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Old 07-10-2013, 05:12 AM   #1
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Default True understanding of calibre?

To me, it doesnt matter who is describing it (I guess I only have my friends opinions and wikipedia) but I just don't understand what a calibre really is. For example, what something like .270 really means, especially when it comes down to something like .30-06 or 7.62x39. Does it have to do with grains, length or width of bullet?



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Old 07-10-2013, 05:41 AM   #2
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It is quite simple really. Caliber is measured by two different scales (as is everything) you have your standard and metric.
5.56mm=.224 caliber
6mm=.243 caliber
6.5mm=.264 caliber
7.62mm=.30 caliber and so on
Caliber is nothing more then the measurement of the width of the bullet. There are often many cases of different sizes in the same caliber such as the 30-30, 30-06, 300 Win Mag, and 7.62x51 which are all 30 calibers but with different cases providing more or less powder charges behind that 30 caliber pill.
Grains is nothing more then the measurement of the weight of the bullet NOTHING ELSE like any other measurement of weight you can convert it to oz, lbs or kg.
Hope that helps, feel free to ask anything else you like. Oh and not to confuse you but a 270 is actualy .277" sometimes the factory name is not an exact measurement.



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Old 07-10-2013, 06:12 AM   #3
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Calibers are determined in so many different ways and for historical reasons it is mind shattering. The 30-06 a 30 caliber adopted for our military in 1906. There are many 30 calibers this one happens to be .308 nominal. The .270 is a bore dimension firing a .277 caliber bullet. Many metric designations include the case length and bullet diameter and or first boring of the unfinished bore. The 7.62 X 39 refers to a .311 bullet in a 39 MM length case.
It is very confusing and no set rules. An American .44 caliber weapon is a a .43 caliber a 45 caliber can be a .450,.451,.452,.457,.458 etc. The 44-40 refers to a .429 caliber bullet with a charge of 40 grs. of black powder. This goes on for ever. Grain weight as used in firearms refers to a base line of 7,000 grains to one 16 ounce pound. Bullets and powders are generally referred to in grain weights. A pound of powder is 7,000 grains if you load 100 grs per cartridge you be able to load 70 rounds from one pound. Bullet weights are referred to by grain weights also. A .224 caliber bullet may be described as a 50 grain HP or a .22 caliber 50 gr. Hollow point.

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Old 07-10-2013, 09:13 AM   #4
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Just to add to the confusion- CALIBER is actually a comparison of bore diameter to barrel length- but only for cannon! An old Navy Gunner's mate may have fired a "3 inch 50" or a "5 inch 38". These were ship mounted guns that had a 3 inch diameter bore, and a barrel 50 times as long as it was wide, or a 5 inch bore, and a barrel 38 times as long. But in small arms, were use it to mean the bore diameter, or to a particular cartridge.

NAMING of small arms cartridges is highly imprecise- blurred by marketing hacks and by egos of cartridge developers. The .38 Special does not shoot a .38 bullet (.357) But it DOES have the same cartridge case diameter that .38s used to use. Bullets were- at one time- heel seated- like a .22 LR- and case and bullet were same diameter. But then the design changed to a bullet recessed into the case, so outer diameter of bullet was less than diameter of case. The .44 Magnum- just sounds so much better than ".429 Magnum"- the true bullet diameter.

As he said up there- Europeans used the bore diameter and case length in millimeters- like 7.62x39- BUT Europeans measure TOP of land to TOP of land, Americans measure BOTTOM of groove to BOTTOM of groove. The result is that a Russian 7.62 is .312 inches, American 7.62 is .308

BLACK POWDER American cartridges were named after caliber and powder weight- like the .45-70 (45 cal bullet over 70 grains of black powder).

Study of the history of a cartridge is a LARGE field that can only be scratched here on a forum- but once you get in to it, you begin to understand that there are more than a dozen different "38" cartridges- most of which do not interchange. And a t least 8 different 9mm cartridges- again- do not interchange.

A good introductory work is Cartridges of the World. About the size of a phone book, and THAT does not cover ALL the cartridges, and has room for only a brief mention.

Shotgun GAUGE is different- refers to the number of round lead balls that make up a pound that each ball would fit the bore- 12 g balls would be 12 balls to the pound- 20 g, 20 balls to make one pound, etc (410 in not a gauge, but a caliber)

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Old 07-10-2013, 02:41 PM   #5
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Trying to understand caliber designations is a PIA.

