why 7.92mm and not 8mm?

One reason that I can give is you do not want the enemy to capture your ammo and be able to use it. Do you have an odd size.

7.92x57 was the standard rifle caliber for the Germans in World War 2, which they used in their Mauser rifles and MG42.

Up until after WW2, there were all sorts of crazy rifle calibers out there because almost every country built and produced their own weapons. There was no NATO or Soviet standard at the time. For that reason, even the smallest measurements have to be properly annotated.

There are two types of countries in the world - Those that use the metric system, and those that landed on the moon. So stop being a loser and get off the metric system.

7.62 mm is 30 caliber (0.30 inch diameter). Just that simple. When I was in the Navy, I worked on the weapon system for the 5 inch guns. Guess what those pansy Europeans called it? 127 mm gun.

There's nothing specially about 7.92mm because there is no such thing (at least since WWII).

It's 7.62. SIX, not nine. 7.62mm happens to correspond very nicely to .30 caliber.

For your reading pleasure:

https://en.wikipedia.org/wiki/7.62_mm_caliber

Think of bullets as print ink cartridges when it comes to profit making. As a firearms maker, you want to make money off of selling the ammo too so you design a caliber or cartridge that only you provide.

it is probably a metric conversion of a non metric standard

What's so special about round numbers?

7.92 mm bullets are used in many of the machine guns!

Depends how accurate you want the number to be. For example when building something - you want some fairy exact measurements.