What External Ballistics Studies
External ballistics is the study of a projectile's behavior from the moment it leaves the muzzle until it reaches the target. It concentrates on the forces acting during free flight, chiefly gravity and aerodynamic drag, and on the secondary effects that steer a projectile off a straight line over distance.
The Two Primary Forces
Gravity accelerates the projectile downward at a constant rate regardless of how fast it is moving forward, producing the drop that defines the trajectory's arc. Drag acts opposite the direction of motion, continuously slowing the projectile and lengthening its time of flight. The interplay of these two forces produces the curved path that trajectory tables describe.
Velocity and Time of Flight
Time of flight is the master variable of external ballistics because both drop and drift scale with it. As drag erodes velocity, each additional increment of distance takes longer to cross, so effects that depend on elapsed time grow faster than distance alone. Retained velocity at range therefore matters as much as muzzle velocity.
The Role of the Ballistic Coefficient
The ballistic coefficient summarizes how efficiently a projectile overcomes drag by comparing it to a standard reference form. A higher BC means slower velocity loss, which flattens the trajectory, shortens flight time, and reduces sensitivity to wind. It is the single most useful number for predicting downrange behavior.
Secondary Effects
Beyond gravity and drag, external ballistics accounts for smaller influences that emerge at distance: wind deflection, spin drift from the projectile's rotation, transonic instability as speed falls toward the speed of sound, and the Coriolis effect from Earth's rotation. Each is minor near the muzzle but can become significant at long range.
Environmental Inputs
Air density ties the whole model together, since drag rises and falls with it. Altitude, temperature, and barometric pressure change density, so the same projectile flies flatter in thin mountain air than in dense sea-level air. Serious solvers require these atmospheric inputs to predict a trajectory accurately.
Worked Example
Consider two loadings launched at the same 2,900 feet per second, one with a G7 BC of 0.180 and one with 0.280. Near the muzzle they behave almost identically. By 600 yards the higher-BC projectile has retained more velocity, so it has dropped less, drifted less in a crosswind, and reached the target sooner, all traceable to its slower rate of velocity loss over the same distance.
A Common Misconception
A frequent error is treating muzzle velocity as the dominant factor in long-range performance. Raw speed helps early, but a fast projectile with a poor BC sheds velocity quickly and is soon overtaken in drop and drift resistance by a slower projectile with a superior shape. Downrange, aerodynamic efficiency usually matters more than starting velocity.