6.5mm?

[keeki]

just wondering where do they get this meassurement? was reading an article comparing several cartridges in 6.5 and the 25-06 was one of the cartridges in the article. the article went on to say that a 25-06 wasnt a true 6.5mm? I dont know about their math but where i went to school .257 is 6.5278mm which makes it the only 6.5 in the comparison. Can anyone shine some light on this for me because im confused?

[earl39]

Not having a real answer i ran the measurements and this is what i came up with. Hope you can follow what i am saying.

Rifles using standard measurement (ie..foot,inch,ect) get caliber from groove diameter.
Rifles using metric measurement get caliber from top of land diameter and have a standard groove depth for easy uniformity.
WHAT THEY ARE / WHAT THEY SAY
.223=5.66 ..................5.56MM
.243=6.18 ..................6.00MM
.264-6.71 ..................6.5MM
.308-7.84 ..................7.62MM
.323=8.21 ..................8.0MM


DISCLAIMER
Remember this is just what i figured out on my own and has absolutely no bases in any provable fact that i can find nor did i do more than a passing bit of research on the subject.

Gary

[earl39]

Went back and did a little more looking and found this on wiki.

In guns including firearms, caliber or calibre is the approximate internal diameter of the barrel in relation to the extension of the projectile used in it.

In a rifled barrel, the distance is measured between opposing lands or grooves; groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere. It is important to performance that a bullet should closely match the groove diameter of a barrel to ensure a good seal. When the barrel diameter is given in inches, the abbreviation "cal" is used in place of "inches." For example, a small bore rifle with a diameter of 0.22 inch is a .22 cal; however, the decimal point is generally dropped when spoken, making it "twenty-two caliber" or a "two-two caliber". Calibers of firearms can be referred to in millimeters, as in a "caliber of eighty-eight millimeters" (88 mm) or "a hundred and five-millimeter caliber gun" (often abbreviated as "105 mm gun").

While modern cartridges and cartridge firearms are generally referred to by the cartridge name, they are still lumped together based on bore diameter. For example, a firearm might be described as a ".30 caliber rifle", which could be any of a wide range of cartridges using a roughly .30 inch projectile; or a ".22 rimfire", referring to any rimfire cartridge using a .22 caliber projectile.

In some contexts, e.g. guns aboard a warship, "caliber" is used to describe the barrel length as multiples of the bore diameter. A "5-inch 50 caliber" gun has a bore diameter of 5 inches (127 mm) and a barrel length of 50 times 5 inches = 250 inches (6.35 m).



Gary

[keeki]

I understand that caliber is thousanths of inches and is the bore of the barrel and the diameter of the bullet. But when you measure a 6.5mm bullet it measures .264 which is 6.7mm. when you measure a .257 bullet you will get 6.5mm. I was just wondering why they didnt call a 264 a 6.7mm which is what it is instad of a 6.5.

so are you saying that the lands of a 264 barrel is 6.5mm and the grooves are .264?

[earl39]

I was just wondering why they didnt call a 264 a 6.7mm which is what it is instad of a 6.5.
Because the round was developed and named in the United States which uses the imperial measurement standard.



so are you saying that the lands of a 264 barrel is 6.5mm and the grooves are .264?


You seem to be getting the ideal. Within manufacturing tolerances.