A carbine is a firearm similar to a rifle or musket, but generally shorter and of lesser power. Many carbines, especially modern designs, were developed from rifles, being essentially shortened versions of full rifles firing the same ammunition, although usually at a lower velocity. There have also been many where the carbine and rifle adopted by a particular nation were not technically related, such as using completely different ammunition or internal operating systems (though the carbine still being weaker, or of smaller size). Either may be more common, depending on the time period. There are also a limited number of pistol-caliber carbines, designed by integrating the action of a handgun such as a revolver or autoloader into a longer weapon with a rifle barrel and stock. They are generally employed as a more accurate alternative to a traditional handgun, and are used by some police teams and by civilian sport shooters.
In the 1800s, carbines were generally smaller firearms for cavalry. The foot soldiers would have a longer, more powerful firearm, and cavalry a shorter, lighter firearm.
The shorter length and lighter weight of carbines makes them easier to handle in close-quarter combat situations (such as urban or jungle warfare), or when deploying from vehicles. Carbines have higher penetration capabilities than submachine guns, while retaining standardized ammunition unlike personal defense weapons that use proprietary cartridges. The disadvantages of carbines, when compared with their longer counterparts, are generally poorer long-range accuracy and shorter effective range, while also generally being larger than submachine guns and thus harder to maneuver in close quarters. Like full-sized assault rifles, carbines have higher muzzle blast and recoil and may overpenetrate.
 Early history: before the 1900s
The carbine was originally a lighter, shortened, rifled weapon developed for the cavalry, for whom a full-length musket or rifle was too heavy and awkward to fire from horseback. Some sources derive the name of the weapon from the name of its first users—cavalry troopers called "carabins", hence French "carabine". Carbines were usually less accurate and powerful than the longer rifles of the infantry, due to a shorter sight plane and lower velocity of bullets fired from the shortened barrel. With the advent of fast-burning smokeless powder, the velocity disadvantages of the shorter barrels became less of an issue (see internal ballistics). Eventually, the use of horse-mounted cavalry would decline, but carbines continued to be issued and used by many who preferred a lighter, more compact weapon even at the cost of reduced long-range accuracy and power.
During the nineteenth century, carbines were often developed separately from the infantry rifles, and in many cases did not even use the same ammunition, which made for supply difficulties. A notable weapon developed towards the end of the American Civil War by the Union was the Spencer carbine. It had a spring-powered magazine in the stock which held seven rounds. In the late 1800s it became common for a number of nations to make bolt-action rifles in both full-length and carbine versions. One of the most popular and recognizable carbines was the Winchester lever-action carbine, with several versions using revolver cartridges. This made it an ideal choice for cowboys and explorers, who could carry a revolver and a carbine, both using the same ammo.
 Shorter rifles, shorter carbines: World War I and World War II
In the decades preceding World War I, the standard battle rifle used by armies around the world had been growing shorter, either by redesign or by the general issue of carbine versions instead of full-length rifles. For example, the Russian Model 1891 rifle with a 800 mm (31.5 inch) barrel was shortened to 730 mm (28.75 in.) in 1930, and to 510 mm (20 in.) in 1938; the German Mauser 98 rifles went from 740 mm (29 in.) in 1898 to 600 mm (23.6 in.) in 1935 as the Karabiner Kurz (K98k or Kar98k), or "short carbine". The barrel lengths in rifles used by the United States did not change between the bolt-action M1903 rifle of World War I and the World War II M1 Garand rifle, but then the 610 mm (24 in.) barrel on the M1903 was short for its day. The US M1 Carbine was more of a traditional carbine in that it was significantly shorter and lighter, with an 460 mm (18 in. barrel), than the M1 Garand rifle. The M1 Carbine was not a shorter version of the M1 Garand, but a wholly different design firing a smaller, less-powerful cartridge, as was common in the 1800s.
The United Kingdom also developed a "Jungle Carbine" version of their Lee-Enfield service rifle, featuring a shorter barrel, flash hider, and manufacturing modifications designed to decrease the rifle's weight. Officially titled Rifle, No. 5 Mk I, it was introduced in the closing months of WWII, but did not see widespread service until the Korean War, the Mau Mau uprising, and the Malayan Emergency.
 After World War II
A shorter weapon was more convenient when riding in a truck, armored personnel carrier, helicopter or aircraft, and also when engaged in close-range combat. Based on the combat experience of WWII, the criteria used for selecting infantry weapons began to change. Unlike previous wars, which were often fought mainly from fixed lines and trenches, WWII was a highly mobile war, often fought in cities, forests, or other areas where mobility and visibility were restricted. In addition, improvements in artillery made moving infantry in open areas even less practical than it had been.
