| Writers often need to know just what kind of | | | | Magnum is not .44 inches in diameter. It is .429 inches |
| weaponry their protagonists are using. Here's a useful | | | | in diameter. A .38 Special is not .38 inches. It is .357 |
| primer as to the different terms and what they | | | | inches. The .357 Magnum, though, is truly .357 inches |
| mean. | | | | in diameter. A 9mm is .355 inches. Some calibers are |
| CALIBER (or calibre) is "usually" a numerical term, | | | | also confusing. A .45 Colt is a revolver cartridge. A .45 |
| without the decimal point, in a cartridge's name to | | | | ACP (Automatic Colt Pistol) is an automatic cartridge. |
| indicate the bullet diameter. | | | | They are not interchangeable. |
| Alternatively it can be a measurement of the | | | | Caliber also has an alternative meaning when it relates |
| diameter of the gun's barrel. | | | | to the length of the barrel in larger, artillery, pieces eg |
| It is described in most glossaries as: "The internal | | | | an artillery piece may be described as 5 inch/50 |
| diameter of a gun's barrel." This can be measured | | | | caliber, which would mean it has a barrel 50 times the |
| either in English units or in metric. The measurement | | | | bore diameter, or 250 inches long. |
| can be taken in a rifled arm either land to land or | | | | GAUGE refers to shotguns and is the bore size of a |
| groove to groove. E.g., in most US .30-caliber rifles, | | | | shotgun determined by the number of round lead |
| the diameter of the bore land to land is .300 inches, | | | | balls of bore diameter that equals a pound |
| while groove to groove it is .308 inches ( Land is the | | | | For example, twelve lead balls that fit the diameter |
| raised portion of rifling in the barrel, groove is the | | | | of a 12 gauge shotgun equal one pound. The most |
| recessed portion of rifling) | | | | common sizes of shotgun gauge are 10 gauge, 12 |
| In the US the measurement is usually expressed in | | | | gauge, 16 gauge, 20 gauge, and .410. ( Again, just to |
| hundreds of an inch; in Great Britain in thousandths; in | | | | confuse matters, the .410 is the exception in |
| Europe and elsewhere in millimetres. That's why you | | | | shotguns in that it is measured as a caliber not a |
| see the calibre of handguns often expressed in | | | | gauge, so the .410 has a barrel .410 inches in width.) |
| different units. | | | | So, using this system, as the gauge goes up, the |
| Just to confuse matters, even in the same country | | | | bore diameter goes down. This is the opposite of |
| the quoted calibre does not always indicate the true | | | | calibre. Just remember, as a writer, your duty is to |
| bore diameter but measure the diameter of the | | | | get it right. And also remember, there's always going |
| "outer" barrel. | | | | to be somebody out there who'll point out your |
| Not all calibers indicate bore diameter. The .44 | | | | mistakes, so try to get it right first time. |