This article is about the projectile, for other uses see bullet (disambiguation).

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.

Material

Bullets are classically molded from a mixture of lead and tin. Typesetter's lead (used to mold Linotype), works very well.

Some bullets are jacketed with copper or steel to make them harder.

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

Bismuth 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

Bullet designs have to solve several problems:

• The bullet must form a seal with the gun's bore. If it does not, the 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 not tumble in flight as this would cause a loss of speed and thus kinetic energy. The solutions to this vary depending on the design speed.

Supersonic bullets are pointed, smoothly sloping back to the rear. The longest-range supersonic bullets have a boat-tail, a narrowing and rounding-off toward the end to reduce vacuum on the back of the bullet. The modern supersonic bullet shape is known as a Spitzer bullet, and first became popular around the beginning of the 20th century. Supersonic bullets with rounded or flat noses may also be used for hunting, in order to assure expansion.

Transonic bullets, such as deer slugs and air-gun pellets are double cones, going wide to narrow to wide. The narrow waist prevents auxiliary shockwaves from forming, and causing the bullet to bullet. This 'coke bottle' shape is also apparent in high speed aircraft.

Subsonic bullets generally have rounded fronts.

The bullet must accomplish its mission: usually, penetrate the target. Bullets either cut tissue, or damage it by causing a hydrostatic shockwave.

Since subsonic bullets lack a shock wave, they have to cut the biggest possible hole in order to maximize their damage.

One way is to drill the front of the bullet, creating a hollow point bullet, and possibly scribe the copper shell. When the bullet hits it will unfold into a sharp-edged flower-shape that cuts through flesh.

The 'dum dum' is also an expanding bullet. It has a hard metal outer shell, and a soft lead interior and back. When it hits, the lead cracks the metal shell, and flows into a wide, mushroom shape.

The Russian ammunition for the AK-74 had a bullet with a hard steel shell, a soft lead interior. a steel penetrator, and a bubble in the nose. The bullet is dynamically stable in flight. After it hits the interior lead deforms, causing the bullet to unbalance and tumble. The tumble was designed to cause the bullet to make exactly two flips in 40 cm, roughly the thickness of a human body. This maximizes hydrodynamic shock, but does not violate the Geneva Accords on Humane Weaponry. This design was copied by NATO, in the form of the 5.56 mm NATO standard round. The USA has since improved the design to increase its effectiveness against armour, and the SS109 bullet can now pierce several millimeters of armour plate, due to its steel penetrator design, which still tumbles on soft tissue contact.

Subsonic bullets with rounded fronts often ricochet off their target if it is at an angle. To overcome this problem wadcutters or semi wadcutters were developed with flattened noses, or "hollow point", with a concave nose. As the flat nose interferes with feeding a self-loading gun, full wadcutters are usually only shot from revolvers or single-shot guns. A variation is to have a ring of small teeth, covered by a soft plastic nose so that the bullet will feed correctly in self-loading guns. The teeth engage a sloping surface.

At close to moderate ranges, an explosive bullet is only slightly more effective than an expanding bullet. In most cases, they are not worth the extra expense and danger to the user. PETN is the standard explosive used in bullets.

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.

The bullet must engage the rifling without damaging the gun's bore. Usually there's 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.

Lead is the typical material. Lead is relatively cheap, expands well, and can be hardened alloys 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 (http://www.rcbs.com/default.asp?menu=1&s1=4&s2=9&s3=83) are one of many makers, and the link will let you see many different designs, starting with the basic round ball. With a mould, bullet heads can be made at home for reloading your own bullets, where local laws allow.

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 manufactures 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 till 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 initially 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 in between 1500-1800, 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 only 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, 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.

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 speed, as they only protect the base of the bullet from melting, not the sides. They are normally a very thin copper disc.

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