2 years after George Luger submitted a 9x19 to the US for trials.

In 1904 the Imperial German Navy adopted a version of the Luger known as the Pistole 04 (P-04) and in 1908 the German army replaced their M1879 Reichsrevolver with the Pistole 08, the famous P-08 Luger. Thus the birth of the most popular handgun round in the world today!