View source for Caliber

{{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]].[https://www.tncourts.gov/sites/default/files/docs/firearmshandout_1.pdf]<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> 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}}

==Cartridge naming conventions==
While modern firearms are generally referred to by the name of the [[cartridge (firearms)|cartridge]] the gun is chambered for, they are still categorized together based on bore diameter.{{Citation needed|date=October 2024}}  For example, a firearm might be described as a "[[.30 caliber|30 caliber]] rifle", which could accommodate any of a wide range of cartridges using a roughly {{convert|0.30|in|mm}} projectile; or as a "22 rimfire", referring to any [[Rimfire ammunition|rimfire]] firearms firing cartridges with a [[22 caliber]] projectile. However, there can be significant differences in nominal bullet and bore dimensions, and all cartridges so "categorized" are not automatically identical in actual caliber.

For example, [[.303 British|303 British]] firearms and projectiles are often "categorized" as ".30-caliber" alongside several dozen U.S. "30-caliber" cartridges despite using bullets of {{convert|.310|–|.312|in|adj=on|sigfig=3}} diameter while all U.S. "30-caliber" centerfire rifle cartridges use a common, standard {{convert|.308|in|adj=on|sigfig=3}} bullet outside diameter. Using bullets larger than design specifications causes excessive pressures, while undersize bullets cause low pressures, insufficient muzzle velocities and fouling that will eventually lead to excessive pressures.

Makers of early cartridge arms had to invent methods of naming cartridges since no established convention existed then.<ref name="cotw">{{cite book | last = Barnes | first = Frank C. | editor = McPherson, M. L. | title = Cartridges of the World | url = https://archive.org/details/cartridgesworld00barn | url-access = limited | edition = 8th | orig-year = 1965 | year = 1997 | publisher = DBI Books | pages =[https://archive.org/details/cartridgesworld00barn/page/n9 8]–12 | isbn = 0-87349-178-5}}</ref> One of the early established cartridge arms was the [[Spencer repeating rifle]], which [[Union (American Civil War)|Union]] forces used in the [[American Civil War]]. It was named based on the [[Chamber (firearms)|chamber]] dimensions, rather than the bore diameter, with the earliest cartridge called the "No. 56 cartridge", indicating a chamber diameter of .56&nbsp;in; the bore diameter varied considerably, from .52 to .54&nbsp;in.  Later various [[wildcat cartridge|derivatives]] were created using the same basic cartridge, but with smaller-diameter bullets; these were named by the cartridge diameter at the base and mouth. The original No. 56 became the .56-56, and the smaller versions, .56-52, .56-50, and .56-46. The 56–52, the most common of the new calibers, used a 50-cal bullet.

Other black powder-era cartridges used naming schemes that appeared similar, but measured entirely different characteristics; [[.45-70|45-70]], [[.44-40|44-40]], and [[.32-20|32-20]] were designated by bullet diameter to hundredths of an inch and standard [[black powder]] charge in [[grain (measure)|grain]]s. Optionally, the bullet weight in grains was designated, such as 45-70-405.<ref name=barnes2016-p9/> This scheme was far more popular and was carried over after the advent of early [[smokeless powder]] cartridges such as the [[.30-30 Winchester|30-30 Winchester]] and [[.22 Long|22 Long]]. Later developments used terms to indicate relative power, such as [[.44 Special]] and [[.44 Magnum]]. Variations on these methods persist today, with new cartridges such as the [[.204 Ruger|204 Ruger]] and [[.17 HMR|17 HMR]] (Hornady Magnum Rimfire).

Metric diameters for small arms refer to cartridge dimensions and are expressed with an "×" between the bore diameter and the length of the cartridge case; for example, the [[6.5×55mm|6.5×55mm Swedish]] cartridge has a bore diameter of 6.5&nbsp;mm and a case length of 55&nbsp;mm.<ref name=barnes2016-p9/>

