Just to add
A lot of european cartridges have names like 7 X 57 or 6.5 X 55. These mean the bullet dia. in mm (7mm) X the case length in mm (57mm).

Often if a cartridge came about as a "wild cat" (some one modified a cartridge by making the neck bigger or smaller [necking up or down] then loading a larger or smaller dia. bullet in the case) then the name will be (in North America) the new bullet dia. then the old or parent cartridge. As in 7mm/08 (a .308 necked down to take a 7mm bullet) or a .25/06.

Down here in Australia they use the old .303 british cartridge a lot. And they neck it down to .25 cal. but call it a .303/25. A favorite round for shooting kangaroos.

Some British rounds arenamed (parent cartridge)/(bullet or bore dia.)

Robin