1. .303 British
2. 6.5x55mm Swede
3. 7x57mm Mauser
4. 7.5x54mm French
5. 7.65x54mm Argentine
6. .30-06 US Springfield
What do these numbers mean, like three-oh-three and seven-by-fifty-seven?
However, some standard calibres are expressed as metric (mm = millimeter), while others (US and UK) are expressed as a decimal fraction of an inch, usually in hundredths, e.g., .30 calibre is just under 1/3 inch, or in thousandths, e.g. .270 Winchester. For a metric example, the Argentine Mauser ammunition (7.65x54) is designed for a rifle barrel bore diameter of 7.65 millimeters, and the chamber is precisely sized to fil a cartridge case of 54 millimeters length. Notice cartridge numbers 4 and 5 in the picture above? Exact same length of brass case, but number 5's projectile appears to be just a bit fatter (.15mm to be precise).
But that second number is not always the case length, especially in inch calibres. As "inch" examples, the standard UK calibre for more than 50 years was .303 British, referred to as "three-oh-three British," and the US cartridge of nearly the same duration, the familiar "thirty-aught-six" Springfield. This is still slightly different, in that the -06 refers to the year (1906) of cartridge design, approval, and selection as the new standard US rifle ammunition.
To add more confusion, the Springfield's immediate predecessor US rifle, the US Springfiield Model 1896 ("Krag"), used .30-40 ammunition, also referred to as .30-40 Army,.30-40 Government, .30-40 Krag, and just "Krag." In this usage, the -40 refers to the number of grains of gunpowder that was the standard load per case.
NOTE: The US Springfield Model 1896 was and is still commonly referred to as a "Krag" or "Krag-Jorgensen," from the last names of the rifle's two Norwegian designers. However, although the US chambered their Krags in .30-40, the Norwegian-Swedish (at that time a union of two countries) joint army ammunition selection board had recently designated 6.5mm as the standard cartridge for their ammunition. So any Krag rifles in 6.5 are legitimate, but rarely seen in the USA.