What does caliber measure and how does that transfer to the mm x mm system?

Question

What does caliber measure and how does that transfer to the mm x mm system?

Answer 1

Caliber measures the diameter of the projectile, 7.62 = .30 caliber, 5.56 = .223

Answer 2

I'm going to reuse some of what I posted last time someone asked what caliber is.

(Q1.What does caliber measure?

(A1. Quoted Answer from previous post on this topic:

Caliber is simply the diameter of the bullet, expressed in hundredths of an inch, thousands of an inch, or in mm. For example, a .22 LR is nominally .22 hundredths of an inch in diameter. A .303 English is nominally .303 thousandths of an inch in diameter. A 9mm is 9 millimeters in diameter. In your example, both bullets are nominally .38 hundredths of an inch. The reason I included the nominally in the explanation is that the actual diameter can vary a bit depending on where and how the measurement is taken. For example a .38 Special is actually .357 thousandths of an inch, a .22 LR is actually around .223 thousands of an inch in diameter and a .44-40 is actually .427 thousandths of an inch in diameter.

(Q2. how does that transfer to the mm x mm system?

(A2. In the metric system of cartridge nomenclature, the first mm represents the diameter of the bullet and takes the place of the "English" measurement we use to denote caliber. The second mm describes the length of the shell. So a 7.62x39mm cartridge fires a bullet 7.62 mm in diameter and has a case 39mm long.

Answer 3

Caliber is a rough approximation of bore diameter, but you'll drive yourself crazy if you expect exact correlations, and the names are often purposefully off to avoid confusion with others of similar dimensions.

For one, there's the question of whether to measure between the lands, or between the grooves. A .300" (7.62mm) bore from land to land is also .308" or .309" (7.85mm) from groove to groove.

And you'll find lots of youngsters who don't know any better telling you silly stuff about .223" stuff, simply because that's the designation of a popular cartridge, the 223 Remington. But they developed that cartridge in a day when the 222 Remington was extremely popular, and the naming was for convenience. In fact, with a couple of exceptions, the 22 caliber rifles all shoot .224" (5.7mm) bullets, whether they're named 218 Bee or 225 Winchester.

If you really want to know all the minutiae for a wide variety of cartridges, you'll have to do some research on each individually. The best single source I know of, though far from complete, is the Vihta Vuori handloading manual, which gives CIP dimensions in both English and metric systems.

Answer 4

Caliber is essentially the diameter of the bore in hundredths of an inch....the metric measure does the same using a different scale.

See the chart.

Answer 5

The standard in the united states is that the caliber is the diameter of the bullet i.e. the caliber .308 is .308" in diameter. However it is sometimes misleading. For example the .38 Special  in fact has a bullet diameter of .358".

The metric standard is different in that it has two dimensions, the first being the bullet diameter in millimeters and the second being the case length. It is a much more effective system as it makes it easier to approximate power and sizes of cartridges in your head. The 7.62x51mm (.308) is obviously going to be a much more powerful round than the 7.62x33mm (.30 carbine) or the 7.62x39mm (the AK round), be about the same as the 7.62x54mm (the rimmed SVD/mosin nagant round), and less than the 7.62x63mm (.30-06)... you could go on and on with this...  7.62x25mm (Tokarev), 7.62x38mm (Nagant revolver), 7.62x67mm (.300 WSM also known as the 7.62 magnum), 7.62x56mm (.303) etc. In the caliber standard, they would all simply be .30 caliber or some variation. The metric nomenclature is much more accurate and useful.