Table of Contents
What Is Ballistic Coefficient?
The ballistic coefficient (BC) is a measure of a projectile ability to overcome air resistance in flight. A higher BC means the projectile retains velocity better, shoots flatter at long range, and is less affected by wind. It depends on the projectile mass, caliber (diameter), and aerodynamic form factor (shape compared to a standard reference projectile).
BC is critically important for long-range shooting, military ballistics, and ammunition design. It determines trajectory, wind drift, remaining energy at the target, and time of flight. Modern long-range bullets are designed with high BC values using boat-tail shapes, polymer tips, and optimized ogive profiles.
Ballistic Coefficient Formula
Where BC is the ballistic coefficient, SD is sectional density, i is the form factor relative to the G1 or G7 standard projectile, m is mass in pounds, and d is caliber in inches. The form factor compares the bullet drag to a standard shape.
Common Ballistic Coefficients (G1)
| Bullet | Weight | BC (G1) |
|---|---|---|
| .223 55gr FMJ | 55 gr | 0.243 |
| .308 168gr HPBT | 168 gr | 0.462 |
| .308 175gr SMK | 175 gr | 0.505 |
| .338 300gr SMK | 300 gr | 0.768 |
| .50 BMG 750gr | 750 gr | 1.050 |
FAQ
What is the difference between G1 and G7 BC?
G1 uses a flat-base reference projectile, traditional in sporting ammunition. G7 uses a boat-tail reference more similar to modern long-range bullets. G7 BC values are lower numerically but more consistent across velocities. G7 is preferred for precision long-range calculations.
Does BC change with velocity?
Yes. Because the drag coefficient changes with Mach number (especially near transonic), the actual BC varies with velocity. G1 BCs can change 10-20% across a bullet trajectory. G7 BCs remain more constant. Some ballistic software uses multiple BC values for different velocity bands.
How does BC affect wind drift?
A bullet with twice the BC will experience approximately half the wind drift at the same range. This is because the higher BC bullet spends less time in flight, giving the wind less time to push it off course. This is why long-range competitors favor high-BC bullets.