[AlexDatig]

These are so-called lines ( http://en.wikipedia.org/wiki/Line_(length) ), tenths of an inch. So 7.62 is supposed to be 0.3, but different cartridges of similar calibre have often slightly different diameters. If you develop a new cartridge with a new length and internal structure, why care about the perfectly equal diameter?

[teodaschwartia]

7.62 mm by 51 mm is a .308. .308 Winchester is roughly the same as the 7.62 NATO. So he's partially right.

[boizzones]

7.62 mm by 51 mm is a .308. .308 Winchester is roughly the same as the 7.62 NATO. So he's partially right.

Then somebody needs to work on their math. 7.62/25.4 = 0.300

[GenryDont]

25.4 * 0.3 = 7.5 + .12 = 7.62

Looks right to me. 0.300

[indocrew]

7.62 mm = 0.30 in, 7.65 mm = 0.31 in, yet the 7.62x57 and 7.65x53 slugs can actually be used almost interchangeably.

.30 carbine, 30-06, .300, .301, .303, .307 and .308 are all much the same in diameter, they are given different names by convention. Some were originally designated by the size of the bullet itself (I think this is the case for the .308). Some were originally designated by the intended barrel design across the lands, which is necessarily smaller than the bullet size. That may be the case for the .303. The 30-06 was named for the 1906 Winchester rifle for which it was designed.

When bullets are made of different materials by the same manufacturer (simple lead ball vs copper jacketed) the bullet size is slightly different to get the same ballistic performance despite the difference in the way the metal will register in the rifling. Each manufacturer's bullets may be of slightly different size, and match shooters will swear this round works better with this rifle make but not with another rifle make, etc.

[mtvlover571]

30-06 was the standard when the garand was designed (between mid 20's to mid 30's). 7.62 NATO was developed in 1950's and adopted by the US for use with the M14.

[dxpfmP0l]

Shape of the bullet matters a great deal, both to match the bore of the barrel, and to minimize air resistance. Round balls haven't been used since the mid 19th century, right around the time that mass produced, interchangeable parts became standard.