Bullet

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For other uses, see Bullet (disambiguation).
0.357 Magnum cartridges, containing bullets.
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0.357 Magnum cartridges, containing bullets.

A bullet is a projectile shot by a gun, usually made of a metal alloy. In contrast to a shell, a bullet does not contain explosives. The term bullet refers specifically to the metal slug that is propelled from a firearm. Although the term is occasionally used to refer to the combination of bullet, case, gunpowder, and primer, such an item is properly called a cartridge. A cartridge without a bullet is called a blank.

Contents

Material

Bullets are classically molded from a mixture of lead and tin. Typesetter's lead (used to mold Linotype), works very well. Lead is a popular choice because it is highly dense (providing a high amount of mass — and thus, kinetic energy — for a given volume) and is cheap to obtain and fabricate.

However, a projectile in flight undergoes significant external forces which may dissipate its ability to provide maximum energy to the target. At speeds of greater than 300 m/s (1000 ft/s) (common in most handguns), lead begins to deform during flight due to its softness and low tensile strength. At speeds greater than 600 m/s (2000 ft/s) (such as high-powered rifles, which may have muzzle velocities exceeding 1000 m/s (3500 ft/s)), unjacketed lead bullets may experience structural failure and disintegrate in flight due to very high centrifugal force imparted by the spin used to stabilize its flight. Also, as velocities are increased, the bullet will begin to leave very thick and heavy residues of lead in the barrel that damages the bullet and destroys accuracy on subsequent shots. Alloying the lead with a small percentage of tin and/or antimony helps this situation somewhat, but is not completely effective as velocities are increased.

Bullets intended for high-velocity applications generally have a lead core jacketed with copper or steel; the thin layer of copper protects the lead core during flight, delivering it intact to the target. There, the heavy lead core delivers its kinetic energy to the target. Some premium bullets may be made entirely of copper or brass.

Steel jacketed bullets are actually copper-dipped so that the steel will not damage the rifling in the gun barrel.

Armor Piercing bullets are jacketed designs where the core material is a very hard and high-density metal such as tungsten, tungsten carbide, depleted uranium, or steel.

Frangible bullets are the opposite of Armor Piercing designs, and are intended to disintegrate into tiny particles upon impact to minimize their penetration. An example is the Glaser Safety Slug.

Bismuth and Tungsten bullet alloys are available, and prevent release of toxic lead into the environment. Neither tin nor copper are toxic to mammals.

Rubber bullets, plastic bullets, and beanbags are designed to be non-lethal, for example for use in riot control.

Wax bullets are often used by quick draw shooters for their own safety.

Design

A modern cartridge is made up of five components:1.  the bullet itself, which serves as the projectile; 2. the casing, which holds all parts together; 3. the explosive, for example gunpowder or cordite, which serves as a propellant; 4. the rim, at the base of the cartridge; 5. the primer, which ignites the gunpowder.
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A modern cartridge is made up of five components:
1. the bullet itself, which serves as the projectile;
2. the casing, which holds all parts together;
3. the explosive, for example gunpowder or cordite, which serves as a propellant;
4. the rim, at the base of the cartridge;
5. the primer, which ignites the gunpowder.

Bullet designs have to solve several problems:

  • The bullet must form a seal with the gun's bore. The worse the seal, the more gas generated by the explosion leaks past the bullet reducing the efficiency. There are two types of seals (gas checks) in common use. One is a slight indentation in the back of the bullet. Gas pressure forces the metal lip against the bore. Another type is a basic labyrinthine seal: one or two bands of raised material go around the bullet.
  • The bullet must engage the rifling without damaging the gun's bore. Usually there is a raised band or two of material around its middle. This will be something soft, such as copper, tin, plastic or lead, which will prevent wear of the harder steel gun barrel. AP (Armour Piercing) rounds made of bronze, hardened steel or tungsten (and even depleted uranium) cause barrel wear, and so are normally covered. Where these bands are a different material, they are called "driving bands" as they drive the bullet around the rifling.

Bullets must have a surface which works well with the surface; it must seal, but not have too much friction. What happens to a bullet inside of a gun, is part of the internal ballistics. In smooth-bore firearms there the bullet must minimize the gas the goes around it, but not so tight it cannot leave the barrel. In rifled barrels, the bullet must do the latter, but also engage the rifling so spin is imparted to the bullet. The spin stabilizes the bullet in flight (see physics). It is important that the bullet have its center of mass at its center, otherwise the spin will cause it to wobble. Materials which have a low friction (compared to other metals) in relation to steel are used to sheath modern bullets and engage the rifling;they must be soft enough but minimize wear on the rifling.

Once it leaves the barrel, it is governed by external ballistics. Here, the bullets shape is very important for aerodynamics, as is the physics of spinning things (for bullets that are spun). In the past, with smooth-bore firearms, a sphere shape was common because no matter how it turned, it presented a uniform front. This shape was also used for early rifled firearms as well. It offers the lowest overall surface area for a given volume as well.

Rifling stabilizes the bullet in flight, which allowed for new shapes with advantages over spheres. This allowed for more cylinder shaped rounds, and ones shaped for lower drag. With less drag, the bullet can go farther. A parabolic or pointed head can result in less drag, as can features like a boat tail. However, this must be balanced against increasing the surface area of the bullet too much. If the bullets cross-section is decreased, it must then be longer for a given volume and weight of material. Also, as a practical matter there is no perfect shape since the bullet must travel at different speeds and must travel in different atmospheric conditions. This aspect must be balanced against other aspects, especially what happens when the bullet hits something and what happens inside the firearm.

