Editing Caliber (section)

==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>