Drag Coefficient
Also: Cd
The dimensionless number describing a projectile's air resistance as a function of speed, the core quantity behind any external ballistics calculation.
The drag coefficient is a dimensionless number that expresses how strongly the air resists a projectile at a given speed. It is not constant: it changes with velocity, rising sharply as a bullet approaches and crosses the transonic region, then settling at higher and lower speeds. A plot of this coefficient against velocity forms the drag model that a solver uses to predict flight.
In external ballistics the drag coefficient ties directly to the form factor, which compares a real bullet’s drag to that of a standard reference projectile. Combined with sectional density, that comparison produces the ballistic coefficient, the single figure most shooters use to summarize how well a bullet holds its velocity downrange.