A firearm is a portable gun (a barreled ranged weapon) designed for use by a single individual. It inflicts damage on targets by launching one or more projectiles driven by rapidly expanding high-pressure gas produced by exothermic combustion (deflagration) of chemical propellant within an ammunition cartridge.^[1][2][3] If gas pressurization is achieved through mechanical gas compression rather than through chemical propellant combustion, then the gun is technically an air gun, not a firearm.^[4]

The first primitive firearms originated in 10th-century China when bamboo tubes containing gunpowder and pellet projectiles were mounted on spears into the one-person-portable fire lance,^[5] which was later used as a shock weapon to good effect in the Siege of De'an in 1132. In the 13th century the Chinese invented the metal-barrelled hand cannon, widely considered the true ancestor of all firearms. The technology gradually spread through the rest of East Asia, South Asia, the Middle East, and Europe. Older firearms typically used black powder as a propellant, but modern firearms use smokeless powder or other propellants. Most modern firearms (with the notable exception of smoothbore shotguns) have rifled barrels to impart spin to the projectile for improved flight stability.

Modern firearms can be described by their caliber (i.e. their bore diameter; this is given in millimeters or inches e.g. 7.5 mm, .357 in.) or in the case of shotguns by their gauge (e.g. 12 ga.); by the type of action employed (muzzleloader, breechloader, lever, bolt, pump, revolver, semi-automatic, fully automatic, etc.) together with the usual means of deportment (hand-held or mechanical mounting). Further classification may make reference to the type of barrel used (rifled) and to the barrel length (24 inch), to the firing mechanism (e.g. matchlock, wheellock, flintlock, percussion lock), to the design's primary intended use (e.g. hunting rifle), or to the commonly accepted name for a particular variation (e.g. Gatling gun).

Shooters aim firearms at their targets with hand-eye coordination, using either iron sights or optical sights. The accurate range of pistols generally does not exceed 110 yards (100 m), while most rifles are accurate to 550 yards (500 m) using iron sights, or to longer ranges using optical sights (firearm rounds may be dangerous or lethal well beyond their accurate range; the minimum distance for safety is much greater than the specified range). Purpose-built sniper rifles and anti-materiel rifles are accurate to ranges of more than 2,200 yards (2,000 m).

Firearms include a variety of ranged weapons and there is no agreed upon definition.

In the United States, Under 26 USCA § 861 (a), The term ‘‘firearm’’ means (1) a shotgun having a barrel or barrels of less than 18 inches in length; (2) a weapon made from a shotgun if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 18 inches in length; (3) a rifle having a barrel or barrels of less than 16 inches in length; (4) a weapon made from a rifle if such weapon as modified has an overall length of less than 26 inches or a barrel or barrel of less than 16 inches in length; (5) any other weapon, as defined in subsection (e); (6) a machinegun; (7) any silencer (as defined in section 921 of title 18, United States Code); The term ‘‘firearm’’ shall not include an antique firearm or any device (other than a machinegun or destructive device) which, although designed as a weapon, the Secretary finds by reason of the date of its manufacture, value, design, and other characteristics is primarily a collector’s item and is not likely to be used as a weapon.

The term "small arms" generally refers to any kinetic projectile firearm small and light enough to be carried and operated by a single infantryman. These include handguns such as revolvers, pistols and derringers, and long guns such as rifles (of which there are many subtypes such as anti-material rifles, sniper rifles, designated marksman rifles, battle rifles, assault rifles and carbines), shotguns, submachine guns, personal defense weapons, squad automatic weapons and light machine guns.^[6]

The world's top small arms manufacturing companies are Browning, Remington, Colt, Ruger, Smith & Wesson, Savage, Mossberg (USA), Heckler & Koch, SIG Sauer, Walther (Germany), ČZUB (Czech Republic), Glock, Steyr-Mannlicher (Austria), FN Herstal (Belgium), Beretta (Italy), Norinco (China), Tula Arms and Kalashnikov (Russia), while former top producers were Mauser, Springfield Armory, and Rock Island Armory under Armscor (Philippines).

In 2018, Small Arms Survey reported that there are over one billion small arms distributed globally, of which 857 million (about 85 percent) are in civilian hands.^[7][8] U.S. civilians alone account for 393 million (about 46 percent) of the worldwide total of civilian held firearms.^[8] This amounts to "120.5 firearms for every 100 residents."^[8] The world's armed forces control about 133 million (about 13 percent) of the global total of small arms, of which over 43 percent belong to two countries: the Russian Federation (30.3 million) and China (27.5 million).^[7] Law enforcement agencies control about 23 million (about 2 percent) of the global total of small arms.^[7]

The smallest of all firearms is the handgun. There are two common types of handguns: revolvers and semi-automatic pistols. Revolvers have a number of firing chambers or "charge holes" in a revolving cylinder; each chamber in the cylinder is loaded with a single cartridge or charge. Semi-automatic pistols have a single fixed firing chamber machined into the rear of the barrel, and a magazine so they can be used to fire more than one round. Each press of the trigger fires a cartridge, using the energy of the cartridge to activate the mechanism so that the next cartridge may be fired immediately. This is opposed to "double-action" revolvers which accomplish the same end using a mechanical action linked to the trigger pull.

