Rifling spins the bullet so that gyroscopic stability keeps its nose forward. That stability comes at a price: a spinning body under an aerodynamic force does not respond in the direction of the force but at right angles to it. Over a long flight the accumulated sideways response is spin drift.
Where it comes from
A bullet on a curving trajectory is always slightly nose-high relative to its flight path — the path bends downward under gravity faster than the gyroscopically stiff bullet re-aligns. That small angle produces an aerodynamic force, and gyroscopic precession converts it into a steady sideways drift.
The direction follows the twist. A right-hand twist drifts the bullet to the right; a left-hand twist drifts it left. It does not depend on hemisphere, wind or which way you are facing — only on which way the rifling turns.
What makes it larger
| Factor | Effect on spin drift |
|---|---|
| Faster twist (lower number) | More — higher spin rate, more precession |
| Longer time of flight | Much more — the effect accumulates non-linearly |
| Higher stability factor | More |
| Longer, heavier bullet for calibre | More, because it needs more spin to stabilise |
| Crosswind | No direct effect — it is a separate, additive drift |
The dependence on time of flight is what makes spin drift a long-range concern specifically. It is negligible at 300 m, small at 500 m, and by 1,000 m it is commonly comparable to the width of a torso plate — a real correction, on the same order as a modest wind error.
Stability factor, briefly
Gyroscopic stability factor, usually written Sg, expresses how much spin the bullet has relative to how much it needs. Below about 1.0 the bullet is unstable; between 1.0 and 1.4 it is marginal; 1.4 and above is comfortable.
Sg is driven by twist rate, bullet length, bullet mass, calibre and air density. Thin air raises it — a bullet marginally stable at sea level is more stable at altitude. The app derives it from the twist rate and load data you have already entered, which is also what feeds the spin drift estimate.
Over-stabilising costs a little more spin drift and a little more jacket stress. Under-stabilising costs accuracy, and it costs it worst in the transonic region. Given that trade, aiming for comfortable rather than minimal stability is the sound choice.
How it interacts with wind
Spin drift and wind drift are independent and simply add. A right-hand-twist rifle in a wind from 9 o'clock has both effects pushing right, and the total horizontal correction is larger than either alone. In a wind from 3 o'clock they oppose each other and partly cancel.
This is why shooters who ignore spin drift often develop an apparent bias in their wind calls: they compensate for the constant sideways offset by systematically over- or under-calling wind in one direction. Modelling spin drift separately removes the bias and makes wind-calling practice honest.
Should you switch it on?
- Under 500 m — it does not matterThe correction is smaller than your group. Leave it on; it costs nothing and changes almost nothing.
- 500 to 800 m — worth includingIt is becoming a measurable offset, and including it makes your recorded wind calls cleaner.
- Past 800 m — include it and check the twist directionIt is now a real part of your horizontal solution. A wrong twist direction here is a double-magnitude error.
- Verify against still-air dataThe only honest check is a genuinely calm day. If your impacts sit consistently to one side at distance, that is spin drift you have not accounted for — or accounted for twice.
The app treats spin drift as a switchable correction that is applied on top of the wind solution, so you can see its contribution on its own rather than having it silently folded into a single windage number.
Frequently asked
Which way does spin drift push the bullet? +
In the direction of the rifling twist. Right-hand twist drifts right, left-hand twist drifts left. It is independent of hemisphere and of shooting direction.
How is it different from Coriolis? +
Spin drift comes from the bullet's own rotation and depends on the barrel. Coriolis comes from the Earth's rotation and depends on your latitude and the direction you are shooting. They are separate effects that add together.
Can I just fold spin drift into my wind call? +
You can, and many shooters do — but then your wind calls carry a hidden constant, and they stop being comparable between rifles or between directions. Modelling it separately is more useful if you keep records.