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Old 07-10-2013, 04:53 PM   #6
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Then to make things more complicated there are sometimes 2 names for the same cartridge. 7mm Mauser, 7x57, 275 Mauser are the same. 7mm Express and 280 Remington are the same. There are many examples.

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Old 07-10-2013, 06:45 PM   #7
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Quote:
Originally Posted by c3shooter View Post
Just to add to the confusion- CALIBER is actually a comparison of bore diameter to barrel length- but only for cannon! An old Navy Gunner's mate may have fired a "3 inch 50" or a "5 inch 38". These were ship mounted guns that had a 3 inch diameter bore, and a barrel 50 times as long as it was wide, or a 5 inch bore, and a barrel 38 times as long. But in small arms, were use it to mean the bore diameter, or to a particular cartridge.

NAMING of small arms cartridges is highly imprecise- blurred by marketing hacks and by egos of cartridge developers. The .38 Special does not shoot a .38 bullet (.357) But it DOES have the same cartridge case diameter that .38s used to use. Bullets were- at one time- heel seated- like a .22 LR- and case and bullet were same diameter. But then the design changed to a bullet recessed into the case, so outer diameter of bullet was less than diameter of case. The .44 Magnum- just sounds so much better than ".429 Magnum"- the true bullet diameter.

As he said up there- Europeans used the bore diameter and case length in millimeters- like 7.62x39- BUT Europeans measure TOP of land to TOP of land, Americans measure BOTTOM of groove to BOTTOM of groove. The result is that a Russian 7.62 is .312 inches, American 7.62 is .308

BLACK POWDER American cartridges were named after caliber and powder weight- like the .45-70 (45 cal bullet over 70 grains of black powder).

Study of the history of a cartridge is a LARGE field that can only be scratched here on a forum- but once you get in to it, you begin to understand that there are more than a dozen different "38" cartridges- most of which do not interchange. And a t least 8 different 9mm cartridges- again- do not interchange.

A good introductory work is Cartridges of the World. About the size of a phone book, and THAT does not cover ALL the cartridges, and has room for only a brief mention.

Shotgun GAUGE is different- refers to the number of round lead balls that make up a pound that each ball would fit the bore- 12 g balls would be 12 balls to the pound- 20 g, 20 balls to make one pound, etc (410 in not a gauge, but a caliber)
So what do you mean by top of land to top of land and bottom of groove to bottom of groove
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Old 07-10-2013, 09:08 PM   #8
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Most rifling is created by either:

cutting one groove at a time with a machine tool (cut rifling or single point cut rifling);
cutting all grooves in one pass with a special progressive broaching bit (broached rifling);
pressing all grooves at once with a tool called a "button" that is pushed or pulled down the barrel (button rifling);
forging the barrel over a mandrel containing a reverse image of the rifling, and often the chamber as well (hammer forging);
flow forming the barrel preform over a mandrel containing a reverse image of the rifling (rifling by flow forming)

The grooves are the spaces that are cut out, and the resulting ridges are called lands. These lands and grooves can vary in number, depth, shape, direction of twist (right or left), and twist rate (see below). The spin imparted by rifling significantly improves the stability of the projectile, improving both range and accuracy. Typically rifling is a constant rate down the barrel, usually measured by the length of travel required to produce a single turn. Occasionally firearms are encountered with a gain twist, where the rate of spin increases from chamber to muzzle. While intentional gain twists are rare, due to manufacturing variance, a slight gain twist is in fact fairly common. Since a reduction in twist rate is very detrimental to accuracy, gunsmiths who are machining a new barrel from a rifled blank will often measure the twist carefully so they may put the faster rate, no matter how minute the difference is, at the muzzle end (see internal ballistics for more information on accuracy and bore characteristics).

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Old 07-10-2013, 11:44 PM   #9
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Here is a cross section of a rifled barrel. Europeans measure like THIS-

barrel-rifling-pictures.jpg. What they CALL a 7.62mm actually is about .312, so it fills the low spots.

WE measure low spot to low spot- so what WE call 7.62 mm is about .308 inches across. See why this stuff is a PITA?

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Old 07-11-2013, 12:17 AM   #10
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Oh ok thank makes sense know! Thanks for the image!



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