Prior to the 19th century, virtually all handguns were single-shot muzzleloaders. With the invention of the revolver in 1818, handguns capable of holding multiple rounds became popular. Certain designs of auto-loading pistol appeared beginning in the 1870s and had largely supplanted revolvers in military applications by the end of World War I. By the end of the 20th century, most handguns carried regularly by military, police and civilians were semi-automatic, although revolvers were still widely used. Generally speaking, military and police forces use semi-automatic pistols due to their high magazine capacities and ability to rapidly reload by simply removing the empty magazine and inserting a loaded one. Revolvers are very common among handgun hunters because revolver cartridges are usually more powerful than similar caliber semi-automatic pistol cartridges (which are designed for self-defense) and the strength, simplicity and durability of the revolver design is well-suited to outdoor use. Revolvers, especially in .22 LR and 38 Special/357 Magnum, are also common concealed weapons in jurisdictions allowing this practice because their simple mechanics make them smaller than many autoloaders while remaining reliable. Both designs are common among civilian gun owners, depending on the owner's intention (self-defense, hunting, target shooting, competitions, collecting, etc.).

A long gun is generally any firearm that is larger than a handgun and is designed to be held and fired with both hands, either from the hip or the shoulder. Long guns typically have a barrel between 10 and 30 inches (there are restrictions on minimum barrel length in many jurisdictions; maximum barrel length is usually a matter of practicality), that along with the receiver and trigger group is mounted into a wood, plastic, metal or composite stock, composed of one or more pieces that form a foregrip, rear grip, and optionally (but typically) a shoulder mount called the butt. Early long arms, from the Renaissance up to the mid-19th century, were generally smoothbore firearms that fired one or more ball shot, called muskets or arquebus depending on caliber and firing mechanism.

Most modern long guns are either rifles or shotguns. Both are the successors of the musket, diverging from their parent weapon in distinct ways. A rifle is so named for the spiral fluting (rifling) machined into the inner surface of its barrel, which imparts a self-stabilizing spin to the single bullets it fires. Shotguns are predominantly smoothbore firearms designed to fire a number of shot; pellet sizes commonly ranging between 2 mm #9 birdshot and 8.4 mm #00 (double-aught) buckshot. Shotguns are also capable of firing single slugs, or specialty (often "less lethal") rounds such as bean bags, tear gas or breaching rounds. Rifles have a very small impact area but a long range and high accuracy. Shotguns have a large impact area with considerably less range and accuracy. However, the larger impact area can compensate for reduced accuracy, since shot spreads during flight; consequently, in hunting, shotguns are generally used for flying game.

Rifles and shotguns are commonly used for hunting and often to defend a home or place of business. Usually, large game are hunted with rifles (although shotguns can be used, particularly with slugs), while birds are hunted with shotguns. Shotguns are sometimes preferred for defending a home or business due to their wide impact area, multiple wound tracks (when using buckshot), shorter range, and reduced penetration of walls (when using lighter shot), which significantly reduces the likelihood of unintended harm, although the handgun is also common.

There are a variety of types of rifles and shotguns based on the method they are reloaded. Bolt-action and lever-action rifles are manually operated. Manipulation of the bolt or the lever causes the spent cartridge to be removed, the firing mechanism recocked, and a fresh cartridge inserted. These two types of action are almost exclusively used by rifles. Slide-action (commonly called 'pump-action') rifles and shotguns are manually cycled by shuttling the foregrip of the firearm back and forth. This type of action is typically used by shotguns, but several major manufacturers make rifles that use this action.

Both rifles and shotguns also come in break-action varieties that do not have any kind of reloading mechanism at all but must be hand-loaded after each shot. Both rifles and shotguns come in single- and double-barreled varieties; however due to the expense and difficulty of manufacturing, double-barreled rifles are rare. Double-barreled rifles are typically intended for African big-game hunts where the animals are dangerous, ranges are short, and speed is of the essence. Very large and powerful calibers are normal for these firearms.

Rifles have been in nationally featured marksmanship events in Europe and the United States since at least the 18th century, when rifles were first becoming widely available. One of the earliest purely "American" rifle-shooting competitions took place in 1775, when Daniel Morgan was recruiting sharpshooters in Virginia for the impending American Revolutionary War. In some countries, rifle marksmanship is still a matter of national pride. Some specialized rifles in the larger calibers are claimed to have an accurate range of up to about 1 mile (1,600 m), although most have considerably less. In the second half of the 20th century, competitive shotgun sports became perhaps even more popular than riflery, largely due to the motion and immediate feedback in activities such as skeet, trap and sporting clays.

