A bullet is a kinetic projectile and the component of firearm ammunition that is expelled from the gun barrel during shooting. The term is from Middle French and originated as the diminutive of the word boulle (boullet), which means "small ball". Bullets are made of a variety of materials such as copper, lead, steel, polymer, rubber and even wax. They are available either singly as in muzzleloading and cap and ball firearms ^[undefined] or as a component of paper cartridges ^[undefined], but much more commonly in the form of metallic cartridges. ^[undefined] Bullets are made in a large number of shapes and constructions depending on the intended applications, including specialized functions such as hunting, target shooting, training and combat.

Though the word "bullet" is often used incorrectly in colloquial language to refer to a cartridge round, a bullet is not a cartridge but rather a component of one. ^[undefined] A round of ammunition cartridge is a combination package of the bullet (which is the projectile), the case (which holds everything together), the propellant (which provide majority of the energy to launch the projectile) and the primer (which ignites the propellant). This use of the term "bullet" when intending to describe a cartridge often leads to confusion when the components of a cartridge are specifically referred to.

Bullet sizes are expressed by their weights and diameters (referred to as "calibers") in both imperial ^[undefined] and metric measurement systems. For example: 55 grain.223 caliber bullets are of the same weight and caliber as 3.56 gram 5.56mm caliber bullets.

The bullets used in many cartridges are fired at muzzle velocities faster than the speed of sound ^{[undefined] [undefined]} — about 343 metres per second (1,130 ft/s) in dry air at 20 °C (68 °F) — and thus can travel a substantial distance to a target before a nearby observer hears the sound of the shot. The sound of gunfire (i.e. the muzzle report) is often accompanied with a loud bullwhip-like crack as the supersonic bullet pierces through the air creating a sonic boom. Bullet speeds at various stages of flight depend on intrinsic factors such as its sectional density, aerodynamic profile and ballistic coefficient, and extrinsic factors such as barometric pressure, humidity, air temperature and wind speed. ^{[undefined] [undefined]} Subsonic cartridges fire bullets slower than the speed of sound so there is no sonic boom. This means that a subsonic cartridge, such as .45 ACP, can be substantially quieter than a supersonic cartridge such as the .223 Remington, even without the use of a suppressor. ^[undefined]

Bullets do not normally contain explosives, ^[666666] but damage the intended target by transferring kinetic energy upon impact and penetration (see terminal ballistics).

The first use of gunpowder in Europe was recorded in 1247. It had been used in China for hundreds of years. The cannon appeared in 1327. Later in 1364, the hand cannon appeared. Early projectiles were made of stone. Stone was used in cannon and hand cannon. In cannon it was eventually found that stone would not penetrate stone fortifications which gave rise to the use of heavier metals for the round projectiles. Hand cannon projectiles developed in a similar fashion following the failure of stone from siege cannon. The first recorded instance of a metal ball from a hand cannon penetrating armor occurred in 1425. ^[undefined] In this photograph of shot retrieved from the wreck of the Mary Rose which was sunk in 1545 and raised in 1982. The round shot are clearly of different sizes and some are stone while others are cast iron. ^[undefined]

The development of the hand culverin and matchlock arquebus brought about the use of cast lead balls as projectiles. "Bullet" is derived from the French word boulette, which roughly means "little ball". The original round musket ball was smaller than the bore of the barrel. It was loaded into the barrel first, just resting upon the powder, and later using some sort of material as a wadding, between the ball and the powder as well as over the ball to keep it in place. ^[undefined] that held the bullet firmly in the barrel and against the powder. (Bullets not firmly on the powder risked exploding the barrel, with the condition known as a "short start".) ^[undefined]

The loading of muskets was, therefore, easy with the old smooth-bore Brown Bess and similar military muskets. The original muzzle-loading rifle, however, was loaded with a piece of leather or cloth wrapped around the ball, to allow the ball to engage the grooves in the barrel. Loading was a bit more difficult, particularly when the bore of the barrel was fouled from previous firings. For this reason, and because rifles were not often fitted for a bayonet, early rifles were not generally used for military purposes.

The first half of the nineteenth century saw a distinct change in the shape and function of the bullet.

In 1826, Henri-Gustave Delvigne, a French infantry officer, invented a breech with abrupt shoulders on which a spherical bullet was rammed down until it caught the rifling grooves. Delvigne's method, however, deformed the bullet and was inaccurate.

Square bullets have origins that almost pre-date civilization and were used by slingers in slings.

They were typically made out of copper or lead.

The most notable use of square bullet designs was done by, James Puckle and Kyle Tunis who patented them, where they were briefly used in one version of the Puckle gun. The early use of these in the black-powder era was soon discontinued due to irregular and unpredictable flight patterns.

Delvigne continued to develop bullet design and by 1830 had started to develop cylindro-conical bullets.