The means of measuring a rifled bore varies, and may refer to the diameter of the lands or the grooves of the rifling.<ref name=barnes2016-p9/> For example, the [[.257 Roberts|257 Roberts]] and [[.250-3000 Savage|250 Savage]] both use a .257 inch projectile; both 250 Savage and 257 Roberts rifle bores have a .250 inch land diameter and .257 inch groove diameter.<ref name=VanZwoll>{{cite book| last=Van Zwoll| first=Wayne| title=Shooter's bible guide to rifle ballistics| page=18| publisher=[[Skyhorse Publishing]]| date=2011| isbn=978-1-61608-224-6}}</ref> The [[.308 Winchester]] is measured across the grooves and uses a .308-in diameter (7.82-mm) bullet; the military-specification version is known as [[7.62×51mm NATO|7.62 × 51 mm NATO]], so called because the bore diameter measured between the lands is 7.62&nbsp;mm, and the cartridge has a case 51&nbsp;mm long.<ref name=Barnes2016-p670>{{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=670}}</ref>

==Rifle caliber and cartridge conversions==
Converting a rifle to fire a different cartridge in the same bore diameter, often involves merely re-chambering the barrel to the new cartridge dimensions, if the rim diameter of the new cartridge matches that of the old cartridge. Converting a rifle to fire a different cartridge in a different caliber and bore as what it initially was, means that the barrel of the rifle will also need to be changed. Because many competitive precision rifle shooters often shoot thousands of rounds per year both for practice and competitions, and they more often reach the end of their barrel life, whereby the [[rifling]] is worn down to a point where a rifle loses some of its [[accuracy]], the choice to make a caliber or cartridge change is often done at the same time as when a new rifle [[Gun barrel|barrel]] is fitted to the rifle by a [[gunsmith]]. There are a few important factors to consider when converting a rifle to a different caliber or cartridge. The action of the rifle should be long enough to contain the new cartridge, the magazine should also be able to hold the new cartridge, the bolt face should be the correct diameter<ref>{{cite web |url=https://www.loaddevelopment.com/bolt-face-action-database/ |title=Bolt Face Database |publisher=LoadDevelopment.com |year=2020 |access-date=2020-09-22}}</ref> and the extractor the correct size to hold the head of the new cartridge. The most common of these caliber conversions on rifles, are usually done to change from a parent cartridge to a new cartridge based on it, like when converting a rifle to a [[6.5mm Creedmoor|6.5 mm Creedmoor]] from a [[.308 Winchester|308 Winchester]] on which it is based.

==Metric and US customary==
{{see|Metrication in the United States#Firearms}}
The following table lists some of the commonly used calibers where both metric and US customary units are used as equivalents. Due to variations in naming conventions, and the whims of the cartridge manufacturers, bullet diameters can vary widely from the diameter implied by the name. For example, a difference of 0.045&nbsp;in (1.15&nbsp;mm) occurs between the smallest and largest of the several cartridges designated as ".38 caliber".  <!-- And it may be noted that 0.38 inch is more than 9 {{Frac|1|2}}&nbsp;mm. -->