In military use, bolt-action rifles with high-power scopes are common as sniper rifles, however by the Korean War the traditional bolt-action and semi-automatic rifles used by infantrymen had been supplemented by select-fire designs known as "automatic rifles".

A carbine is a firearm similar to a rifle in form and intended usage, but generally shorter or smaller than the typical "full-size" hunting or battle rifle of a similar time period, and sometimes using a smaller or less-powerful cartridge. Carbines were and are typically used by members of the military in roles that are expected to engage in combat, but where a full-size rifle would be an impediment to the primary duties of that soldier (vehicle drivers, field commanders and support staff, airborne troops, engineers, etc.). Carbines are also common in law enforcement and among civilian owners where similar size, space and/or power concerns may exist. Carbines, like rifles, can be single-shot, repeating-action, semi-automatic or select-fire/fully automatic, generally depending on the time period and intended market. Common historical examples include the Winchester Model 1892, Lee–Enfield "Jungle Carbine", SKS, M1 carbine (no relation to the larger M1 Garand) and M4 carbine (a more compact variant of the current M16 rifle). Modern U.S. civilian carbines include compact customizations of the AR-15, Ruger Mini-14, Beretta Cx4 Storm, Kel-Tec SUB-2000, bolt-action rifles generally falling under the specifications of a scout rifle, and aftermarket conversion kits for popular pistols including the M1911 and Glock models.

A machine gun is a fully automatic emplaceable weapon, most often separated from other classes of automatic weapons by the use of belt-fed ammunition (though some designs employ drum, pan or hopper magazines), generally in a rifle-inspired caliber ranging between 5.56×45mm NATO (.223 Remington) for a light machine gun to as large as .50 BMG or even larger for crewed or aircraft weapons. Although not widely fielded until World War I, early machine guns were being used by militaries in the second half of the 19th century. Notables in the U.S. arsenal during the 20th century included the M2 Browning .50 caliber heavy machine gun, M1919 Browning .30 caliber medium machine gun, and the M60 7.62×51mm NATO general-purpose machine gun which came into use around the Vietnam War. Machine guns of this type were originally defensive firearms crewed by at least two men, mainly because of the difficulties involved in moving and placing them, their ammunition, and their tripod. In contrast, modern light machine guns such as the FN Minimi are often wielded by a single infantryman. They provide a large ammunition capacity and a high rate of fire, and are typically used to give suppressing fire during infantry movement. Accuracy on machine guns varies based on a wide number of factors from design to manufacturing tolerances, most of which have been improved over time. Machine guns are often mounted on vehicles or helicopters, and have been used since World War I as offensive firearms in fighter aircraft and tanks (e.g. for air combat or suppressing fire for ground troop support).

The definition of machine gun is different in U.S. law. The National Firearms Act and Firearm Owners Protection Act define a "machine gun" in the United States code Title 26, Subtitle E, Chapter 53, Subchapter B, Part 1, § 5845 as: "... any firearm which shoots ... automatically more than one shot, without manual reloading, by a single function of the trigger". "Machine gun" is therefore largely synonymous with "automatic weapon" in the U.S. civilian parlance, covering all automatic firearms.

The definition of a sniper rifle is disputed among military, police and civilian observers alike, however most generally define a “sniper rifle” as a high powered, semi-automatic/bolt action, precision rifle with an accurate range further than that of a standard rifle. These are often purpose-built for their applications. For example, a police sniper rifle may differ in specs from a military rifle. Police snipers generally do not engage targets at extreme range, but rather, a target at medium range. They may also have multiple targets within the shorter range, and thus a semi-automatic model is preferred to a bolt action. They also may be more compact than milspec rifles as police marksmen may need more portability. On the other hand, a military rifle is more likely to use a higher powered cartridge to defeat body armor or medium-light cover. They are more commonly (but not a lot more) bolt-action, as they are simpler to build and maintain. Also, due to fewer moving and overall parts, they are much more reliable under adverse conditions. They may also have a more powerful scope to acquire targets further away. Overall, sniper units never became prominent until World War 1, when the Germans displayed their usefulness on the battlefield. Since then, they have become irrevocably embedded in warfare. Examples of sniper rifles include the Accuracy International AWM, Sako TRG-42 and the CheyTac M200. Examples of specialized sniper cartridges include the .338 Lapua Magnum, .300 Winchester Magnum, and .408 CheyTac rounds.

A submachine gun is a magazine-fed firearm, usually smaller than other automatic firearms, that fires pistol-caliber ammunition; for this reason certain submachine guns can also be referred to as machine pistols, especially when referring to handgun-sized designs such as the Škorpion vz. 61 and Glock 18. Well-known examples are the Israeli Uzi and Heckler & Koch MP5 which use the 9×19mm Parabellum cartridge, and the American Thompson submachine gun which fires .45 ACP. Because of their small size and limited projectile penetration compared to high-power rifle rounds, submachine guns are commonly favored by military, paramilitary and police forces for close-quarters engagements such as inside buildings, in urban areas or in trench complexes.

