Some cartridges like the .30-40 Krag are named for their bore size, .300 and their capacity in powder 40 grains. Some like the .30-06 have the size of the bore and the year of adoption, 1906. Some use the size of the bullet rather than the bore size. The .250(bore) Savage and the .257(bullet) Remiington Roberts are similar cartridges that use the same bullet. Metric cartridges use the size of the bore or bullet as the first number and the length of the empty case as the second, the 6.5x55 was a cartrige using a 6.5 mm diameter bore with a case 55 millimeters long. The British were for a long time insistent on naming cartridges for the size of the bore when the cartridge was actually a metric cartridge from some European company i. e. the .276 Rigby is actually the same as the 7x57 Mauser. In other words there is no rhyme of reason to these names but after awhile you'll be able to make a reasonable stab at the size of the bullet and perhaps the capacity of the case, just by reading the name.