{| class="wikitable sortable" border="1"
|+ Common calibers in inch and their metric equivalents<ref name="accurate">{{cite book | last = Accurate  | title = Accurate Smokeless Powders Loading Guide | edition = Number Two (Revised)  | year = 2000 | publisher =Wolfe Publishing | location =Prescott, AZ | page = 392 | id =  barcode 94794 00200}}</ref><ref>{{cite web |url=http://www.blue-star-inc.com/catalog/lprbullet.htm |title=Pistol and Rifle Lead Bullets |access-date=2007-12-05 |archive-date=2020-10-05 |archive-url=https://web.archive.org/web/20201005090622/http://www.blue-star-inc.com/catalog/lprbullet.htm |url-status=dead }}</ref><ref>{{cite web |url=http://www.blue-star-inc.com/catalog/lprbullet.htm |title=Rifle Bullets |access-date=2007-12-05 |archive-date=2020-10-05 |archive-url=https://web.archive.org/web/20201005090622/http://www.blue-star-inc.com/catalog/lprbullet.htm |url-status=dead }}</ref><ref>{{cite web |url=http://www.rainierballistics.com/mainframe.htm |title=LeadSafe Total Copper Jacket ('TCJ') Bullet List |access-date=2007-12-05 |archive-url=https://web.archive.org/web/19990218162127/http://www.rainierballistics.com/mainframe.htm |archive-date=1999-02-18 |url-status=dead }}</ref><ref>{{cite book|last1=Frank C Barnes|title=Cartridges of the World|publisher=Gun Digest Books|edition=14th}}</ref>
|-
! Caliber !! Metric caliber !! Typical bullet diameter!! class="unsortable"| Common cartridges !! class="unsortable"| Notes
|-
| 0.172 || [[4 mm caliber|4 mm]] || 0.172 in || [[.17 HMR|17 HMR]], 17 Hornet, 17 Ackley Hornet, 17 Winchester Super Magnum, 17-32 Magnum, 17 VHA, 17 Remington, 17/222, 17 Mach III-IV, 17 Ackley Improved Bee, 17-357 RG, 17 Remington Fireball, 17 Incinerator, 4.39×39R mm SPS ||
|-
| 0.204 || [[5 mm caliber|5 mm]] || 0.204 in  ||  [[.204 Ruger|204 Ruger]], [[5mm Remington Rimfire Magnum]] ||
|-
| 0.221 || 5.45&nbsp;mm || 0.221 in || [[5.45×39mm]] Russian family || Russian Mil Std 
|-
| 0.223 ||[[5.56 mm caliber|5.56 mm]]||0.224 in||[[22 Long Rifle]], [[.223 Remington]],  [[5.56 NATO  ]], 297/230 Morris Extra Long, [[.22 Hornet|22 Hornet]], [[22 Rem Automatic]], [[5.66 x39 MPS]], [[22 Rem Jet]]||
|-
| 0.224 || 5.7&nbsp;mm || 0.224 in|| [[.218 Bee|218 Bee]], [[.219 Zipper|219 Zipper]], 22 Hornet-K, [[.220 Swift|220 Swift]], [[.222 Remington|222 Remington]], [[.222 Remington Magnum|222 Remington Magnum]], [[.223 Remington|223 Remington]], [[5.56×45mm NATO|5.56×45 mm NATO]],  [[5.7×28mm|5.7×28 mm]], [[.22 TCM]], [[5.8×42mm DBP87|5.8 × 42 Chinese]], [[.224 Weatherby Magnum|224 Weatherby Magnum]], [[.225 Winchester|225 Winchester]], [[.223 Winchester Super Short Magnum|223 Winchester Super Short Magnum (Obsolete)]] 223 Ackley Improved, [[.219 Donaldson Wasp|219 Donaldson Wasp]], [[.221 Remington Fireball|221 Remington Fireball]], [[.22-250 Remington|22-250 Remington]], and more ||
|- || 
