Found this 9mm Luger ammo vs 9mm

[Kyrie]

Originally Posted by DonVoigt

Ok,
great references.
But the 9mm does headspace on the case mouth, no matter what sammi "x'ed".
Just as soon as one has a minimum case in a maximum chamber, the "x" moves immediately to the case mouth.
The "x" only enters the equation in the imaginary world of engineeers. JMHO

Don is generally correct, if a little off conceptually.

"Headspace" is a characteristic of a firearm's chamber, and not a characteristic of firearm's cartridge.

In a 9 mm Luger barrel that is properly dimensioned with a correctly cut chamber the headspace datum line will be located in the chamber between .754 and .776 inches from the standing breech when the firearm is in battery.

A correctly dimensioned 9 mm Luger cartridge will have a case with an LOA (case head to case mouth) between .754 and .776 inches in length.

The verbal shorthand for those two paragraphs is "A 9 mm Luger cartridge takes its headspace off the case mouth."