A bullet is a solid projectile propelled by a firearm or air gun and is normally made from metal (usually lead, but not always). A bullet does not contain explosives, but damages the intended target by tissue disruption and impact. The word "bullet" is sometimes erroneously used to refer to a cartridge, which is the combination of bullet, casing (case or shell), gunpowder and primer. See ammunition. The Oxford English Dictionary definition of a bullet is "a projectile of lead ... for firing from a rifle, revolver etc."  However, bullets for air guns are not part of a cartridge.
The history of bullets parallels the history of firearms. It's no surprise that advances in one resulted from or precipitated advances in the other. Originally, bullets were metallic or stone balls placed in front of an explosive charge of gun powder at the end of a closed tube. As firearms became more technologically advanced, from 1500 to 1800, bullets changed very little. They remained simple round lead balls, called rounds, differing only in their diameter.
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 musket bullet was a spherical lead ball two sizes smaller than the bore, wrapped in a loosely-fitted paper patch which served to hold the bullet in the barrel firmly upon the powder. (Bullets that were not firmly upon the powder upon firing risked causing the barrel to explode, with the condition known as a "short start".) The loading of muskets was, therefore, easy with the old smooth-bore Brown Bess and similar military muskets. The original muzzle-loading rifle, on the other hand, with a more closely fitting ball to take the rifling grooves, was loaded with difficulty, particularly when the bore of the barrel was dirty from previous firings ("fouled"). For this reason, early rifles were not generally used for military purposes. Early rifle bullets required cloth or leather patches to grip the rifling grooves, and to hold the bullet securely against the powder.
The first half of the nineteenth century saw a distinct change in the shape and function of the bullet. In 1826, Delirque, a French infantry officer, invented a breech with abrupt shoulders on which a spherical bullet was rammed down until it caught the rifling grooves. Delirque's method, however, deformed the bullet and was inaccurate.
 Pointed bullets
Among the first pointed or "conical" bullets were those designed by Captain John Norton of the British Army in 1823. Norton's bullet had a hollow base which expanded under pressure to engage with a barrel's "rifling" (internal grooves) at the moment of being fired; the British Board of Ordnance rejected it because spherical bullets had been in use for the last 300 years.
Renowned English gunsmith William Greener invented the Greener bullet in 1836. It was very similar to Norton's bullet except that the hollow base of the bullet was fitted with a wooden plug which more reliably forced the base of the bullet to expand and catch the rifling. Tests proved that Greener's bullet was extremely effective but it too was rejected for military use because, being two parts, it was judged as being too complicated to produce.
The soft lead bullet that came to be known as the Minié ball, (or minnie ball) was first introduced in 1847 by Claude Étienne Minié (1814? - 1879), a captain in the French Army. It was nearly identical to the Greener bullet. As designed by Minié, the bullet was conical in shape with a hollow cavity in the rear, which was fitted with a little 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, thereby expanding the sides of the bullet to grip and engage the rifling. In 1855, the British adopted the Minié ball for their Enfield rifles.
It was in the American Civil War, however, that the Minié ball first saw widespread use. Roughly 90% of the battlefield casualties in this war were caused by Minié balls fired from rifles.
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.
About 1862 and later, W. E. Metford had carried out an exhaustive series of experiments on bullets and rifling, and had invented the important system of light rifling with increasing spiral, and a hardened bullet. The combined result of the above inventions was that in December 1888 the Lee-Metford small-bore (0.303") rifle, Mark I, (photo of cartridge on right) was finally adopted for the British army. The Lee-Metford was the predecessor of the Lee-Enfield.
 The modern bullet
The next important change in the history of the rifle bullet occurred in 1883, when Major Rubin, director of the Swiss Laboratory at Thun, invented the copper jacketed bullet; an elongated bullet with a lead core in a copper envelope or jacket.
The copper jacketed bullet allows much higher muzzle velocities than lead alone, as copper has a much higher melting point, greater specific heat capacity, and is harder. Lead bullets fired at high velocity may suffer surface melting due to hot gases behind and friction with the bore.
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 increased the lethality of the battlefield drastically.