| 0.243 || [[6 mm caliber|6 mm]] || 0.243 in || [[.243 Winchester|243 Winchester]], [[6mm Remington|244 Remington]], [[6mm Remington|6 mm Remington]], 6&nbsp;mm Whisper, [[6mm PPC|6 mm PPC]], [[6mm BR|6 mm Bench Rest Remington]], [[6×45mm|6 × 45 mm]], [[6.5×47mm Lapua#Variants|6 × 47 mm]], 6&nbsp;mm Cheetah, 240 Weatherby, 6 × 62 Freres, [[6mm BR|6 mm Norma BR]], [[6 mm XC|6mm XC Tubb]], 6&nbsp;mm JDJ, 6&nbsp;mm SAW, 6-250 Walker, 6.17 Spitfire, 6.17 Flash, 6&nbsp;mm Lee Navy, and more ||
|-
| 0.25 || 6.35&nbsp;mm ||0.257 in, 6.35&nbsp;mm ||[[.25 ACP|25 ACP]] (0.251"), [[.250-3000 Savage|250/3000 Savage]], [[257 Roberts]], [[.25-06|25-06]] (0.257"),|| also called .25 Auto and 6.35mm Browning
|-
| 0.26 || 6.5&nbsp;mm || 0.264 in, 6.7&nbsp;mm  ||  [[6.5×55mm Swedish|6.5 × 55 mm Swedish]], [[.260 Remington|260 Remington]], [[.26 Nosler|26 Nosler]], [[6.5mm Creedmoor|6.5 mm Creedmoor]], [[6.5×47mm Lapua|6.5×47 mm Lapua]], [[6.5mm Grendel|6.5 mm Grendel]] || cartridges commonly known as "6.5&nbsp;mm"
|-
| 0.27 || 6.8&nbsp;mm || 0.277 in, 7.035&nbsp;mm  ||  [[.270 Winchester|270 Winchester]], [[6.8 SPC]], [[.277 Fury]]||
|-
| 0.284 || [[7 mm caliber|7 mm]] || 0.284 in, 7.213&nbsp;mm  ||  [[.280 Remington|280 Remington]], [[7mm-08 Remington|7 mm-08 Remington]], [[7mm Weatherby Magnum|7 mm Weatherby Magnum]],  [[7mm Remington Magnum|7 mm Remington Magnum]], [[7×57mm Mauser|7 × 57 mm Mauser]], [[7×64mm|7 × 64 mm]] || commonly called "7&nbsp;mm"
|-
| 0.308 || 7.62&nbsp;mm || 0.308 in, 7.82&nbsp;mm || 30 Luger (7.65×21mm Luger), [[.30-30 Winchester]], 30 Herrett, 300 Whisper, 30-378 Weatherby, 7.63 Mannlicher–Schoenauer, 7.63 Mauser, 30 USA Rimless, 308 Corbon, .3-9 Savage, 30 Kurz, [[300 AAC Blackout (7.62×35mm)|300 BLK (7.62 × 35 mm)]], 7.5mm Schmidt–Rubin, [[.300 Winchester Magnum|300 Winchester Magnum]], [[.30 Carbine|30 Carbine]], 309 JDJ, [[.30-03 Springfield]], [[.30-06 Springfield]], [[.30-06 JDJ]], .307 GNR, [[7.62×51mm NATO|308 Winchester (7.62 × 51 mm NATO)]], [[.300 Weatherby Magnum|300 Weatherby Magnum]], 30 Army (30-40 Krag),  7.82&nbsp;mm Lazzeroni, and more ||
|-
| 0.307 || 7.8&nbsp;mm || 0.307 in, 7.8&nbsp;mm || [[7.5 FK|7.5 FK (aka 7.5 × 27mm)]]||
|-
| 0.311 || 7.9&nbsp;mm || 0.311 in, 7.92&nbsp;mm || [[.303 British|303 British]], [[7.62×39mm Soviet|7.62 × 39 mm Soviet]], [[7.62×54mmR|7.62 × 54 mmR]], [[7.62×25mm Tokarev|7.62 × 25 mm]], [[7.7×58mm Arisaka|7.7 × 58 mm]] || 7.62×54mmR is actually 7.92&nbsp;mm (Mosin, SVD, PKM, etc.) The same applies to 7.62×39mm (AK-47, AKM, etc.) 
|-
| 0.312 || 7.94&nbsp;mm || 0.312 in, 7.94&nbsp;mm || [[.32 ACP|32 ACP]], [[32-20 Winchester]] || Also known as 7.65×17mm Browning
|- 
| 0.323 || [[8 mm caliber|8 mm]] || 0.323 in, 8.20&nbsp;mm || [[7.92×57mm Mauser|8×57 mm Mauser]], [[.325 WSM|325 WSM]], [[8mm Remington Magnum|8 mm Remington Magnum]], 8&nbsp;mm plastic ([[airsoft]]) BBs || .32 caliber rifle cartridges 
|-
| 0.327 || [[8 mm caliber|8 mm]] || 0.327 in, 8.30&nbsp;mm || [[8 mm Lebel]] || 8x51 mm R (ex 8x50 mm R)