Submachine guns were originally about the size of carbines. Because they fire pistol ammunition, they have limited long-range use, but in close combat can be used in fully automatic in a controllable manner due to the lighter recoil of the pistol ammunition. They are also extremely inexpensive and simple to build in time of war, enabling a nation to quickly arm its military. In the latter half of the 20th century, submachine guns were being miniaturized to the point of being only slightly larger than some large handguns. The most widely used submachine gun at the end of the 20th century was the Heckler & Koch MP5. The MP5 is actually designated as a "machine pistol" by Heckler & Koch (MP5 stands for Maschinenpistole 5, or Machine Pistol 5), although some reserve this designation for even smaller submachine guns such as the MAC-10 and Glock 18, which are about the size and shape of pistols.

An automatic rifle is a magazine-fed firearm, wielded by a single infantryman, that is chambered for rifle cartridges and capable of automatic fire. The M1918 Browning Automatic Rifle was the first U.S. infantry weapon of this type, and was generally used for suppressive or support fire in the role now usually filled by the light machine gun. Other early automatic rifles include the Fedorov Avtomat and the Huot Automatic Rifle. Later, German forces fielded the Sturmgewehr 44 during World War II, a light automatic rifle firing a reduced power "intermediate cartridge". This design was to become the basis for the "assault rifle" subclass of automatic weapons, as contrasted with "battle rifles", which generally fire a traditional "full-power" rifle cartridge.

In World War II, Germany introduced the StG 44, and brought to the forefront of firearm technology what eventually became the class of firearm most widely adopted by the military, the assault rifle. An assault rifle is usually slightly smaller than a battle rifle such as the American M14, but the chief differences defining an assault rifle are select-fire capability and the use of a rifle round of lesser power, known as an intermediate cartridge.

Soviet engineer Mikhail Kalashnikov quickly adapted the German concept, using a less-powerful 7.62×39mm cartridge derived from the standard 7.62×54mmR Russian battle rifle round, to produce the AK-47, which has become the world's most widely used assault rifle. Soon after World War II, the Automatic Kalashnikov AK-47 assault rifle began to be fielded by the Soviet Union and its allies in the Eastern Bloc, as well as by nations such as China, North Korea, and North Vietnam.

In the United States, the assault rifle design was later in coming; the replacement for the M1 Garand of WWII was another John Garand design chambered for the new 7.62×51mm NATO cartridge; the select-fire M14, which was used by the U.S. military until the 1960s. The significant recoil of the M14 when fired in full-automatic mode was seen as a problem as it reduced accuracy, and in the 1960s it was replaced by Eugene Stoner's AR-15, which also marked a switch from the powerful .30 caliber cartridges used by the U.S. military up until early in the Vietnam War to the much less powerful but far lighter and light recoiling .223 caliber (5.56mm) intermediate cartridge. The military later designated the AR-15 as the "M16". The civilian version of the M16 continues to be known as the AR-15 and looks exactly like the military version, although to conform to B.A.T.F.E. regulations in the U.S., it lacks the mechanism that permits fully automatic fire.

Variants of both of the M16 and AK-47 are still in wide international use today, though other automatic rifle designs have since been introduced. A smaller version of the M16A2, the M4 carbine, is widely used by U.S. and NATO tank and vehicle crews, airbornes, support staff, and in other scenarios where space is limited. The IMI Galil, an Israeli-designed weapon based on the action of the AK-47, is in use by Israel, Italy, Burma, the Philippines, Peru, and Colombia. Swiss Arms of Switzerland produces the SIG SG 550 assault rifle used by France, Chile, and Spain among others, and Steyr Mannlicher produces the AUG, a bullpup rifle in use in Austria, Australia, New Zealand, Ireland, and Saudi Arabia among other nations.

Modern designs call for compact weapons retaining firepower. The bullpup design, by mounting the magazine behind the trigger, unifies the accuracy and firepower of the traditional assault rifle with the compact size of the submachine gun (though submachine guns are still used); examples are the French FAMAS and the British SA80.

A recently developed class of firearm is the personal defense weapon or PDW, which is in simplest terms a submachine gun designed to fire ammunitions with ballistic performance similar to rifle cartridges. While a submachine gun is desirable for its compact size and ammunition capacity, its pistol cartridges lack the penetrating capability of a rifle round. Conversely, rifle bullets can pierce light armor and are easier to shoot accurately, but even a carbine such as the Colt M4 is larger and/or longer than a submachine gun, making it harder to maneuver in close quarters. The solution many firearms manufacturers have presented is a weapon resembling a submachine gun in size and general configuration, but which fires a higher-powered armor-penetrating round (often specially designed for the weapon), thus combining the advantages of a carbine and submachine gun. This also earned the PDWs an infrequently used nickname — the submachine carbines. The FN P90 and Heckler & Koch MP7 are most famous examples of PDWs.

