I decided to start with a relatively benign acronym today. The abbreviation standing for Millimeter which is what one of the most common calibers for handguns is measured in.
Developed in Austria by Georg Luger in 1901, the 9mm Luger or 9mm Parabellum is by far the most readily accessible handgun and cartridge in the United States. (8 out of 10 guns. com 2021 Top ten selling handguns)
Here’s what I found. Basically, bullets designed in any country other than the US would be measure in millimeters.
Metric diameters for small arms refer to cartridge dimensions and are expressed with an “×” between the bore diameter and the length of the cartridge case; for example, the 6.5×55mm Swedish cartridge has a bore diameter of 6.5 mm and a case length of 55 mm.
Barnes, Frank C. (2016). Woodard, W. Todd (ed.). Cartridges of the World: A Complete and Illustrated Reference for More than 1500 Cartridges (15th ed.). Iola, Wis.: Krause Publications. p. 9. ISBN978-1440246425. OCLC934886116.