A bullet is a projectile, often a pointed metal cylinder, that is shot from a firearm. The bullet is usually part of an ammunition cartridge, the object that contains the bullet and that is inserted into the firearm. Cartridges are often called bullets, but this article will discuss only the projectiles fired from small or personal firearms (such as pistols, rifles, and shotguns).
Though there were cast lead bullets used with slings thousands of years ago, the history of the modern bullet starts with the history of firearms. Sometime after A.D. 1249, it was realized that gunpowder could be used to fire projectiles out of the open end of a tube. The earliest firearms were large cannons, but personal firearms appeared in the mid-fourteenth century. Early projectiles were stone or metal objects that could fit down the barrel of the firearm, though lead and lead alloys (mixtures of metals) were the preferred materials by 1550. As manufacturing techniques improved, firearms and lead bullets became more uniform in size and were produced in distinct calibers (the diameter of the bullet).
The Industrial Revolution produced further improvements. Firearms with rifled barrels (spiral grooves inside of the firearm barrel that impart stabilizing spinning motion to the bullet) led to the familiar conical bullet. More powerful smokeless powders replaced gunpowder (now called black powder) in the late nineteenth century, but they also required harsher firearm and bullet materials. Lead bullets left lead residue in the barrel; jacketed bullets (a harder metal layer surrounds the softer lead core) were developed to stop this. The familiar metal ammunition cartridge (containing a bullet, a case, a primer, and a volume of propellant) was common by World War I.
Bullets are made of a variety of materials. Lead or a lead alloy (typically containing antimony) is the traditional bullet core material. Traditional bullet jackets are made of copper or gilding metal, an alloy of copper and zinc. There are many other materials that are used in bullets today, including aluminum, bismuth, bronze, copper, plastics, rubber, steel, tin, and tungsten.
Bullet lubricants include waxes (traditionally carnauba wax made from the carnauba palm), oils, and molybdenum disulfide (moly). Modern wax and oil formulas are generally not made public. Moly is a recent innovation; this naturally occurring mineral sticks to metal on contact. The bullet making process can also use grease and oils to lubricate the bullet during machining and pressing steps. This lubrication prevents damage to the bullet or the machinery by allowing the bullet and machinery to move against each other without sticking. Solvents are used to remove grease and oil from the bullet afterward.
There are several different uses for ammunition, such as military, law enforcement, hunting, marksmanship/target shooting, and self-defense, each requiring different bullet performance. There are also legal and public relations design considerations, such as lethality, threats to innocent bystanders, environmental impact, and appearance.
Bullet design is dependent on firearm design and vice versa. The bullet must fit into the barrel correctly. A bullet that is too small will not engage the rifling in the barrel, or it will bounce around in the barrel and not exit in a straight line. A bullet that is too large will jam in the barrel, possibly causing the firearm to explode from the pressure. The bullet weight must also match the amount of powder in the cartridge, so that it is fired at the correct speed.
Bullets are designed using calculations and data gathered from previous testing (firing) of bullets. This data can include variables such as accuracy (whether it hit the target), precision (whether more than one of the same bullet type produced similar results), speed of the bullet, effectiveness at a given range (distance to the target), penetration into the target, and damage to the target. Bullets are then tested against a target which resembles what they will be used against. There are several materials used to simulate the intended target, including bullet gelatin, a recently developed material used to simulate flesh.
Modern bullets can have many different features. Some of these features concern the shape of the bullet and others the materials of construction. Most bullets look like a cylinder with a pointed end. The cylindrical section to the rear of the bullet is the shank and the pointed section to the front of the bullet is the tip, though the tip may be flat instead of pointed. Bullets can be made of one or more materials.
Bullets made out of only soft material (such as lead) expand on impact causing more damage to the target. Bullets made out of only a harder material (such as steel) penetrate further into thicker targets, but do not expand much. A softer core can be enclosed or partially enclosed in a layer of harder metal called a jacket. This jacket can completely enclose the bullet or it can leave the softer tip exposed for expansion purposes. Varying the amount of jacketing alters the amount of penetration versus expansion.
The shank can have a flat base or a tapered base (boat tail). The flat base is heavier and provides greater penetration, but the boat tail provides greater accuracy over distance. The base of the shank can also have a base plate of harder metal to prevent deformation of the bullet during firing. The base sometimes has a conical indentation (a gas check) that expands on firing to seal the base of the bullet against the firearm barrel and trap all of the energy from firing to propel the bullet forward. The shank may also have grooves used to contain lubricating grease that helps the bullet move freely in the firearm barrel. Sometimes a single groove, called a cannelure, is cut into the bullet to mark how far the bullet is to be inserted into the cartridge and to provide a feature to crimp the cartridge to the bullet.
The tip of the bullet is usually pointed. This point may be curved (called an ogive). Sharper tips provide greater penetration. Wadcutters are bullets with no point or a sharp shoulder behind the point used in target shooting to cut paper targets cleanly. Semiwadcutter bullets have a flat-tipped cone tip and can be used for target shooting, hunting, or self-defense. Target bullets are light and designed for speed and accuracy in a shooting range. They are usually not appropriate for other purposes.
Some tips are designed to expand on impact. This kind of bullet is banned from military use, but can be used for law enforcement, self-defense, and hunting. The tip or the entire bullet may be made of a soft material such as lead, but there are other design features that can aid bullet expansion. Hard material behind the softer tip provides more penetration and pushes the softer tip forward to expand more. The harder material can be the shank, a section of the tip, a partition of hard metal between the tip and the shank, or even a hard point on the tip that is driven backward on impact to expand the softer tip material.