Battle rifles are another subtype of rifle, usually defined as selective fire rifles that use full power rifle cartridges, examples of which include the 7.62x51mm NATO, 7.92x57mm Mauser, and 7.62x54mmR. These serve similar purposes to assault rifles, as they both are usually employed by ground infantry. However, some prefer battle rifles due to their more powerful cartridge, despite added recoil. Some semi-automatic sniper rifles are configured from battle rifles.

Firearms are also categorized by their functioning cycle or "action" which describes its loading, firing, and unloading cycle.

The earliest evolution of the firearm, there are many types of manual action firearms. These can be divided into two basic categories: single shot and repeating.

A single shot firearm can only be fired once per equipped barrel before it must be reloaded or charged via an external mechanism or series of steps. A repeating firearm can be fired multiple times, but can only be fired once with each subsequent pull of the trigger. Between trigger pulls, the firearm's action must be reloaded or charged via an internal mechanism.

A semi-automatic, self-loading, or "auto loader" firearm is one that performs all steps necessary to prepare it for firing again after a single discharge, until cartridges are no longer available in the weapon's feed device or magazine. Auto loaders fire one round with each pull of the trigger. Some people confuse the term with "fully automatic" firearms. (See next.) While some semi-automatic rifles may resemble military-style firearms, they are not properly classified "Assault Weapons" which refers to those that continue to fire until the trigger is no longer depressed.

An automatic firearm, or "fully automatic", "fully auto", or "full auto", is generally defined as one that continues to load and fire cartridges from its magazine as long as the trigger is depressed (and until the magazine is depleted of available ammunition.) The first weapon generally considered in this category is the Gatling gun, originally a carriage-mounted, crank-operated firearm with multiple rotating barrels that was fielded in the American Civil War. The modern trigger-actuated machine gun began with various designs developed in the late 19th century and fielded in World War I, such as the Maxim gun, Lewis Gun, and MG 08 "Spandau". Most automatic weapons are classed as long guns (as the ammunition used is of similar type as for rifles, and the recoil of the weapon's rapid fire is better controlled with two hands), but handgun-sized automatic weapons also exist, generally in the "submachine gun" or "machine pistol" class.

Selective fire, or "select fire", means the capability of a weapon's fire control to be adjusted in either semi-automatic or fully automatic firing modes. The modes are chosen by means of a selector, which varies depending on the weapon's design. Some selective-fire weapons have burst fire mechanisms built in to limit the maximum number of shots fired in fully automatic mode, with most common limits being two or three rounds per trigger pull. The presence of selective-fire modes on firearms allows more efficient use of ammunition for specific tactical needs, either precision-aimed or suppressive fire. This capability is most commonly found on military weapons of the 20th and 21st centuries, most notably the assault rifles.

Assault by firearm resulted in 173,000 deaths, globally, in 2015, up from 128,000 deaths in 1990,^[9][10] however this represents a drop in rate from 2.41/100,000 in 1990 to 2.35/100,000 in 2015, as world population has increased by more than two billion.^[11] Additionally, there were 32,000 unintentional firearm global deaths in 2015.^[9] In 2017, there were 39,773 gun-related deaths in the United States, and over 60% of suicides in the United States are from firearms.^[12] Firearms are the second leading mechanism of injury deaths, after motor vehicle accidents.^[13][14] To prevent unintentional injury, gun safety training includes education on proper firearm storage and firearm-handling etiquette.^[15][16]

The first primitive firearms were invented about 1250 AD in China when the man-portable fire lance (a bamboo or metal tube that could shoot ignited gunpowder) was combined with projectiles such as scrap metal, broken porcelain, or darts/arrows.^[5][17]

The earliest depiction of a firearm is a sculpture from a cave in Sichuan, China. The sculpture dates to the 12th century and is of a figure carrying a vase-shaped bombard, with flames and a cannonball coming out of it.^{[18] []} The oldest surviving gun, a hand cannon made of bronze, has been dated to 1288 because it was discovered at a site in modern-day Acheng District, Heilongjiang, China, where the Yuan Shi records that battles were fought at that time.^[19] The firearm had a 6.9 inch barrel of a 1-inch diameter, a 2.6 inch chamber for the gunpowder and a socket for the firearm's handle. It is 13.4 inches long and 7.8 pounds without the handle, which would have been made of wood.^{[18] []}

The Arabs and Mamluks had firearms in the late 13th century.^[20][21][22] In the 14th century, firearms were obtained by the Ottoman Turks^[23][24] and the Europeans.^{[18] []} The Koreans adopted firearms from the Chinese in the 14th century. The Iranians (first Aq Qoyunlu and Safavids) and Indians (first Mughals) all got them no later than the 15th century, from the Ottoman Turks. The Japanese did not acquire firearms until the 16th century, and then from the Portuguese rather than the Chinese.^{[18] []}

The development behind firearms accelerated during the 19th and 20th centuries. Breech-loading became more or less a universal standard for the reloading of most hand-held firearms and continues to be so with some notable exceptions (such as mortars). Instead of loading individual rounds into weapons, magazines holding multiple munitions were adopted—these aided rapid reloading. Automatic and semi-automatic firing mechanisms meant that a single soldier could fire many more rounds in a minute than a vintage weapon could fire over the course of a battle. Polymers and alloys in firearm construction made weaponry progressively lighter and thus easier to deploy. Ammunition changed over the centuries from simple metallic ball-shaped projectiles that rattled down the barrel to bullets and cartridges manufactured to high precision. Especially in the past century has particular attention been devoted to accuracy and sighting to make firearms altogether far more accurate than ever before. More than any single factor though, firearms have proliferated due to the advent of mass production—enabling arms manufacturers to produce large quantities of weaponry to a consistent standard.

