Caliber

In guns including firearms, caliber or calibre is the approximate internal diameter of the barrel in relation to the diameter of the projectile used in it.
In a rifled barrel, the distance is measured between opposing lands or grooves; groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere. It is important to performance that a bullet should closely match the groove diameter of a barrel to ensure a good seal. When the barrel diameter is given in inches, the abbreviation "cal" is used in place of "inches." For example, a small bore rifle with a diameter of 0.22 inch is a .22 cal; however, the decimal point is generally dropped when spoken, making it "twenty-two caliber" or a "two-two caliber". Calibers of firearms can be referred to in millimeters, as in a "caliber of eighty-eight millimeters" (88 mm) or "a hundred and five-millimeter caliber gun" (often abbreviated as "105 mm gun").
While modern cartridges and cartridge firearms are generally referred to by the cartridge name, they are still lumped together based on bore diameter. For example, a firearm might be described as a ".30 caliber rifle", which could be any of a wide range of cartridges using a roughly .30 inch projectile; or a ".22 rimfire", referring to any rimfire cartridge using a .22 caliber projectile.
In some contexts, e.g. guns aboard a warship, "caliber" is used to describe the barrel length as multiples of the bore diameter. A "5-inch 50 caliber" gun has a bore diameter of 5 inches (127 mm) and a barrel length of 50 times 5 inches = 250 inches (6.35 m).

 Cartridge naming conventions

Makers of early cartridge arms had to invent methods of naming^[1] the cartridges, since there was at the time no established convention. One of the early established cartridge arms was the Spencer repeating rifle, which saw service in the American Civil War. It was named based on the chamber dimensions, rather than the bore diameter, with the earliest cartridge called the "No. 56 cartridge," indicating a chamber diameter of .56 inch; the bore diameter varied considerably, from .52 to .54 inch. Later various 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 caliber bullet.
Other early white powder-era (Ballistite and Poudre blanche) cartridges used naming schemes that appeared similar, but measured entirely different characteristics. .45-70, .38-40, and .32-20 were designated by bullet diameter in hundredths of an inch and standard black powder charge in grains. Optionally the bullet weight in grains was designated, e.g. .45-70-405. This scheme was far more popular and was carried over after the advent of early smokeless powder cartridges such as the .30-30 Winchester and .22 Long; or a relative power, such as .44 Special and .44 Magnum. Variations on these methods persist today, with new cartridges such as the .204 Ruger and .17 HMR (Hornady Magnum Rimfire).
Metric calibers for small arms are usually expressed with an "x" between the width and the length; for example, 7.62x51 NATO. This indicates that the cartridge uses a 7.62 mm diameter bullet, loaded in a case 51 mm long. Similarly, the 6.5x55 Swedish cartridge has a bullet diameter of 6.5 mm and a case length of 55 mm. To convert from the diameter in mm to caliber divide the mm diameter by 25.4. If you want to convert from caliber to mm you multiply by 25.4. The means of measuring a rifled bore varies, and may refer to the diameter of the lands or the grooves of the rifling; this is why the .303 British, measured across the lands, actually uses a .311 inch bullet (7.70 mm vs. 7.90 mm), while the .308 Winchester, while dimensionally similar to (but should not be considered interchangeable with) the 7.62x51 mm NATO cartridge, is measured across the grooves and uses a .308" diameter (.308 times 25.4 = 7.82 mm) bullet. An exception to this rule are the proprietary cartridges used by U.S. maker Lazzeroni, which are named based on the groove diameter in millimeters, such as the 7.82 Warbird.^[1][2]
Modern small arms range in bore size from approximately .17 (4.5 mm) up to .50 caliber (12.7 mm). Arms used to hunt large dangerous game, such as those used in express rifles, may be as large as .80 caliber. In the middle of the 19th century, muskets and muzzle-loading rifles were .58 caliber or larger; the Brown Bess flintlock, for example, had a bore diameter of about .75 caliber (19 mm). Paintball guns (or "markers") are typically .68 caliber (17 mm).

Metric versus Imperial

The following table lists some commonly used calibers with their metric and imperial 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, there is a difference of as much as 0.045 inches (1.15 mm) between the smallest and largest of the several cartridges designated as '.38 caliber'. And it may be noted that .38 inches is more than 9 1/2 mm.

