The ballistic coefficient of a projectile is a measurement of its resistance to drag when fired through air. Any bullet or missile fired from a weapon will travel a given distance based on several factors. The mass of the projectile, the ability of its shape to overcome drag, which is called the form factor, and the density of the air are all factors.
Understanding the behavior of a fired bullet is an important part of weapons design. The amount of powder used in the cartridge will accelerate the bullet to a certain speed when it leaves the barrel. At this point, gravity and drag work together to pull the bullet toward the ground and slow it down. Wind will also affect the bullet's path, or trajectory, by moving it in different directions as the bullet travels down range to a target.
The shape of a bullet, or any projectile up to a large missile, is designed to minimize drag by having an aerodynamic, or drag-reducing, shape. Ballistics calculations use a measured value called the form factor to represent the shape of the bullet. Form factor calculations use a measured drag coefficient divided by a value for a standard industry reference shape.
After determining the form factor, the ballistic coefficient can be determined as a mathematical equation. The mass of the bullet, its form factor and a diameter of the bullet are used in the calculation. A ballistic coefficient can vary greatly above or below one for different bullet designs, but a coefficient of one is assumed for the standard industry projectile as a reference. Many tests were performed from the 1870s through the 1930s to develop ballistic information used as industry standards for projectile testing.
Bullet manufacturers publish ballistic coefficient data for their ammunition. Many sport shooters and hunters use bullets with higher coefficients, because in theory they will provide better results. A higher coefficient will normally result in bullets that travel with a flat trajectory, or path above ground, and are less sensitive to wind and air effects.
Published coefficient data may be used for comparison, but some differences can exist. Variations in manufacturing of projectiles can cause differences in mass or shape of the bullet. These differences can result in actual performance that is less than published data. Although these differences may be slight, they can be significant for precision shooters or shooting over larger distances.
Ballistic coefficient data have also been used for development of spacecraft since the 1950s. The behavior of a space vehicle such as a capsule will depend on having a very low coefficient, or a great deal of drag, which allows it to slow down in the atmosphere so it lands gently. On the other hand, a ballistic missile needs to travel very quickly through the atmosphere with little effect from weather or air drag, so it needs to have a very high coefficient.