Drag Model Standards
A ballistic coefficient (BC) expresses how efficiently a projectile overcomes air resistance, but the number is only meaningful when paired with the reference drag model it was measured against. The two most common standards are G1 and G7, and reporting a BC without naming its model leaves the figure ambiguous.
What a Reference Projectile Is
Each drag model is anchored to an idealized standard projectile with a precisely defined shape and a measured drag curve. A real projectile's BC is the ratio of the standard projectile's deceleration to the real one's, so the number literally says "how many times better than the reference this projectile flies." Different references therefore produce different BC numbers for the exact same projectile.
The G1 Standard
The G1 model is based on a short, flat-based reference form with a blunt nose. It is the most widely published standard and works reasonably for shorter, flat-based designs. Its weakness is that its drag curve changes shape across velocity, so a single G1 number can drift with speed and tends to overstate efficiency for long, sleek projectiles at distance.
The G7 Standard
The G7 model uses a long, boat-tailed reference profile that closely resembles modern low-drag projectiles. For streamlined match projectiles, a G7 BC stays far more constant across the flight envelope, making it the more reliable choice for long-range prediction. A given projectile's G7 number is numerically much smaller than its G1 number.
Velocity Banding and Consistency
Because the G1 curve mismatches modern shapes, some data is published as velocity-banded G1 values, listing a different BC for each speed range. A well-matched G7 value usually needs no banding, which is a practical signal that the reference fits the projectile.
Worked Example
Suppose a long boat-tail projectile is listed with a G1 BC of 0.500 and a G7 BC of 0.256. These are not contradictory; they describe the same projectile against two different references. A solver expecting G7 input that is fed the 0.500 G1 figure will badly underpredict drop and drift. Always match the number to the model the solver requests.
A Common Misconception
It is a mistake to assume a higher BC number always means a more efficient projectile. A G1 value of 0.500 and a G7 value of 0.256 can belong to the same projectile, so comparing a G1 number against a G7 number is meaningless. Only compare BCs measured against the same reference model, and for long-range work prefer G7 where it is available.