Generally, kinetic energy can be enhanced in two ways:

Velocities of bullets increased with the use of a "jacket" of a metal such as copper or copper alloys that covered a lead core and allowed the bullet to glide down the barrel more easily than exposed lead. Such bullets are designated as "full metal jacket" (FMJ). Such FMJ bullets are less likely to fragment on impact and are more likely to traverse through a target while imparting less energy. Hence, FMJ bullets impart less tissue damage than non-jacketed bullets that expand. (Dougherty and Eidt, 2009) This led to their adoption for military use by countries adhering to the Hague Convention in 1899.

That said, the basic principle behind firearm operation remains unchanged to this day. A musket of several centuries ago is still similar in principle to a modern-day assault rifle—using the expansion of gases to propel projectiles over long distances—albeit less accurately and rapidly.^[25]

The Chinese fire lance was the direct predecessor to the modern concept of the firearm. It was not a gun itself, but an addition to the soldiers' spears. Originally it consisted of paper or bamboo barrels that would have incendiary gunpowder within it, that could be lit one time and would project flames at the enemy. Sometimes the Chinese troops would place small projectiles within the barrel that would also be projected when the gunpowder was lit, but most of the explosive force would create flames. Later, the barrel was changed to be made of metal, so that a more explosive gunpowder could be used and put more force into the propulsion of the projectile.^{[18] []}

The original predecessor of all firearms, the Chinese fire lance and hand cannon were loaded with gunpowder and the shot (initially lead shot, later replaced by cast iron) through the muzzle, while a fuse was placed at the rear. This fuse was lit, causing the gunpowder to ignite and propel the cannonball. In military use, the standard hand cannon was tremendously powerful, while also being somewhat useless due to relative inability of the gunner to aim the weapon, or control the ballistic properties of the projectile. Recoil could be absorbed by bracing the barrel against the ground using a wooden support, the forerunner of the stock. Neither the quality or amount of gunpowder, nor the consistency in projectile dimensions were controlled, with resulting inaccuracy in firing due to windage, variance in gunpowder composition, and the difference in diameter between the bore and the shot. The hand cannons were replaced by lighter carriage-mounted artillery pieces, and ultimately the arquebus.

Muzzle-loading muskets (smooth-bored long guns) were among the first firearms developed. The firearm was loaded through the muzzle with gunpowder, optionally some wadding and then a bullet (usually a solid lead ball, but musketeers could shoot stones when they ran out of bullets). Greatly improved muzzleloaders (usually rifled instead of smooth-bored) are manufactured today and have many enthusiasts, many of whom hunt large and small game with their guns. Muzzleloaders have to be manually reloaded after each shot; a skilled archer could fire multiple arrows faster than most early muskets could be reloaded and fired, although by the mid-18th century, when muzzleloaders became the standard small armament of the military, a well-drilled soldier could fire six rounds in a minute using prepared cartridges in his musket. Before then, effectiveness of muzzleloaders was hindered by both the low reloading speed and, before the firing mechanism was perfected, the very high risk posed by the firearm to the person attempting to fire it.

One interesting solution to the reloading problem was the "Roman Candle Gun" with superposed loads. This was a muzzleloader in which multiple charges and balls were loaded one on top of the other, with a small hole in each ball to allow the subsequent charge to be ignited after the one ahead of it was ignited. It was neither a very reliable nor popular firearm, but it enabled a form of "automatic" fire long before the advent of the machine gun.^[26]

Most early firearms were muzzle-loading. This form of loading has several disadvantages, such as a slow rate of fire and having to expose oneself to enemy fire to reload as the weapon had to be pointed upright so the powder could be poured through the muzzle into the breech followed by the ramming the projectile into the breech. As effective methods of sealing the breech were developed through the development of sturdy, weatherproof, self-contained metallic cartridges, muzzle-loaders were replaced by single-shot breech loaders. Eventually single-shot weapons were replaced by the following repeater type weapons.

