Editing Caliber (section)

{{short description|Internal diameter of the barrel of a gun}}
{{hatnote group|{{redirect|Calibre}}
{{distinguish|Calipers}}
}}
{{more footnotes needed|date= September 2014}}
{{Use American English |date = July 2019}}
[[File:Rifle, bolt-action (AM 745326-14).jpg|thumb|7,92 mm caliber rifle barrel ([[Swedish Mauser|Mauser m/96]])]]
[[File:Rifle cartridge comparison.jpg|thumb|right|Rifle cartridges: from left: [[.50 BMG|50 BMG]] • [[.300 Winchester Magnum|300 Win Mag]] • [[.308 Winchester|308 Winchester]], [[7.62×39mm|7.62 × 39 mm]] • [[5.56×45mm NATO|5.56 × 45 mm NATO]] • [[22 LR]]]]
[[File:22-45.jpg|thumb|right|A [[.45 ACP|45 ACP]] [[Hollow-point bullet|hollowpoint]] ([[Federal Cartridge|Federal]] HST) with two [[.22 Long Rifle|22 LR]] cartridges for comparison]]

In [[gun]]s, particularly [[firearm]]s, but not [[#As a measurement of length|artillery, where a different definition may apply]], '''caliber''' (or '''calibre'''; sometimes abbreviated as "'''cal'''") is the specified nominal internal [[diameter]] of the [[gun barrel]] [[Gauge (firearms)|bore]] – regardless of how or where the bore is measured and whether the finished bore matches that specification.<ref>{{cite book |last= Brown |first= Edmund G. |author-link = Jerry Brown |date = 2009 |title = Handgun Safety Certificate |location = West Sacramento, California |publisher = [[California Department of Justice]] |page = 52 }}</ref> It is measured in [[inch]]es or in [[millimetres|millimeters]].{{Clarify|date=January 2024|reason=E.g. Imperial Japanese Navy used centimeters after 1917}}<ref name=barnes2016-p9>{{Cite book |last= Barnes |first= Frank C. |editor-last= Woodard |editor-first= W. Todd |year= 2016 |title= Cartridges of the World: A Complete and Illustrated Reference for More than 1500 Cartridges |edition= 15th |location= Iola, Wis. |publisher= Krause Publications |isbn= 978-1440246425 |oclc= 934886116 |page= 9}}</ref>  In the United States it is expressed in hundredths of an inch; in the United Kingdom in thousandths; and elsewhere in millimeters. For example, a US "[[45 caliber]]" firearm has a barrel diameter of roughly 0.45 inches (11.43mm).{{citation needed|date=February 2024}}  Barrel diameters can also be expressed using metric dimensions. For example, a "9 mm pistol" has a barrel diameter of about 9 millimeters. Since metric and US customary units do not convert evenly at this scale, metric conversions of caliber measured in decimal inches are typically approximations of the precise specifications in non-metric units, and vice versa.

In a [[rifling|rifled]] barrel, the distance is measured between opposing [[Rifling#Construction and operation|lands]] or between opposing [[Rifling#Construction and operation|grooves]]; groove measurements are common in cartridge designations originating in the [[United States]], while land measurements are more common elsewhere in the world. Measurements "across the grooves" are used for maximum precision because [[rifling]] and the specific caliber so measured is the result of final machining process which cuts grooves into the rough bore, leaving the "lands" behind.

Good performance requires a concentric, straight bore that accurately centers the projectile within the barrel, in preference to a "tight" fit which can be achieved even with off-center, crooked bores that cause excessive friction, fouling and an out-of-balance, wobbling projectile in flight.

Calibers fall into four general categories by size:{{Citation needed|date=October 2024}}

* [[miniature-bore]] historically refers to calibers with a diameter of {{convert|0.22|in|mm}} or smaller
* [[small-bore]] refers to calibers with a diameter of {{convert|0.32|in|mm}} or smaller
* medium-bore refers to calibers with a diameter of {{convert|0.33|in|mm}} to {{convert|0.39|in|mm}}
* large-bore refers to calibers with a diameter of {{convert|0.40|in|mm}} or larger

There is much variance in the use of the term "small-bore", which over the years has changed considerably,  with anything under {{convert|0.577|in|mm}} considered "small-bore" prior to the mid-19th century.{{Citation needed|date=October 2024}}