His bullet designs were improved by Francois Tamisier with the addition of "ball grooves" which are known as "cannelures", these moved the resistance of air behind the center of gravity of the bullet. ^[undefined]

Tamisier also developed progressive rifling.

The rifle grooves were deeper toward the breech, becoming shallower as they progressed toward the muzzle.

This causes the bullet to be progressively molded into the grooves which increased range and accuracy.

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The Thouvenin rifle barrel had a forcing plug in the breech of the barrel to mold the bullet into the rifling with the use of a special ramrod. While successful in increasing accuracy it was extremely hard to clean. These improvements were the basis for the development of the Minié Ball.

Among the first pointed or "conical" bullets were those designed by Captain John Norton of the British Army in 1832. Norton's bullet had a hollow base made of lotus pith that, on firing, expanded under pressure to engage with a barrel's rifling. ^[undefined] The British Board of Ordnance rejected it because spherical bullets had been in use for the previous 300 years. ^[undefined]

Renowned English gunsmith William Greener invented the Greener bullet in 1836. Greener fitted the hollow base of an oval bullet with a wooden plug that more reliably forced the base of the bullet to expand and catch the rifling. Tests proved that Greener's bullet was extremely effective, but the military rejected it too because, being two parts, they judged it as too complicated to produce. ^[undefined]

The soft lead Minié ball was first introduced in 1847 by Claude-Étienne Minié, a captain in the French Army. It was another improvement of the work done by Delvigne. As designed by Minié, the bullet was conical in shape with a hollow cavity in the rear, which was fitted with a small iron cap instead of a wooden plug. When fired, the iron cap would force itself into the hollow cavity at the rear of the bullet, thus expanding the sides of the bullet to grip and engage the rifling. In 1855, the British adopted the Minié ball for their Enfield rifles. In 1855 James Burton, a machinist at the US Armory at Harper's Ferry, West Virginia improved the Minié Ball further by eliminating the metal cup in the bottom of the bullet. ^{[undefined] [undefined]} The Minié ball first saw widespread use in the American Civil War. Roughly 90% of the battlefield casualties in this war were caused by Minié balls fired from rifled muskets. ^[undefined]

A similar bullet called the Nessler ball was also developed for smoothbore muskets. ^[undefined]

Between 1854 and 1857, Sir Joseph Whitworth conducted a long series of rifle experiments, and proved, among other points, the advantages of a smaller bore and, in particular, of an elongated bullet. The Whitworth bullet was made to fit the grooves of the rifle mechanically. The Whitworth rifle was never adopted by the government, although it was used extensively for match purposes and target practice between 1857 and 1866, when it was gradually superseded by Metford's.

In 1861 W.B. Chace approached President Abraham Lincoln with an improved ball design for muskets.

In firing over the Potomac river where the Chace ball and the round ball were alternated Lincoln observed that the Chace design carried a third or more farther fired at the same elevation.

Although Lincoln recommended testing it never took place.

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About 1862 and later, W. E. Metford carried out an exhaustive series of experiments on bullets and rifling, and invented the important system of light rifling with increasing spiral, and a hardened bullet. The combined result was that in December 1888 the Lee–Metford small-bore (0 .303", 7.70 mm) rifle, Mark I, was finally adopted for the British army. The Lee–Metford was the predecessor of the Lee–Enfield. ^[undefined]

The next important change in the history of the rifle bullet occurred in 1882, when Lt. Colonel Eduard Rubin, director of the Swiss Army Laboratory at Thun, invented the copper-jacketed bullet — an elongated bullet with a lead core in a copper jacket. It was also small bore (7.5mm and 8mm) and it is the precursor of the 8mm Lebel bullet adopted for the smokeless powder ammunition of the Mle 1886 Lebel rifle.

The surface of lead bullets fired at high velocity may melt due to hot gases behind and friction with the bore.

Because copper has a higher melting point, and greater specific heat capacity and hardness, copper-jacketed bullets allow greater muzzle velocities.

European advances in aerodynamics led to the pointed spitzer bullet. By the beginning of the twentieth century, most world armies had begun to transition to spitzer bullets. These bullets flew for greater distances more accurately and carried more energy with them. Spitzer bullets combined with machine guns greatly increased the lethality of the battlefield.

The latest advancement in bullet shape was the boat tail, a streamlined base for spitzer bullets. The vacuum created as air moving at high speed passes over the end of a bullet slows the projectile. The streamlined boat tail design reduces this form drag by allowing the air to flow along the surface of the tapering end. The resulting aerodynamic advantage is currently seen as the optimum shape for rifle technology. The first combination spitzer and boat-tail bullet, named Balle "D" from its inventor (a lieutenant-colonel Desaleux), was introduced as standard military ammunition in 1901, for the French Lebel Model 1886 rifle.