Another feature that provides expansion is a hollow tip (or hollow point), an empty cone in the tip that points toward the rear of the bullet. When the bullet hits the target, the thin sides of the hollow tip expand outward. Even harder metals can expand, especially if they are scored (have grooves cut in them) to provide places to split apart.
Few bullets have separable parts. Some bullets have sabots, sleeves that surround the bullet while it is being fired but that fall off after leaving the firearm. Sabots allow smaller bullets to be fired from larger firearms at higher velocities than they would be fired from smaller firearms. Bullets can also contain multiple pellets or other particles that exit the bullet in a spray on impact or on leaving the target. This provides a higher chance of hitting something (from the many particles) or can cause many wounds in an easily damaged target.
Shotguns often fire shot (many small round pellets) or solid slugs (large, often soft bullets) out of an unrifled barrel, though some shotguns have rifled barrels. Air guns fire solid round or hourglass-shaped pellets.
Military bullets have special features, sometimes also used in law enforcement and self-defense. In order to get around the prohibition on expanding bullets, military bullets can be designed with heavier than normal back ends so that they tumble into the target on impact to create a larger wound. They can also be designed to break apart on impact with a similar effect. Some military bullets have incendiary (flammable) material in the base of the bullet that leaves a visible trail. This is known as a tracer bullet because it allows the shooter to track the bullet. Incendiary material can also be placed in the tip of the bullet so that it can start a fire on impact. Military bullets are usually made of harder materials or are fully jacketed. They are often designed for penetration. "Non-lethal" plastic or rubber bullets are sometimes used by the military and in law enforcement. These bullets are designed to temporarily incapacitate rioters and demonstrators, but they have the ability to kill.
Law enforcement and self-defense bullets should incapacitate the target. Many of these bullets are designed to expand or shatter after hitting the target, causing maximum damage. These bullets can be made of harder material that has greater penetration through materials such as heavy clothing and body armor. Police and self-defense bullets should not over penetrate (go through the target) and endanger bystanders.
Hunters have different requirements for different types of targets. Fast moving targets require faster, often lighter, bullets. Larger targets with heavy hides and large bones require bullets that can penetrate and inflict enough damage to drop the animal quickly. There are several different designs that address these conflicting demands. Many hunting bullets are designed to expand. Partitioned bullets and partially jacketed bullets are common for larger targets.
There are many types of bullet manufacturers, ranging from large companies and governments to smaller custom ammunition manufacturers to individuals who load and reload ammunition with a few simple tools. There are also many different bullet designs and a lack of consensus about which is most effective. Because of this, there is no uniform method of ammunition manufacture. Large ammunition manufacturers, including the United States government, automate some of the manufacturing steps. At appropriate points during the manufacturing process, special features may be added.
The two most common bullet-forming methods are casting and swaging. Hollow points can be formed by either method. Hard (harder than lead) solid bullets can be stamped (a metal punch cuts a bullet-shaped piece out of a bar or sheet of softer metal) and machined from metal stock. Machining includes any process where a machine is used to shape metal by cutting away portions. A typical machine used for bullets is a lathe. A lathe rotates the bullet metal against steel chisels to gradually cut away material.
Some bullets have jackets of harder metal surrounding a softer core.
Many firearm users want consistent performance from their ammunition. The larger ammunition manufacturers responded by instituting quality control programs in the 1980s and 1990s. These programs include statistical process control (SPC), total quality management (TQM), and random testing. SPC involves measuring a manufacturing process and determining statistically how to optimize it so that it produces correct and consistent results. TQM is the application of this kind of quality control to the whole business, not just the manufacturing part of the business.
Random testing involves periodically taking a manufactured part and testing it. Completed bullets are loaded into ammunition and fired to determine if they perform as expected. Unfinished bullets can be examined to determine if they are being produced correctly up to that point in the manufacturing process. Both finished and unfinished bullets can be weighed, measured for symmetry (bullets should be identical along every direction from an imaginary line drawn from the center of the tip to the center of the base), and cut apart to make sure that there are no air spaces and that internal features are correct (such as the thickness of a partition or a jacket). Commercial bullet sizes can vary by thousandths of an inch, but military and high quality bullets are more uniform.
Up to 24 toxic materials have been found in ammunition production. Solvents (often used to remove oil and grease) are dangerous to inhale and can be captured for disposal or purification and reuse, as can any oil. Scrap metal can be reused or disposed.
The most dangerous raw material is lead. Production workers and firearm users can be exposed to dangerous levels of lead from bullets, and firing ranges, including military ones, are being shut down because of high
Companies continue to improve bullet performance to attract buyers, but social and political considerations are becoming more important. Health, safety, and environmental issues are leading to the replacement of toxic materials such as lead with materials such as tungsten, steel, bismuth, and plastic. Newer materials do not have the same performance characteristics as older materials, and this leads to newer ammunition designs.
There has been a legal struggle for decades over the lethality of police and self-defense weapons. Public outcry in the United States has been greatest against so-called "cop-killer" bullets designed to penetrate body armor such as that used by police, and against expanding bullets such as the Black Talon, which has a tip that opens into six sharp "claws" on impact.
Other innovations may be more radical. For example, tanks can fire shells with fins that pop out for stabilization at velocities that are too high for barrel rifling. This innovation could be scaled down for personal firearms. Self-propelled, finned rockets can also be shot out of pistol-sized launchers, though this type of projectile may no longer be called a bullet.
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