Common calibers in inch and their metric equivalents^[3][4][5][6]

Caliber	Metric equivalent	Typical bullet diameter	Common cartridges	Notes
.17	4.4 mm	0.172 in	.17 Remington, .17 HMR
.177	4.5 mm	.177 lead, .175 BB	Airgun .177 caliber
.20, .204	5 mm	0.204 in	.204 Ruger, 5 mm Remington Rimfire Magnum
.22, .218, .219 .220, .221, .222, .223, .224, .225, .226	5.5, 5.56, 5.7 mm	0.223-0.224 in	.22 Long Rifle, .222 Remington, .223 Remington, 5.56×45mm NATO, 5.7 x 28 mm, .22-250 Remington, .22 Airgun	NATO Round, M4/M16
.228	none	0.228 in	.228 Ackley Magnum	Bullets formerly available from Barnes, in heavily constructed 70 and 90 grain weights for medium game use
.24	6 mm	0.243 in	.243 Winchester, 6 mm Remington, 6mm plastic airsoft BBs
.25	6.35 mm	0.25 in, 6.35 mm	.25 ACP, 6.35x16mmSR	a.k.a .25 Auto and 6.35 mm Browning
.257	6.5 mm	0.257 in, 6.527 mm	.257 Roberts, .25-06 Remington, .250 Savage	typical 25 cal, not normally called 6.5
.26	6.5 mm	0.264 in, 6.7 mm	6.5 x 55 mm, .260 Remington	cartridges commonly known as 6.5
.27	6.8 mm, 7 mm	0.277 in, 7.035 mm	.270 Winchester, 6.8 SPC	not called 7 mm
.28	7 mm	0.284 in, 7.213 mm	7 mm Remington Magnum, 7 x 57 mm, 7mm-08 Remington	commonly called 7 mm
.30	7.62 mm	0.308 in	30-06, .308 Winchester (7.62mm NATO), .300 Winchester Magnum	American ".30 caliber"
.30	7.62 mm	0.311 in	.303 British, 7.62x39, 7.62x54R	Other ".30 caliber"
.32, .327	7.65 mm	0.309 - 0.312 in	.32 ACP, .32 S&W, .327 Federal Magnum	.32 caliber handgun cartridges
.32, .325	8 mm	0.323 in	8x57mm IS, .325 WSM, 8 mm Remington Magnum, 8mm plastic (airsoft) BBs	.32 caliber rifle cartridges
.338	8.58 mm	0.338 in	.338 Lapua, .338 Winchester Magnum, .338 Federal	.338 Rifle cartridge
.38, .380, .357, .35	9 mm	0.355-0.357 in	.38 Special, .380 ACP, .357 Magnum, .357 SIG, .35 Remington, 9×19mm Parabellum, 9x18mm Makarov, .357 in certain new Crosman Pre-charged pneumatic (PCP) airguns.	Generally .357 for revolvers and rifles, .355 in autoloaders
.38	10 mm	0.400 in	.38-40	Old black powder cartridge
.40	10 mm	0.400 in	.40 S&W, 10 mm Auto
.404	10.25 mm	0.423 in	.404 Jeffery
.405	10.75 mm	0.411 in	.405 Winchester
.408	10.4 mm	0.408 in	.408 Chey Tac	CheyTac Intervention
.41	10.25 mm	0.410 in	.41 Magnum .41 Action Express
.416	10.6 mm	0.416 in	.416 Barrett, .416 Remington Magnum, .416 Rigby, .416 Weatherby Magnum	Long-range sniper rounds If your target is under 500 Yards away and needs less than 1000 Ft. Lbs for an effective kill.
.43	11 mm	0.43 in Sl	.43 SL large
.44	10.8 mm	0.427 - 0.430 in	.44-40 Winchester, .44 Special, .44 Magnum	.44-40 is 0.427" to 0.428", .44 Special and Magnum are both 0.429" to 0.430"
.45	11.45 mm	0.451-0.452 in	.45 Colt post-WWII, .454 Casull, .45 ACP, 45GAP	Handgun .45 calibers, .451 jacketed bullets and .452 cast lead bullets
.45	11.6 mm	0.458 in	.45-70 Government	Most rifle .45 calibers
.454	11.53 mm	0.454 in	.45 Colt pre-WWII
.458, .46	11.6 mm	0.458 in	.460 Weatherby, .458 Winchester Magnum
.475, .480	12 mm	0.475 in	.480 Ruger, .475 Linebaugh
.50	12.7 mm	0.50 in	.50 AE, .500 S&W, .50 Beowulf, .50 GI	Desert Eagle, S&W X-Frame, Alexander Arms .50 Beowulf, Guncrafter Industries 1911 .50, .50 caliber Paintball markers
.50	12.95 mm	0.510 in	.50 BMG, 12.7 x 108 mm	M2 Browning machine gun and other heavy machine guns, long range rifles typified by Barrett Firearms Manufacturing products
.57	14.5 mm	0.586 in, 14.88 mm	14.5 x 114 mm, 14.5 mm JDJ	The 14.5 JDJ cartridge uses a .50 BMG case with opened neck, so only rifles chambered for the .50 BMG can be converted to this caliber
.68	17.272 mm	0.675-0.695 in	.68 Caliber Paintball markers	Typically .68 Caliber, not measured metrically and not generally tracked for variations; i.e. projectile size may vary (Not actually a firearm)
.79	20 mm	0.787 in	20 x 102 mm, 20x138mmB	Anti-materiel rifle and autocannon caliber,
.95	24.13 mm	0.950 in	.950 JDJ	.950 JDJ is the only known cartridge beyond .79 caliber to be used in a rifle

