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