The final advancement in bullet shape occurred with the development of the boat tail which is a streamlined base for spitzer bullets. A vacuum is created when air strata moving at high speed passes over the end of a bullet. The streamlined boat tail design aims to eliminate this drag-inducing vacuum by allowing the air to flow alongside the surface of the tapering end, thus eliminating the need for air to turn around the 90-degree angle normally formed by the end of shaped bullets. The resulting aerodynamic advantage is currently seen as the optimum shape for rifle technology. The spitzer boat-tailed bullet (Balle "D") was first introduced as standard ammunition in a military rifle in 1901, for the French Lebel Mle 1886 service weapon.
Bullet designs have to solve two primary problems. They must first form a seal with the gun's bore. The worse the seal, the more gas, generated by the rapid combustion of the propellant charge, leaks past the bullet, reducing the efficiency. The bullet must also engage the rifling without damaging the gun's bore. Bullets must have a surface which will form this seal without causing excessive friction. What happens to a bullet inside the bore is termed internal ballistics. A bullet must also be consistent with the next bullet so that shots may be fired precisely.
Once it leaves the barrel, it is governed by external ballistics. Here, the bullet's shape is important for aerodynamics, as is the rotation imparted by the rifling. Rotational forces stabilize the bullet gyroscopically as well as aerodynamically. Any asymmetry in the bullet is largely cancelled as it spins. 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, but the aerodynamic shape changed little giving moderate accuracy. Generally, bullet shapes are a compromise between aerodynamics, interior ballistics necessities, and terminal ballistics requirements. Another method of stabilization is for the center of mass of the bullet to be as far forward as practical as in the minnie ball or the shuttlecock. This allows the bullet to fly front-forward by means of aerodynamics.
See Terminal ballistics and/or Stopping power for an overview of how bullet design affects what happens when a bullet hits something, and how this is affected by its design. What happens to the bullet is dictated as much by what it hits and how it hits, as by the bullet itself (just like how its interaction with air was critical in external ballistics). Bullets are generally designed to penetrate, deform, and/or break apart. For a given material and bullet, which of these happens is determined especially by the strike velocity.
Actual bullet shapes are many and varied, and an array of them can be found in any reloading manual that sells bullet moulds. RCBS, one of many makers, offers many different designs, starting with the basic round ball. 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.
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 300 m/s (1000 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, making it easy to use in fabricating bullets.
- Lead: Simple cast, extruded, swaged, or otherwise fabricated lead slugs are the simplest form of bullets. At speeds of greater than 300 m/s (1000 ft/s) (common in most handguns), lead is deposited in rifled bores at an ever-increasing rate. Alloying the lead with a small percentage of tin and/or antimony serves to reduce this effect, but grows less effective as velocities are increased. A cup made of harder metal, such as copper, placed at the base of the bullet and called a gas check, is often used to decrease lead deposits by protecting the rear of the bullet against melting when fired at higher pressures, but this too does not solve the problem at higher velocities.
- Jacketed Lead: Bullets intended for even higher-velocity applications generally have a lead core that is jacketed or plated with cupronickel, copper alloys, or steel; a thin layer of harder copper protects the softer lead core when the bullet is passing through the barrel and during flight, which allows delivering the bullet intact to the target. There, the heavy lead core delivers its kinetic energy to the target. Full Metal Jacket bullets or Ball bullet have the front and sides of the bullet completely encased in the harder metal jacket. Some bullet jackets do not extend to the front of the bullet to aid in expansion and increase lethality. These are called soft points or hollow point bullets. Steel bullets are often plated with copper or other metals for additional corrosion resistance during long periods of storage. Synthetic jacket materials such as nylon and Teflon have been used with limited success.
- Armor Piercing: Jacketed designs where the core material is a very hard, high-density metal such as tungsten, tungsten carbide, depleted uranium, or steel. A pointed tip is often used, but a flat tip on the penetrator portion is generally more effective.