Firearm calibers outside the range of .17 to .50 (4.5 to 12.7 mm) exist, but are rarely encountered. Wildcat cartridges, for example, can be found in .10, .12, and .14 caliber (2.5, 3.0, & 3.6 mm), typically used for short range varmint hunting, where the high velocity, lightweight bullets provide devastating terminal ballistics with little risk of ricochet. Larger calibers, such as .577, .585, .600, .700, and .729 (14.7, 14.9, 15.2, 17.8, & 18.5 mm) are generally found in proprietary cartridges chambered in express rifles or similar guns intended for use on dangerous game.^[7]

Shotguns

Main article: Gauge (bore diameter)

Shotguns are classed according to gauge, a related expression. The gauge of a shotgun refers to how many lead spheres the diameter of the bore would equal a pound. In the case of a 12-gauge shotgun, it would take twelve spheres the size of the shotgun's bore to equal a pound. A numerically larger gauge indicates a smaller barrel: a 20-gauge 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 twelve caliber." The sixteenth caliber is known as "lordly" (Russian: барский). While shotgun bores can be expressed in calibers (the .410 bore shotgun is in fact a caliber measure of .41 caliber [11 mm]), the nature of shotshells is such that the barrel diameter often varies significantly down the length of the shotgun barrel, with various levels of choke and backboring.

Caliber as measurement of length

Main article: Caliber (artillery)

The length of artillery barrels has often been described in terms of multiples of the bore diameter e.g. a 4-inch gun of 50 calibers would have a barrel 50 x 4 inches = 200 inches long.

Pounds as a measure of cannon bore

Smoothbore cannon and carronade bores are designated by the weight in Imperial pounds of round solid iron shot of diameter to fit the bore. Standard sizes are 6, 12, 18, 24, 32 and 42 pounds, with some 68 pound weapons, and other nonstandard weapons using the same scheme. See Carronade#Ordnance.
From about the middle of the 17th century until the middle of the 19th century, measurement of the bore of large gunpowder weapons was usually expressed as the weight of its iron shot in pounds. Iron 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 3 pounders (pdr.), 4 pdr., 6 pdr., 8 pdr., 9 pdr, 12 pdr, 18 pdr., 24 pdr. and 32 pdr. being the most common sizes encountered; although larger, smaller and intermediate sizes existed.
However, in practice there was significant variation 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 livre, until 1812, had a mass of 489.5 g whilst the contemporary English (Avoirdupois) pound massed approximately 454 g. Thus, a French 32 pdr. at the Battle of Trafalgar threw a shot with 1.138 kg more mass than an English 32 pdr.
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 that 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.

English gun classes c. 1800^{[citation needed]}

gun class (pdr.)	shot diameter (cm)	shot volume (cm3)	approx. service bore (cm)	mass of projectile (kg)
2	6.04	172.76	6.64	0.90846
3	6.91	172.76	7.60	1.36028
4	7.60	230.30	8.37	1.81339
6	8.71	345.39	9.58	2.71957
9	10.00	518.28	11.00	4.08091
12	10.97	691.22	12.07	5.44269
18	12.56	1036.96	13.81	8.16499
24	13.82	1382.65	15.20	10.88696
32	15.21	1843.50	16.73	14.51572
64	19.17	3686.90	21.08	29.03063

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. While guns continued to be classed by the weight of their projectile into the middle of the 20th century, particularly in British service, this value no longer had any relation to the bore diameter as the projectiles themselves were no longer simple spheres and in any case were now more often hollow shells with explosive fillings rather than solid iron shot.