First off, lets be clear, cartridge names do have some relationship with their bore size, but only some relationship, and the actual bore diameter may or may not be in the name.
Some rounds are named for their actual bore size (land to land) some for groove to groove diameter, in both English and metric systems.
Some are named for the size they once were, and are not now, because of technical advances. Some are named by their inventors, some, the marketing department. Some are named for other reasons, such as .30-06 which is the caliber and the year of US govt adoption.
Now, as to the 7.62x39 and the 7.62x51, both follow the usual metric practice of bore size x case length in mm. Both are 7.62 because they have the same diameter bore (.3")
The .308 Winchester is the .308 Winchester because Winchester created the name as the civilian sales name of the 7.62x51.
In general terms, the NATO round was developed to duplicate GI .30-06 performance. The ".30 Russian short" (7.62x39mm) was made to meet a lower velocity and bullet weight standard. Essentially the 7.62x39 does not quite meet the performance of the .30-30. Close, but not quite the same.
Quite enough for many uses, but NOT the same as the .308 Win, by several hundred feet per second and without the ability to use heavier (150gr and up) bullets like the .308 can.
Other than the .30 caliber bore size the two rounds have little in common.
Some rounds are named for their actual bore size (land to land) some for groove to groove diameter, in both English and metric systems.
Some are named for the size they once were, and are not now, because of technical advances. Some are named by their inventors, some, the marketing department. Some are named for other reasons, such as .30-06 which is the caliber and the year of US govt adoption.
Now, as to the 7.62x39 and the 7.62x51, both follow the usual metric practice of bore size x case length in mm. Both are 7.62 because they have the same diameter bore (.3")
The .308 Winchester is the .308 Winchester because Winchester created the name as the civilian sales name of the 7.62x51.
In general terms, the NATO round was developed to duplicate GI .30-06 performance. The ".30 Russian short" (7.62x39mm) was made to meet a lower velocity and bullet weight standard. Essentially the 7.62x39 does not quite meet the performance of the .30-30. Close, but not quite the same.
Quite enough for many uses, but NOT the same as the .308 Win, by several hundred feet per second and without the ability to use heavier (150gr and up) bullets like the .308 can.
Other than the .30 caliber bore size the two rounds have little in common.