- Tracer: These have a hollow back, filled with a flare material. Usually this is a mixture of magnesium perchlorate, and strontium salts to yield a bright red color, although other materials providing other colors have also sometimes been used. Tracer material burns out after a certain amount of time. Such ammunition is useful to the shooter as a means of verifying how close the point of aim is to the actual point of impact, and for learning how to point shoot moving targets with rifles. This type of round is also used by all branches of the United States military in combat environments as a signaling device to friendly forces. Normally it is loaded at a four to one ratio with ball ammunition and is intended to show where you are firing so friendly forces can engage the target as well. The flight characteristics of tracer rounds differ from normal bullets, decreasing in altitude sooner than other bullets, because of increased aerodynamic drag.
- Incendiary: These bullets are made with an explosive or flammable mixture in the tip that is designed to ignite on contact with a target. The intent is to ignite fuel or munitions in the target area, thereby adding to the destructive power of the bullet itself.
- Frangible: Designed to disintegrate into tiny particles upon impact to minimize their penetration for reasons of range safety, to limit environmental impact, or to limit the shoot-through danger behind the intended target. An example is the Glaser Safety Slug.
- Non Toxic: Bismuth, tungsten, steel, and other exotic bullet alloys prevent release of toxic lead into the environment. Regulations in several countries mandate the use of non-toxic projectiles especially when hunting waterfowl. It has been found that birds swallow small lead shot for their gizzards to grind food (as they would swallow pebbles of similar size), and the effects of lead poisoning by constant grinding of lead pellets against food means lead poisoning effects are magnified. Such concerns apply primarily to shotguns, firing pellets (shot) and not bullets, but reduction of hazardous substances (RoHS) legislation has also been applied to bullets on occasion to reduce the impact of lead on the environment at shooting ranges.
- Practice: Made from lightweight materials like rubber, Wax, wood, plastic, or lightweight metal, practice bullets are intended for short-range target work, only. Because of their weight and low velocity, they have limited range.
- Less Lethal, or Less than Lethal: Rubber bullets, plastic bullets, and beanbags are designed to be non-lethal, for example for use in riot control. They are generally low velocity and are fired from shotguns, grenade launchers, paint ball guns, or specially-designed firearms and air gun devices.
- Blanks: Wax, paper, plastic, and other materials are used to simulate live gunfire and are intended only to hold the powder in a blank cartridge and to produce noise. The 'bullet' may be captured in a purpose-designed device or it may be allowed to expend what little energy it has in the air. Some blank cartridges are crimped or closed at the end and do not contain any bullet.
- Blended-Metal: Bullets made using cores made powdered metals other than lead with binder. Sometimes sintered.
The Hague Convention prohibits certain kinds of ammunition for use by uniformed military personnel against those uniformed military personnel of opposing forces. These include projectiles which explode within an individual, poisoned and expanding bullets. Nothing in these treaties prohibits incendiary bullets (tracers) or the use of prohibited bullets on military equipment.
These treaties apply even to .22 LR bullets used in pistols. 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 hollow-point bullets that are more commonly used in .22 LR pistols.
 Bullet Acronyms
 Figurative uses
The word for the bullet, usually because of its speed, is sometimes used figuratively, e.g.:-
- The Japanese Bullet Trains.
- The expression "bullet-headed" for a dolichocephalic shape of the human head.
- The term silver bullet, an extremely effective solution to a problem, comes from the modern addition to werewolf folklore that the monster is highly vulnerable to firearms using silver ammunition.
- The phrase "biting the bullet," meaning (usually mental) preparation for an unpleasant task or experience, refers to a patient doing precisely that to brace himself for a painful medical procedure (such as the removal of another bullet or amputation of a limb) before the advent of anesthesia. This was frequently done on or behind a battlefield, where bullets would be readily available.
- bullet - Definitions from Dictionary.com
- : Hughes, David R. (1990). The History and Development of the M16 Rifle and its Cartridge, Oceanside, CA: Armory Publications.
 See also
- List of rifle cartridges
- List of handgun cartridges
- Table of cartridges by year
- List of Shotgun cartridges
- Percussion cap
- Terminal ballistics, External ballistics
- Tracer ammunition
- Bullet bow shockwave
- Bullet Physics Engine
- Kinetic projectile