thats ingenuity. My take on that is The bullet was built around the casing with 9mm. the casing was built around the bullet in 357 sig.Georg Luger developed his 7.62x51mm Parabellum from the earlier 7.62x25mm Borchardt. Germany requested a larger caliber for their military sidearm, so Luger removed the bottleneck shape of the 7.65×21mm Parabellum case, resulting in a tapered rimless cartridge encasing a bullet that was 9mm in diameter. 9mm wasn't chosen as the bullet caliber, it's just what it worked out to be.