A Subtle Sideways Push
Spin drift is a small, consistent sideways displacement caused by the projectile's own gyroscopic spin. Unlike wind deflection, it occurs even in perfectly still air, and because it is systematic rather than random, it can be predicted and corrected once its cause is understood.
Why a Spinning Projectile Drifts
As a projectile arcs downward along its trajectory, its nose must continually pivot to keep following the descending flight path. A spinning gyroscope resists being reoriented and instead responds at a right angle to the applied force, a behavior called precession. The result is that the nose points slightly to one side of the true path, and this small persistent yaw generates a lateral force that pushes the projectile sideways.
Direction Follows the Twist
The drift direction depends on which way the barrel's rifling spins the projectile. A right-hand twist, which is the most common, produces drift to the right, while a left-hand twist drifts left. The direction is fixed for a given rifle and does not change from shot to shot, unlike wind, which can come from either side.
Growth With Distance
Spin drift accumulates with time of flight and grows faster than distance itself, so it is negligible up close and becomes measurable only at long range. Because it depends on flight time, the same factors that increase drop and wind drift, such as lower velocity and a lower ballistic coefficient, also enlarge spin drift.
Scale Relative to Other Effects
Spin drift is typically much smaller than wind deflection at any given distance, often just a few inches where wind might move a projectile a foot or more. It is one of several minor effects, alongside the Coriolis effect, that precision shooters fold into a solver only after accounting for the dominant factors of drop and wind.
Worked Example
For a typical long-range rifle with a right-hand twist, spin drift might amount to roughly 1 inch at 500 yards, on the order of 3 to 4 inches at 800 yards, and perhaps 8 to 10 inches at 1,000 yards, always to the right. A shooter making a precise cold-bore call at 1,000 yards would hold or dial a small left correction to cancel this consistent rightward push.
A Common Misconception
Spin drift is frequently confused with the Coriolis effect or with wind. It is neither. It arises purely from the projectile's own spin and gyroscopic precession, so its direction is tied to the rifling and is the same in dead-calm air with no regard to compass heading. Wind can push either way and Coriolis depends on latitude and direction of fire, but spin drift is a fixed, one-directional bias for a given barrel.