|-
| 0.338 || 8.6&nbsp;mm || 0.338 in || [[.338 Lapua Magnum|338 Lapua Magnum]], [[.338 Norma Magnum|338 Norma Magnum]], [[.338 Winchester Magnum|338 Winchester Magnum]], [[.338-378 Weatherby Magnum|338-378 Weatherby Magnum]] || C14 Timberwolf (Canadian Forces) 
|-
| 0.355 ||  [[9 mm caliber|9&nbsp;mm]] || 0.355 in, 9.01&nbsp;mm || 9&nbsp;mm Luger (aka [[9×19 mm Parabellum]], aka 9&nbsp;mm NATO), 9&nbsp;mm Ultra, 9&nbsp;mm Bayard Long, [[9mm Mauser|9 mm Mauser]], 9&nbsp;mm Winchester Magnum, 9&nbsp;mm Glisenti, 9 × 21&nbsp;mm, 9 × 23&nbsp;mm Winchester, 9&nbsp;mm Mi-Bullet, [[9mm Steyr|9 mm Steyr]], .356 Team Smith & Wesson, 9&nbsp;mm Federal,  9&nbsp;mm × 25&nbsp;mm Dillon, 9mm Action Express, [[.357 SIG|357 SIG]], [[.380 ACP]] (9mm Short) ||
|-
| 0.356 || 9&nbsp;mm || 0.356 in ||  9×56mm Mannlicher–Schoenauer, 9&nbsp;mm × 57mm Mauser ||
|-
| 0.357 || [[9 mm caliber|9&nbsp;mm]] || 0.357 in, 9.1&nbsp;mm ||  [[.38 Super|38 Super]], [[.38 Special|38 Special]], [[.38 S&W]], [[.357 Magnum|357 Magnum]], [[.35 Remington|35 Remington]], [[9mm Browning Long|9 mm Browning Long]]|| Handgun cartridges known as "38" are .357 caliber. Generally .357 for revolvers and rifles, .355 in autoloaders
|-
| 0.363 || 9&nbsp;mm || 0.365 in, 9.27&nbsp;mm || [[9×18mm Makarov|9 × 18 mm Makarov]] ||
|-
| 0.365 || 9.3&nbsp;mm ||0.365 in || [[9×39mm|9 × 39 mm]], [[9.3×62mm|9.3 × 62 mm]], [[9.3×64mm Brenneke|9.3 × 64 mm Brenneke]], [[9.3×72mmR|9.3 × 72 mmR]], [[9.3×74mmR|9.3 × 74 mmR]] ||
|-
| 0.375 || 9.5&nbsp;mm || 0.375 in, 9.53&nbsp;mm || [[.375 H&H Magnum|375 H&H Magnum]], [[9.5×57mm Mannlicher–Schönauer|9.5 × 57 mm Mannlicher–Schönauer (375 Rimless Nitro Express (RNE) × 2¼)]]||
|-
| 0.40 || [[10 mm caliber|10&nbsp;mm]] || 0.400 in || [[.40 S&W|40 S&W]], [[10mm Auto|10 mm Auto]] || 
|-
|0.410
|10.4&nbsp;mm
|0.410 in
|[[.410 bore|410 bore]]
|
|-
| 0.44 || 10.9&nbsp;mm || 0.429 in ||[[.444 Marlin|444 Marlin]], [[.44 Russian|44 S&W Russian]], [[.44 S&W Special|44 S&W Special]], [[.44 Remington Magnum|44 Remington Magnum]], [[.44 Auto Mag|44 Auto Mag]], [[.440 Cor-Bon|440 Cor-Bon]], 44/454 JDJ Woodswalker ||
|-
| 0.45 || 11.43&nbsp;mm || 0.451–0.454 in || [[.45 ACP|45 ACP]], [[.45 GAP|45 GAP]], [[.454 Casull|454 Casull]], [[.45 Long Colt|45 Long Colt]], [[.455 Webley|455 Webley]], [[.45 Schofield|45 Schofield]], [[.460 S&W Magnum|460 S&W Magnum]] || Bullet diameter depends on bullet type/material. Generally 0.451 in for [[full metal jacket bullet]]s and 0.454 in for lead bullets.
|-
| 0.50 || 12.7&nbsp;mm || 0.510&nbsp;in, 12.95&nbsp;mm ||  [[.50 BMG|50 BMG]], [[.50 Action Express|50 Action Express]], [[12.7×108mm]], [[.500 S&W Magnum|500 S&W Magnum]], [[.50 Beowulf|50 Beowulf]], [[12.7x55mm]]||[[M2 Browning machine gun]] and other heavy machine guns, long-range rifles typified by [[Barrett Firearms Manufacturing|Barrett]] products. [[IMI Desert Eagle|Desert Eagle]] handgun.
|}