Many firearms made in the late 19th century through the 1950s used internal magazines to load the cartridge into the chamber of the weapon. The most notable and revolutionary weapons of this period appeared during the U.S. Civil War and they were the Spencer and Henry repeating rifles. Both used fixed tubular magazines, the former having the magazine in the buttstock and the latter under the barrel which allowed a larger capacity. Later weapons used fixed box magazines that could not be removed from the weapon without disassembling the weapon itself. Fixed magazines permitted the use of larger cartridges and eliminated the hazard of having the bullet of one cartridge butting next to the primer or rim of another cartridge. These magazines are loaded while they are in the weapon, often using a stripper clip. A clip is used to transfer cartridges into the magazine. Some notable weapons that use internal magazines include the Mosin–Nagant, the Mauser Kar 98k, the Springfield M1903, the M1 Garand, and the SKS. Firearms that have internal magazines are usually, but not always, rifles. Some exceptions to this include the Mauser C96 pistol, which uses an internal magazine, and the Breda 30, an Italian light machine gun.

Many modern firearms use what are called detachable or box magazines as their method of chambering a cartridge. Detachable magazines can be removed from the weapon without disassembling the firearms, usually by pushing the magazine release.

A belt or ammunition belt is a device used to retain and feed cartridges into a firearm commonly used on machine guns. Belts were originally composed of canvas or cloth with pockets spaced evenly to allow the belt to be mechanically fed into the gun. These designs were prone to malfunctions due to the effects of oil and other contaminants altering the belt. Later belt designs used permanently connected metal links to retain the cartridges during feeding. These belts were more tolerant to exposure to solvents and oil. Some notable weapons that use belts are the M240, the M249, the M134 Minigun, and the PK Machine Gun.

Matchlocks were the first and simplest firearms firing mechanisms developed. Using the matchlock mechanism, the powder in the gun barrel was ignited by a piece of burning cord called a "match". The match was wedged into one end of an S-shaped piece of steel. As the trigger (often actually a lever) was pulled, the match was brought into the open end of a "touch hole" at the base of the gun barrel, which contained a very small quantity of gunpowder, igniting the main charge of gunpowder in the gun barrel. The match usually had to be relit after each firing. The main parts to the matchlock firing mechanism are the pan, match, arm and trigger.^[27] A benefit of the pan and arm swivel being moved to the side of the gun was it gave a clear line of fire.^[28] An advantage to the matchlock firing mechanism is that it did not misfire. However, it also came with some disadvantages. One disadvantage was if it was raining the match could not be kept lit to fire the weapon. Another issue with the match was it could give away the position of soldiers because of the glow, sound, and smell.^[29]

The wheellock action, a successor to the matchlock, predated the flintlock. Despite its many faults, the wheellock was a significant improvement over the matchlock in terms of both convenience and safety, since it eliminated the need to keep a smoldering match in proximity to loose gunpowder. It operated using a small wheel much like that on cigarette lighters which was wound up with a key before use and which, when the trigger was pulled, spun against a flint, creating the shower of sparks that ignited the powder in the touch hole. Supposedly invented by Leonardo da Vinci, the Italian Renaissance man, the wheellock action was an innovation that was not widely adopted due to the high cost of the clockwork mechanism.

The flintlock action was a major innovation in firearm design. The spark used to ignite the gunpowder in the touch hole was supplied by a sharpened piece of flint clamped in the jaws of a "cock" which, when released by the trigger, struck a piece of steel called the "frizzen" to create the necessary sparks. (The spring-loaded arm that holds a piece of flint or pyrite is referred to as a cock because of its resemblance to a rooster.) The cock had to be manually reset after each firing, and the flint had to be replaced periodically due to wear from striking the frizzen. (See also flintlock mechanism, snaphance, Miquelet lock) The flintlock was widely used during the 17th, 18th, and 19th centuries in both muskets and rifles.

Percussion caps (caplock mechanisms), coming into wide service in the early 19th century, were a dramatic improvement over flintlocks. With the percussion cap mechanism, the small primer charge of gunpowder used in all preceding firearms was replaced by a completely self-contained explosive charge contained in a small brass "cap". The cap was fastened to the touch hole of the gun (extended to form a "nipple") and ignited by the impact of the gun's "hammer". (The hammer is roughly the same as the cock found on flintlocks except that it doesn't clamp onto anything.) In the case of percussion caps the hammer was hollow on the end to fit around the cap in order to keep the cap from fragmenting and injuring the shooter.

Once struck, the flame from the cap in turn ignited the main charge of gunpowder, as with the flintlock, but there was no longer any need to charge the touch hole with gunpowder, and even better, the touch hole was no longer exposed to the elements. As a result, the percussion cap mechanism was considerably safer, far more weatherproof, and vastly more reliable (cloth-bound cartridges containing a premeasured charge of gunpowder and a ball had been in regular military service for many years, but the exposed gunpowder in the entry to the touch hole had long been a source of misfires). All muzzleloaders manufactured since the second half of the 19th century use percussion caps except those built as replicas of the flintlock or earlier firearms.