See Terminal ballistics and/or Stopping power for an overview of how bullet design effect what happens when a bullet hits something, and how this impacts 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.

Types

There are many types of bullets for different applications. For example, here are some of the rounds used in one machine gun series:

  • M61 Armor-piercing, has higher penetration of hard materials like steel.
  • M62 Tracer, has incendiary material in the back of the bullet so that its path in the air is easily visible.
  • M80 Ball, a brass-enclosed lead bullet
  • M82 Blank, has powder but no bullet (used for firing salutes etc.)
  • M63 Dummy, has a bullet but no powder (used for loading and unloading exercises, etc.)

Tracer bullets have a hollow back, filled with a flare material. Usually this is a mixture of magnesium, perchlorate, and chromium, to yield a bright red color. A new design is trying to use a Light Emitting Diode (LED) instead, but the cost and complexity would seem to negate any advantage. Tracer material burns out after a certain amount of time. Tracers are usually mixed in with other kinds of rounds, such as every 7th round for example.

Lead is the typical material used; it is relatively cheap, expands well, and can be hardened by alloying with tin and antimony. Actual bullet shapes are many and varied, and an array of them can be found in any reloading manual that sells bullet moulds. RCBS are one of many makers, and the link will let you see many different designs, starting with the basic round ball. With a mould, bullets can be made at home for reloading your own ammunition, where local laws allow. Cast and jacketed bullets are also commercially available from numerous manufacturers for handloading and are much more convenient than casting bullets from bulk lead.

Manufacture

Lead bullets

Small-scale manufacture is accomplished with individual molds, and hand-file to remove the mold artifacts. Larger scales use multiple molds, and abrasive tumbling to remove separation lines and other mold artifacts. Most lead bullets manufacturers today use a wire and stamp method under extremely high pressure that may reach as high as 80,000 pounds per square inch (550 MPa). The bullets are then swaged through a sizing die to their final dimension.

Jacketed bullets

Lead ingots are extruded into wire under high pressure. Then the wire is stamped into bullet cores and swaged to size. The bullet jacket is punched out of copper plate of appropriate thickness. These copper "plugs" are then extruded into a cup shape by the use of dies. Picture pushing your finger through a piece of gum. The lead core is inserted and the jacketed core is worked through a series of dies until it closes around the core.

Treaties

The Geneva Accords on Humane Weaponry and the Hague Convention prohibit certain kinds of ammunition for use by armies. These include exploding, poisoned and expanding bullets.

History

The first bullets

Almost undoubtedly the first "bullets" were much like crossbow quarrels, fired from metal and wooden guns immediately after the introduction of gunpowder in Europe. Large guns and cannon fired stone balls until the mid-15th century when metal balls began to be cast.

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 leaden ball two sizes smaller than the bore, wrapped in a loosely fitting paper patch which formed a tight seal so the full pressure of the expanding gas would propel the bullet. The loading 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 closely fitting ball to take the rifling grooves, was loaded with difficulty, particularly when foul, and for this reason was not generally used for military purposes.

The bullet takes shape

As firearms became more technologically advanced from 1500 to 1800, the bullets changed little. They remained simple round lead balls, differing only in their size. Even with the advent of rifling the bullet itself didn't change, but was wrapped in a leather patch to grip the rifling grooves.

Nevertheless, many ideas were not pursued, and the history books are full of brilliant ideas that failed to catch on.

The first half of the 19th 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.

Among the first "bullet-shaped" bullets was designed by Captain John Norton of the British Army in 1823. Norton's bullet had a hollow base which expanded under pressure to catch the rifling grooves once fired but the British Board of Ordnance rejected it because spherical bullets has 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 was rejected because, being two parts, it was judged 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 end, which was fitted with a little iron cap instead of a wooden plug. When fired, the cap would force itself into the hollow cavity, forcing the sides of the bullet to expand and engage the rifling. In 1855 the British adopted the minie ball for their Enfield rifles.

It was in the American Civil War, however, that the minie ball saw the most use. Roughly 90% of the battlefield casualties in the war were caused by minie balls fired from rifles.

.303 inch centrefire, rimmed ammunition.
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.303 inch centrefire, rimmed ammunition.

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 small-calibre rifle, one of whose essential features was the employment of an elongated compound bullet, with a lead core in a copper envelope.

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. This can allow the gas past the bullet, deforming it and destroying accuracy. Very rapid acceleration of a lead bullet may cause the rifling to strip, reducing the spin imparted to the bullet, and also destroying accuracy. A gas check may be used for some lead bullets, but are only useful up to a certain muzzle velocity, as they only protect the base of the bullet from melting, not the sides. It normally takes the form of a very thin copper disc attached to the rear of the bullet.

The modern bullet has had minor refinements, but the basic bullet and self-contained cartridge has since remained almost unchanged for over 130 years.

In the late 1950s, engineers noted that a reverse ogive on the rear, a boat-tail increased range on supersonic bullets.

At one point in the 1960s, it looked as though flechettes might replace bullets, but bullets proved more economical, and no less destructive.

Other bullet types: soft point bullet, full metal jacket bullet, armor piercing bullet, Teflon coated bullet, Glaser Safety Slug


See also

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