Bullet

A bullet is a kinetic projectile used in firearms. It is a specific component of a larger system called a cartridge, or a round of ammunition.

While people often use the word "bullet" to describe the whole cartridge, they are technically different. A cartridge is a complete unit that includes the bullet, the case, the propellant, and the primer. The primer ignites the propellant, which provides the energy needed to launch the projectile. Bullets can be made from many materials, including copper, lead, steel, polymer, rubber, or even wax. They are often tapered in shape to make them more aerodynamic as they travel.

Bullets function by transferring kinetic energy upon impact. This energy allows the projectile to strike or penetrate a target.

The speed of a bullet is influenced by both intrinsic and extrinsic factors. Intrinsic factors include the bullet's sectional density, aerodynamic profile, and ballistic coefficient. Extrinsic factors are environmental, such as wind speed, air temperature, humidity, and barometric pressure. Many bullets travel at supersonic speeds, which is faster than the speed of sound. When a bullet pierces the air at these speeds, it creates a loud sonic boom.

In contrast, subsonic cartridges like the .45 ACP travel slower than sound, making them much quieter.

Bullet speeds vary significantly depending on the type of firearm being used. For example, a 9 mm Luger handgun reaches a muzzle velocity of about 350 meters per second. An AK-47 has a much higher muzzle velocity of approximately 715 meters per second. Rifles can be even faster; a Remington 223 firing lightweight varmint projectiles can reach speeds of up to 1,000 meters per second. These differences in speed and mass determine how the bullet carries its energy through the air and how it behaves when it hits a target.

The history of projectiles began with early devices like the Chinese fire lance, which used bamboo tubes to fire porcelain shrapnel. Around 1288, the Yuan dynasty utilized metal hand cannons to achieve military victories.

Early projectiles were often made of stone, but these were not effective at penetrating stone fortifications. This led to the use of denser materials. By 1425, metal balls were recorded penetrating armor. During the era of the matchlock arquebus, cast lead balls became common. These were often loaded with wadding to ensure the bullet sat firmly against the powder, preventing a "short start" that could explode the barrel.

In the 19th century, inventors worked to improve accuracy through better shapes and rifling. Henri-Gustave Delvigne developed cylindro-conical bullets around 1830.

Francois Tamisier later improved these designs by adding cannelures, which are grooves that move air resistance behind the center of gravity. Tamisier also developed progressive rifling, where grooves are deeper at the breech and shallower at the muzzle. In 1847, Claude-Étienne Minié introduced the Minié ball. This conical bullet had a hollow cavity in the rear that an iron cap would expand into upon firing.

This allowed the soft lead to grip the rifling grooves of the barrel perfectly.

The impact of these design changes was massive during historical conflicts. The Minié ball saw widespread use during the Crimean War from 1853 to 1856. During the American Civil War, from 1861 to 1865, roughly 90% of battlefield casualties were caused by Minié balls fired from rifled muskets. Even President Abraham Lincoln observed the effectiveness of improved designs, noting that certain balls could carry a third more distance than traditional round balls. These advancements transformed how firearms functioned on the battlefield, moving from simple smoothbore weapons to highly accurate rifled systems.

While bullets are a specific type of projectile, they belong to a much larger category of tools. Other projectiles include BBs used in BB guns, airsoft pellets, and paintballs.

There are also much larger systems, such as grenade launchers, missile launchers, and RPGs. Even simple tools like slingshots hurl small rocks as projectiles. Whether it is a small lead ball or a large missile, the underlying principle remains the same: using energy to move an object through space to a specific destination.