frizellr has provided the meat of the answer to your question. To this I would add a few general comments: The Europeans are a disciplined, intelligent and systematic lot. There is a general standardized pattern to their small-arms cartridge nomenclature. The first number is the bore size in millimeters, the second numer is the case length in millimeters, and, if the round has a rimmed case, they add an "R" at the end of the designation, such as 7.62X54R, (the rimmed full-power Russian military cartridge).

In the U.S. today, and also to some extent in England, there are no rules for cartridge nomenclature AT ALL, although we did have a sort of system in the 19th Century. Everyone who develops a cartridge design names it to suit himself! In the 19th Century, many designers named a cartridge by giving the nominal caliber, the weight of the powder charge, and often, weight of bullet. IE, .45/70/500, a .45 caliber round holding 70 grains of powder and shooting a 500 grain bullet. Still, there could be straight or bottlenecked .45/70/500's, which were obviously different!! However, today, no such system applies in this country! We often (but not invariably) name a cartridge by caliber, either in the metric system or English system, sometimes by bore diameter, sometimes by bullet diameter, then often add the name of the originator, such as 6mm Remington or .244 Remington (both of these apply to the EXACT SAME CARTRIDGE), or give the caliber and date of introduction, such as .30/'06 for a .30 caliber introduced in 1906, etc. But, sometimes we tack on a muzzle velocity, such as .250/3000 Savage! I believe it is safe to say that in the U.S. today, we have no system at all, and you have to become familiar with a whole lot of different cartridges to know which is which!! For example, there are a lot of very different rounds known as .300's, both Magnums and the .300 Savage, which is certainly no magnum!! HELP!!! The British are about as bad off as we!! To really know your cartridges, buy a copy of Barnes" Cartridges of the World (COTW), and really study it! It is interesting a hell, to say the least!!

Keep yore powder dry!!