Frenchman Louis-Nicolas Flobert invented the first rimfire metallic cartridge in 1845. His cartridge consisted of a percussion cap with a bullet attached to the top.^[30][31] Flobert then made what he called "parlor guns" for this cartridge, as these rifles and pistols were designed to be shot in indoor shooting parlors in large homes.^[32][33] These 6mm Flobert cartridges, do not contain any powder, the only propellant substance contained in the cartridge is the percussion cap.^[34] In English-speaking countries, the 6mm Flobert cartridge corresponds to .22 BB Cap and .22 CB Cap ammunition. These cartridges have a relatively low muzzle velocity of around 700 ft/s (210 m/s).

This was major innovation in firearms ammunition, previously delivered as separate bullets and powder, was combined in a single metallic (usually brass) cartridge containing a percussion cap, powder, and a bullet in one weatherproof package. The main technical advantage of the brass cartridge case was the effective and reliable sealing of high pressure gasses at the breech, as the gas pressure forces the cartridge case to expand outward, pressing it firmly against the inside of the gun barrel chamber. This prevents the leakage of hot gas which could injure the shooter. The brass cartridge also opened the way for modern repeating arms, by uniting the bullet, gunpowder and primer into one assembly that could be fed reliably into the breech by a mechanical action in the firearm.

Before this, a "cartridge" was simply a premeasured quantity of gunpowder together with a ball in a small cloth bag (or rolled paper cylinder), which also acted as wadding for the charge and ball. This early form of cartridge had to be rammed into the muzzleloader's barrel, and either a small charge of gunpowder in the touch hole or an external percussion cap mounted on the touch hole ignited the gunpowder in the cartridge. Cartridges with built-in percussion caps (called "primers") continue to this day to be the standard in firearms. In cartridge-firing firearms, a hammer (or a firing pin struck by the hammer) strikes the cartridge primer, which then ignites the gunpowder within. The primer charge is at the base of the cartridge, either within the rim (a "rimfire" cartridge) or in a small percussion cap embedded in the center of the base (a "centerfire" cartridge). As a rule, centerfire cartridges are more powerful than rimfire cartridges, operating at considerably higher pressures than rimfire cartridges. Centerfire cartridges are also safer, as a dropped rimfire cartridge has the potential to discharge if its rim strikes the ground with sufficient force to ignite the primer. This is practically impossible with most centerfire cartridges.

Nearly all contemporary firearms load cartridges directly into their breech. Some additionally or exclusively load from a magazine that holds multiple cartridges. A magazine is defined as a part of the firearm which exists to store ammunition and assist in its feeding by the action into the breech (such as through the rotation of a revolver's cylinder or by spring-loaded platforms in most pistol and rifle designs). Some magazines, such as that of most centerfire hunting rifles and all revolvers, are internal to and inseparable from the firearm, and are loaded by using a "clip". A clip, often mistakingly used to refer to a detachable "magazine", is a device that holds the ammunition by the rim of the case and is designed to assist the shooter in reloading the firearm's magazine. Examples include revolver speedloaders, the stripper clip used to aid loading rifles such as the Lee–Enfield or Mauser 98, and the en-bloc clip used in loading the M1 Garand. In this sense, "magazines" and "clips", though often used synonymously, refer to different types of devices.

Many firearms are "single shot": i.e., each time a cartridge is fired, the operator must manually re-cock the firearm and load another cartridge. The classic single-barreled shotgun is a good example. A firearm that can load multiple cartridges as the firearm is re-cocked is considered a "repeating firearm" or simply a "repeater". A lever-action rifle, a pump-action shotgun, and most bolt-action rifles are good examples of repeating firearms. A firearm that automatically re-cocks and reloads the next round with each trigger pull is considered a semi-automatic or autoloading firearm.

The first "rapid firing" firearms were usually similar to the 19th century Gatling gun, which would fire cartridges from a magazine as fast as and as long as the operator turned a crank. Eventually, the "rapid" firing mechanism was perfected and miniaturized to the extent that either the recoil of the firearm or the gas pressure from firing could be used to operate it, thus the operator needed only to pull a trigger (which made the firing mechanisms truly "automatic"). An automatic (or "fully automatic") firearm is one that automatically re-cocks, reloads, and fires as long as the trigger is depressed. An automatic firearm is capable of firing multiple rounds with one pull of the trigger. The Gatling gun may have been the first automatic weapon, though the modern trigger-actuated machine gun was not widely introduced until the First World War with the German "Spandau" and British Lewis Gun. Automatic rifles such as the Browning Automatic Rifle were in common use by the military during the early part of the 20th century, and automatic rifles that fired handgun rounds, known as submachine guns, also appeared in this time. Many modern military firearms have a selective fire option, which is a mechanical switch that allows the firearm be fired either in the semi-automatic or fully automatic mode. In the current M16A2 and M16A4 variants of the U.S.-made M16, continuous fully automatic fire is not possible, having been replaced by an automatic burst of three cartridges (this conserves ammunition and increases controllability). Automatic weapons are largely restricted to military and paramilitary organizations, though many automatic designs are infamous for their use by civilians.