==Shotguns==
{{Main|Gauge (firearms)}}

Shotguns are classed according to gauge, a related expression.  The gauge of a shotgun refers to how many lead spheres, each with a diameter equal to that of the bore, that amounts to one pound (454 g (1.0&nbsp;lb)) in weight. In the case of a 12-gauge (18.5&nbsp;mm) shotgun, it would take 12 spheres the size of the shotgun's bore to equal a pound.<ref name=Barnes2016-p629>{{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=629}}</ref> A numerically larger gauge indicates a smaller barrel: a [[20-gauge]] (15.6&nbsp;mm) shotgun requires more spheres to equal a pound; therefore, its barrel is smaller than the 12-gauge. This metric is used in Russia as "caliber number": e.g., "shotgun of the 12 caliber." The 16th caliber is known as "lordly" ({{langx|ru|барский}}). While shotgun bores can be expressed in calibers (the [[.410 bore]] shotgun is measured as {{convert|.410|in|mm|abbr=on}} in diameter,<ref name=Barnes2016-p629/> unlike with rifles the actual bore diameter of a smoothbore shotgun varies significantly down the length of the barrel, with the use of [[Choke (firearms)|chokes]] and back-boring.

In the United Kingdom, "gauge" is referred to as "bore" and in the United States "bore" is referred to as "gauge", e.g. a "12-bore shotgun or 12-gauge shotgun" has a bore or gauge that can accommodate a lead sphere weighing 1/12th of a pound.

==As a measurement of length==
{{Main|Caliber (artillery)#Barrel length}}
The term caliber is used as a measure of length of [[artillery]] barrels from muzzle to breech, expressed as a multiple of the bore diameter.  For example, a 4-inch gun of 50 calibers would have a barrel 4&nbsp;in  × 50 = 200&nbsp;in long (written as 4" L/50 or 4"/50).  A 16-inch gun of 50 calibers (16" L/50) has a barrel length of 50 × 16 = {{convert|800|in|ftin}}.

Both 14-in and 16-in navy guns were common in World War II.  The British Royal Navy insisted on 50-cal guns on ships as it would allow {{convert|1,900|to|2,700|lb|kg|abbr=on}} shells to travel at an initial velocity of up to 1,800&nbsp;mph (2,897&nbsp;km/h) to a distance of 26&nbsp;mi (42&nbsp;km).{{citation needed|date=December 2014}}

==Pounds as a measure of cannon bore==
Smoothbore [[cannon]] and [[carronade]] bores are designated by the weight in imperial pounds of spherical solid iron shot of diameter to fit the bore. Standard sizes are 6, 12, 18, 24, 32, and 42 pounds,<ref>British cannonball sizes. Available at: https://2015fallhw.github.io/arcidau/Cannonballs.html (Accessed: 09 September 2023).</ref> with some non-standard weights using the same scheme. See [[Carronade#Ordnance]].

From about the mid-17th until the mid-19th century, the measurement of the bore of large gunpowder weapons was usually expressed as the weight of its iron shot in [[Pound (mass)|pounds]].  Iron [[round shot|shot]] was used as the standard reference because iron was the most common material used for artillery ammunition during that period, and solid spherical shot the most common form encountered. Artillery was classified thereby into standard categories, with the 3-pounder, 4-pounder, 6-pounder, 8-pounder, 9-pounder, 12-pounder, 18-pounder, 24-pounder, and 32-pounder being the most common sizes encountered, although larger, smaller and intermediate sizes existed.

In practice, though, significant variation occurred in the actual mass of the projectile for a given nominal shot weight. The country of manufacture is a significant consideration when determining bore diameters. For example, the French [[Pound (mass)#French livre|livre]], until 1812, had a mass of {{convert|489.5|g|lb|abbr=on}}, whilst the contemporary English ([[avoirdupois]]) pound massed of approximately {{convert|454|g|lb|abbr=on}}. Thus, a French 32-pounder at the  [[Battle of Trafalgar]] threw a shot with {{convert|1.138|kg|lb|abbr=on}} more mass than an English 32-pounder.

Complicating matters further, muzzle-loaded weapons require a significant gap between the sides of the tube bore and the surface of the shot. This is necessary so the projectile may be inserted from the mouth to the base of the tube and seated securely adjacent the propellant charge with relative ease. The gap, called [[windage]], increases the size of the bore with respect to the diameter of the shot somewhere between 10% and 20% depending upon the year the tube was cast and the [[foundry]] responsible.