The majority of enemy contacts were at ranges of less than 300 meters (325 yards), and the enemy was exposed to fire for only short periods of time as they moved from cover to cover. Most rounds fired were not aimed at an enemy combatant, but instead fired in the enemy's direction to keep them from moving and firing back (see suppressive fire). These situations did not require a heavy rifle, firing full-power rifle bullets with long-range accuracy. A less-powerful weapon would still produce casualties at the shorter ranges encountered in actual combat, and the reduced recoil would allow more shots to be fired in the short amount of time an enemy was visible. The lower-powered round would also weigh less, allowing a soldier to carry more ammunition. With no need of a long barrel to fire full-power ammunition, a shorter barrel could be used. A shorter barrel made the weapon weigh less and was easier to handle in tight spaces, and was easier to shoulder quickly to fire a shot at an unexpected target. Full-automatic fire was also considered a desirable feature, allowing the soldier to fire short bursts of three to five rounds, increasing the probability of a hit on a moving target.
The Germans had experimented with selective-fire carbines firing rifle cartridges during the early years of WWII. These were determined to be less than ideal, as the recoil of full-power rifle cartridges caused the weapon to be uncontrollable in full-automatic fire. They then developed an intermediate-power cartridge round, which was accomplished by reducing the power and the length of the standard 7.92x57 Mauser rifle cartridge to create the 7.92x33 Kurz (Short) cartridge. A selective-fire weapon was developed to fire this shorter cartridge, eventually resulting in the Sturmgewehr 44, later translated as "assault rifle". After WWII, the USSR would adopt a similar weapon, the legendary AK-47, which became the standard Soviet infantry weapon. The United States during WWII also had the M2 Carbine, a selective-fire version of the M1 Carbine firing a 7.62x33mm cartridge. However, the semi-automatic M1 carbine was produced in a 10-to-1 ratio to the M2.
Although the NATO countries did not adopt an intermediate-power round, they continued the trend toward shorter and lighter magazine-fed battle rifles. NATO adopted the 7.62x51 NATO round (which in reality is only slightly different ballistically to the .30-06 Springfield and .303 British cartridges), along with several rifles such as the FN FAL and M14.
By the 1960s NATO had adopted the 5.56 NATO cartridge. This round was even lighter and smaller than the Soviet AK-47 cartridge, but possessed higher velocity. In U.S. service, the M16 assault rifle replaced the M14 as the standard infantry weapon, although the M14 continued to be used by designated marksmen.
Lighter carbines came to be adopted as the standard infantry long rifle. What changed was that only a certain number of soldiers now needed to retain longer range weapons, serving as designated marksmen. Development of lighter assault rifles continued, matched by developments in even lighter carbines. At the same time the infantry switched to 5.56 mm weapons, carbines like the AKS-74U (which fired a Warsaw pact 5.45x39 round) and CAR-15 were being developed.
 Modern history
 Contemporary military forces
By the 1990s, the US had adopted the M4 Carbine, a derivative of the M16 family which was lighter and shorter (in overall length and barrel length), resulting in reduced range and power.
Meanwhile, many armies are experiencing a backlash against carbines and lighter rifles in general, and are equipping selected soldiers, usually called Designated Marksmen, or DM, with higher power rifles. While firing a higher quantity of smaller bullets makes it easier to hit a target (and is therefore good for beginner marksmen), it offers very little to more advanced marksmen. Furthermore, the additional range of the heavier weapons has proven to be necessary in open environments such as deserts. As a result, the focus on more highly trained soldiers equipped with, for example, 7.62 mm NATO firing rifles, such as the U.S. Marine Corps Designated Marksman Rifle variant of the M14, has increased somewhat. A squad of soldiers armed with assault rifles would have a single soldier assigned as DM who would carry a battle rifle for selectively engaging long range targets. The DM differs from the sniper in that the DM moves with his unit, and engages targets at ranges beyond the 500 metre (550 yd) effective range of modern assault rifles, but less than the 600 metre (650 yd) range which is the optimal engagement range for snipers.
 Special operations forces
One bastion of the carbine which is unlikely to be unseated is the special operations forces of the world which need to perform fast, decisive operations. A pistol, though light and quick to operate, is viewed as not having enough power. Consequently, carbines have gained wide acceptance among SOCOM and other communities.
 Personal defense weapons
In both civilian use and among the military's lighter armored soldiers, it is common to use a carbine. However, due to some of the reasons mentioned elsewhere in this article, such as muzzle blast and recoil, these carbines generally chamber lighter rounds than some of their heavier (or shorter) counterparts. Due to the low wounding power of a single round, personal defense weapons depend on high volumes of fire for effectiveness. This may make such carbines successful in military use. In civilian use where fully automatic fire is illegal, acceptance may be lower as a pistol involves single shots. However, firing a pistol effectively at any significant range requires good training, since the absence of a buttstock makes precise ranged aim difficult.
In addition, due to the development of body armor, support personnel required a compact weapon with more stopping power than the traditional submachine gun, leading to the development of the personal defense weapon, or PDW, which uses rounds that have better ballistics than simple pistol rounds but less power and range than full rifle rounds. Examples include the FN P90 and HK MP7. Whether these ultra-light weapons will be widely adopted has yet to be seen. The cartridge used by the FN P90, the 5.7 x 28 mm, for example, fires a 2.9 g (45 grain) armor piercing bullet at velocities of around 700 m/s (2300 ft/s). The H&K MP7 fires an even smaller 4.6 mm round with ballistics similar to the .22 WMR derived .17 Hornady Magnum Rimfire. The small diameter rounds enable the projectile to travel at the high velocity needed to penetrate Kevlar armor, as a light weapon with sufficient energy to push a large caliber bullet through the armor would have prohibitive recoil. The small bullets are generally designed to tumble after penetration.
