Also called a 9mm luger and a 9mm parabolic chamber pressure is significantly higher than a 9×18, commonly used in Eastern Block Makarov pistols or similar 9×18 ultra-round pistols.
What is the difference between a 9×18 and a 9×19 ammo?
One cartridge of 9x18mm ammunition is 18.10mm long; a 9x19mm shell is 19.15mm long. nine itself represents the bullet’s diameter, but it’s not entirely accurate. 9x18mm has a 9.27mm diameter bullet; 9x19mm has a 9.01mm diameter bullet. …mainly to discourage enemy forces from supplying them with ammunition!
The 9x18mm was designed for an inexpensive blowback pistol, where the recoil spring only holds the chamber. This type of operation can only accommodate such a decisive round, and in fact, 9x18mm is the upper limit of a blowback action it can accommodate.