{| class="wikitable"
|-
|+English gun classes c. 1800{{Citation needed|date=April 2010}}
|-
! gun class (pdr.)
!projectile mass (kg)
! shot diameter (cm)
! shot volume (cm<sup>3</sup>)
! approx. service bore (cm)
|-
| align="center"| 2
| align="right"| {{convert|0.9|kg|lb|abbr=on}}
| align="right"| {{convert|6|cm|in|abbr=on}}
| align="right"| {{convert|115|cm3|cuin|abbr=on}}
| align="right"| {{convert|6.6|cm|in|abbr=on}}
|-
| align="center"| 3
| align="right"| {{convert|1.4|kg|lb|abbr=on}}
| align="right"| {{convert|6.9|cm|in|abbr=on}}
| align="right"| {{convert|172|cm3|cuin|abbr=on}}
| align="right"| {{convert|7.6|cm|in|abbr=on}}

|-
| align="center"| 4
| align="right"| {{convert|1.8|kg|lb|abbr=on}}
| align="right"| {{convert|7.6|cm|in|abbr=on}}
| align="right"| {{convert|230|cm3|cuin|abbr=on}}
| align="right"| {{convert|8.4|cm|in|abbr=on}}

|-
| align="center"| 6
| align="right"| {{convert|2.7|kg|lb|abbr=on}}
| align="right"| {{convert|8.7|cm|in|abbr=on}}
| align="right"| {{convert|345|cm3|cuin|abbr=on}}
| align="right"| {{convert|9.6|cm|in|abbr=on}}

|-
| align="center"| 9
| align="right"| {{convert|4.1|kg|lb|abbr=on}}
| align="right"| {{convert|10|cm|in|abbr=on}}
| align="right"| {{convert|518|cm3|cuin|abbr=on}}
| align="right"| {{convert|11|cm|in|abbr=on}}

|-
| align="center"| 12
| align="right"| {{convert|5.4|kg|lb|abbr=on}}
| align="right"| {{convert|10.9|cm|in|abbr=on}}
| align="right"| {{convert|691|cm3|cuin|abbr=on}}
| align="right"| {{convert|12.1|cm|in|abbr=on}}

|-
| align="center"| 18
| align="right"| {{convert|8.2|kg|lb|abbr=on}}
| align="right"| {{convert|12.6|cm|in|abbr=on}}
| align="right"| {{convert|1,037|cm3|cuin|abbr=on}}
| align="right"| {{convert|13.8|cm|in|abbr=on}}

|-
| align="center"| 24
| align="right"| {{convert|10.9|kg|lb|abbr=on}}
| align="right"| {{convert|13.8|cm|in|abbr=on}}
| align="right"| {{convert|1,383|cm3|cuin|abbr=on}}
| align="right"| {{convert|15.2|cm|in|abbr=on}}

|-
| align="center"| 32
| align="right"| {{convert|14.5|kg|lb|abbr=on}}
| align="right"| {{convert|15.2|cm|in|abbr=on}}
| align="right"| {{convert|1,844|cm3|cuin|abbr=on}}
| align="right"| {{convert|16.7|cm|in|abbr=on}}

|-
| align="center"| 64
| align="right"| {{convert|29|kg|lb|abbr=on}}
| align="right"| {{convert|19.2|cm|in|abbr=on}}
| align="right"| {{convert|3,687|cm3|cuin|abbr=on}}
| align="right"| {{convert|21.1|cm|in|abbr=on}}

|}

The relationship between bore diameter and projectile weight was severed following the widespread adoption of rifled weapons during the latter part of the 19th century. Guns continued to be classed by projectile weight into the mid-20th century, [[British standard ordnance weights and measurements|particularly in British service]] with guns, such as the [[Ordnance QF 2-pounder|2-pounder]], [[Ordnance QF 6-pounder|6-pounder]], and [[Ordnance QF 17-pounder|17-pounder]] [[Anti-tank warfare|anti-tank weapons]]. However, this value no longer definitively related to bore diameter, since projectiles were no longer simple spheres—and in any case were more often hollow shells filled with explosives rather than solid iron shot.

==See also==
* [[List of cartridges by caliber]]
* [[List of handgun cartridges]]
* [[List of rifle cartridges]]
* [[List of the largest cannon by caliber]]
* [[Table of handgun and rifle cartridges]]

== Citations ==
{{Reflist|30em}}

==External links==
{{Wiktionary|caliber}}
* [http://gunvgun.com/calibers Search for guns by their caliber]; {{Webarchive|url=https://web.archive.org/web/20110711133932/http://gunvgun.com/calibers |date=2011-07-11 }}

{{Firearms}}

[[Category:Ammunition]]
[[Category:Firearms]]