A ballistic tip bullet is a hollow-point rifle bullet that has a plastic tip on the end of the bullet itself. This improves external ballistics by streamlining the bullet, allowing it to cut through the air more easily, and improves terminal ballistics by allowing the bullet to act as a JHP on impact.

As a side effect, it also feeds better in weapons that have trouble feeding rounds that are not FMJ rounds.

Bullet designs have to solve two primary problems.

In the barrel, they must first form a seal with the gun's bore.

If a strong seal is not achieved, gas from the propellant charge leaks past the bullet, thus reducing efficiency and possibly accuracy. The bullet must also engage the rifling without damaging or excessively fouling the gun's bore, and without distorting the bullet, which will also reduce accuracy. Bullets must have a surface that forms this seal without excessive friction. These interactions between bullet and bore are termed internal ballistics. Bullets must be produced to a high standard, as surface imperfections can affect firing accuracy.

The physics affecting the bullet once it leaves the barrel is termed external ballistics. The primary factors affecting the aerodynamics of a bullet in flight are the bullet's shape and the rotation imparted by the rifling of the gun barrel. Rotational forces stabilize the bullet gyroscopically as well as aerodynamically. Any asymmetry in the bullet is largely canceled as it spins. However, a spin rate greater than the optimum value adds more trouble than good, by magnifying the smaller asymmetries or sometimes resulting in the bullet exploding midway in flight. With smooth-bore firearms, a spherical shape was optimum because no matter how it was oriented, it presented a uniform front. These unstable bullets tumbled erratically and provided only moderate accuracy; however, the aerodynamic shape changed little for centuries. Generally, bullet shapes are a compromise between aerodynamics, interior ballistic necessities, and terminal ballistics requirements. Another method of stabilization is to place the center of mass of the bullet as far forward as is practical, which is how the Minié ball and the shuttlecock are designed. This makes the bullet fly front-forward by means of aerodynamics.

See the articles on terminal ballistics and/or stopping power for an overview of how bullet design affects what happens when a bullet impacts with an object. The outcome of the impact is determined by the composition and density of the target material, the angle of incidence, and the velocity and physical characteristics of the bullet itself. Bullets are generally designed to penetrate, deform, or break apart. For a given material and bullet, the strike velocity is the primary factor that determines which outcome is achieved.

Bullet shapes are many and varied, and an array of them can be found in any reloading manual that sells bullet moulds.

Mould manufacturers such as RCBS, ^[undefined] Paul Jones Moulds, and David Mos offer many different calibers and designs.

With a mould, bullets can be made at home for reloading one's own ammunition, where local laws allow.

Hand-casting, however, is only time- and cost-effective for solid lead bullets.

Cast and jacketed bullets are also commercially available from numerous manufacturers for hand loading and are much more convenient than casting bullets from bulk lead.

Propulsion of the ball can happen via several methods:

Bullets for black powder, or muzzle-loading firearms, were classically molded from pure lead. This worked well for low-speed bullets, fired at velocities of less than 450 m/s (1475 ft/s). For slightly higher-speed bullets fired in modern firearms, a harder alloy of lead and tin or typesetter's lead (used to mold Linotype) works very well. For even higher-speed bullet use, jacketed coated lead bullets are used. The common element in all of these, lead, is widely used because it is very dense, thereby providing a high amount of mass—and thus, kinetic energy —for a given volume. Lead is also cheap, easy to obtain, easy to work, and melts at a low temperature, which results in comparatively easy fabrication of bullets.

Poisonous bullets were a subject to an international agreement as early as the Strasbourg Agreement (1675).

The St. Petersburg Declaration of 1868 prohibited the use of explosive projectiles weighing less than 400 grams. ^[undefined] The reasoning was that they figured if bullets were more deadly there would be less suffering. ^[undefined]

The Hague Convention prohibits certain kinds of ammunition for use by uniformed military personnel against the uniformed military personnel of opposing forces. These include projectiles that explode within an individual, poisoned and expanding bullets.

Protocol III of the 1983 Convention on Certain Conventional Weapons, an annexe to the Geneva Conventions, prohibits the use of incendiary ammunitions against civilians.

These treaties apply even to .22 LR bullets used in pistols, rifles and machine guns. Hence, the High Standard HDM pistol, a.22 LR suppressed pistol, had special bullets developed for it during World War II that were full metal jacketed, in place of the soft-point and hollow-point bullets that are otherwise ubiquitous for.22 LR rounds.

Some jurisdictions acting on environmental concerns have banned hunting with lead bullets and shotgun pellets.

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In December 2014, a federal appeals court denied a lawsuit by environmental groups that the EPA must use the Toxic Substances Control Act to regulate lead in shells and cartridges. The groups sought to regulate "spent lead", yet EPA could not regulate spent lead without also regulating cartridges and shells, per the court. ^[undefined]