 Future acceptance and use
To what extent armies will adopt even lighter carbines, and to what extent they will be avoided, has yet to be seen entirely. It is likely that harder hitting, or at least higher penetrating, weapons will become more common, due to a rise in use of body armor, making weaker weapons ineffective. Stacked against this is mainly the amount of urban warfare that is required, which favors lighter carbine weapons.
The modern usage of the term carbine covers much the same scope as it always had, namely lighter weapons (generally rifles) with barrels of less than about 460 mm (18 inches). These weapons can be considered carbines, while rifles with barrels of 510 mm (20 inches) or more are generally not considered carbines unless specifically named so, and depending on the weapon's power. Modern carbines use ammunition ranging from that used in light pistols up to powerful rifle cartridges, with the usual exception of high velocity magnum cartridges. In the more powerful cartridges, the short barrel of a carbine has significant disadvantages in velocity, and the high residual pressure when the bullet exits the barrel results in substantially greater muzzle blast. Flash suppressors and muzzle brakes are common solutions to this problem, which may ease their acceptance.
 Pistol-caliber carbines
One of the more unusual classes of carbine is the pistol caliber carbine. These first appeared soon after metallic cartridges became common. These were developed as "companions" to the popular revolvers of the day, firing the same cartridge but allowing more velocity and accuracy than the revolver. These were carried by cowboys, lawmen, and others in the Old West. The classic combination would be a Winchester lever action carbine and a Colt revolver in .44-40 or .45 Colt. Another example is the DeLisle carbine that was chambered for .45 ACP.
Modern equivalents also exist, such as the discontinued Ruger Police Carbine, which uses the same magazine as the Ruger pistols of the same caliber. The Beretta Cx4 Storm shares magazines with many Beretta pistols, and shares its design with the Beretta Px4 Storm pistol. The Hi-Point 995 carbine is a cheaper alternative to other pistol caliber carbines in the United States and shares magazines with the Hi-Point C-9 pistol, other examples are the Kel-Tec Sub-2000 series which accepts Glock, Beretta and S&W pistol magazines chambered in either 9mm Luger or .40S&W. The recent introduction of such products may indicate that there is a growing demand for these companion carbines.
The primary advantages of a pistol caliber carbine are increased accuracy due to the buttstock or shoulder rest, higher muzzle velocity and energy of a longer barrel for increased wounding potential and penetration, and the ability to add accessories such as night scopes, tactical lights, and/or laser sights. Additionally, many pistol calibers, such as .357 Magnum, .41 Magnum, .44 Magnum, and .45 ACP, have much heavier projectiles, such as 15 g (230 grain), than rifles do, 3.6 g (55 grains) being common in 22-caliber rounds like .22-250 and .223.
Recently, kits have appeared on the market which consist of a carbine length barrel, bolt (usually blowback operated), and stock that attach to a pistol frame. Attaching the pistol frame provides the feed mechanism (through the pistol's magazine) and trigger mechanism, and produces a completed carbine. Kits of this type are available for Colt M1911 pattern and Glock pistols.
 Short barrelled rifles
Firearms with shoulder stocks and barrels less than 406 mm (16 in) in length are classified as "short barreled rifles" (under the US National Firearms Act or NFA), and are sometimes restricted in the same way that sawed off shotguns and machine guns are. Because of this, rifles with barrels of less than 406 mm (16 in), or pistols with shoulder stocks, are rare. A list of firearms not covered by the NFA due to their antique or Curio and Relic status may be found here; this list includes a number of carbines with barrels less than the minimum legal length. Firearms classified as machine guns are also not subject to the barrel length restriction.
Kits exist which will convert many pistols into carbines by the addition of a shoulder stock; notable examples are the long barrelled Colt Buntline revolver stock, the Mauser C96 "Broomhandle" holster/stock, and various others for models such as the Browning Hi-Power, Luger, Colt M1911, and the Heckler & Koch VP70. Since these stock additions retain the short pistol barrel (as short as 100 mm (4 inches) they are highly restricted under the NFA; many pistols which had attachments for the stocks, including rare wartime models, were altered to remove the attachment point.
 Other carbines
Another class of carbine is a semi-automatic version of a submachine gun, with an extended barrel, just over 406 mm (16 inches) long, which will escape ban by some "assault weapon" legislation. While functionally identical to other carbines, these are banned in some places as "assault weapons" based on their cosmetic similarity to submachine guns. However, they may not accept certain parts (such as magazines or collapsing stocks) from the submachine guns they resemble. These are a popular compromise for (American) shooters who would like to own a submachine gun but cannot due to local restrictions or the prohibitive cost of buying a civilian legal submachine gun. Many of the more vocal proponents of personal defense firearms typically recommend a pistol and a higher power firearm, like a carbine or a shotgun.
In some historical cases the term machine carbine was the official title for sub-machine guns. Examples are the Sten and Owen guns. Equally to confuse the non-automatic version of the Sterling submachine gun was also a "Carbine".