Ballistic coefficient (BC)

Definition and key components

BC is derived from three principal factors: mass, cross-sectional area (related to diameter), and aerodynamic shape, which is expressed through a drag or form factor. In practical terms BC is often expressed as the ratio of sectional density to a form factor: BC = SD / i, where sectional density (SD) is mass divided by diameter squared and i is a dimensionless form factor comparing the bullet's drag to a standard reference projectile.

Models, units and variability

Different drag functions are used to tabulate BC; the most common are the G1 and G7 models. Because BC is referenced to an aerodynamic model and changes with velocity, it is not a single immutable property: the quoted BC for a bullet often applies to a specific velocity band and reference model. Manufacturers frequently publish BC values as simple numbers for comparison, but those figures depend on measurement method — chronograph or Doppler radar — and on which drag model was chosen. For manufacturer specifications and further data see bullet manufacturer data.

Uses and practical effects

• Trajectory prediction: BC enters ballistics calculators to estimate drop and time of flight.
• Wind drift and stability: higher BC reduces the projectile's susceptibility to crosswinds.
• Energy retention: a higher BC means more retained kinetic energy at range, important for hunting and terminal performance.
• Ammunition selection: shooters compare BCs when choosing bullets for long-range accuracy or specific ballistic performance.

History, typical values and notable cautions

The idea of a coefficient expressing resistance to air has long roots in classical ballistics and was refined with standard drag functions in the 20th century. Typical BC values for common small-arms bullets often lie roughly between about 0.12 and 1.00, though exact figures vary by caliber, shape, and reference model. Users should remember that BC is model-dependent and velocity-dependent: the same bullet can have different BC values under different conditions, so real-world shooting often requires measured or model-specific BCs rather than relying on a single quoted number.