Originally Posted by grit
Right idea. 7mmm-08 is the 308 case necked to 7mm. 7mm is the .284 bullets. 270 is .277. I know .277 is closer to 7mm. One of those confusing things...
It's less confusing when you realize they're all just names to differentiate the cartridges. There are no legal rules which govern the names other than trademark and copyright laws. Cartridges which shoot .224 diameter bullets have names ranging from 218 to 225 with 2 or 3 digits. The number may be from less than the bore diameter to larger than the bullet diameter. Some names try make the cartridge seem more impressive. A 38 Special shoots a .357" bullet. A 44 Magnum shoots a .429 diameter bullet. "Magnum" means very little. A 256 Win Mag is a tiny little case with 22 grains capacity while a 264 Win Mag has 82 grains, huge for it's caliber.
When there are two numbers the second number can be the bullet diameter of it's parent case, like the 30-378 Weatherby or 7mm-08, a year it was introduced (to distinguish the the 30-06 from the 30-03) the year of the parent like a 6.5-06, a black powder case capacity like the 45-70 or a smokeless case capacity like the 30-30. Maybe other things.
Metric cartridges like the 5.56x45 are usually the bore diameter and the case length in millimeters rounded off though there are exceptions when numbers pairs are already used.
Many cartridges get their names simply to promote marketing. The 50 Beowulf is a catchier name than the now discontinued 499 Leitner-Wise, though they're very similar and use the same bullets. Sometimes a cartridge gets renamed. The 244 Remington was renamed the 6mm Remington. though they are the same case other than the headstamp. Rifles marked to shoot the 6mm usually but not necessarily have a faster twist. The Remington 280 (a 30-06 necked down to shoot .284 bullets) was renamed in 1979 to 7mm Express to help sales. It didn't and caused confusion with the 7mm Remington Magnum, so Remington renamed it back to 280 Remington in 1981. Several cartridges have two names for the European and American markets.
They're still just names. It's important to use the right name particularly when buying ammo. Also when posting on the web so others will know what your talking about.