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Wind Drift, Spin Drift & Coriolis

Past a few hundred yards, three things push your bullet sideways: wind, the bullet's own spin, and the Earth's rotation. Here's what each does and how AHL Sight corrects for them.

Wind drift — the big one

Wind is by far the largest horizontal error at distance. What matters is the crosswind component: a wind from 3 or 9 o'clock is "full value," while a headwind or tailwind barely deflects the bullet. Drift grows faster than range because the bullet spends more time in the wind as it slows. In AHL Sight you set wind speed and the clock direction it blows from, and the DOPE table and reticle show the wind hold in MIL or MOA. Remember: the app models a single wind value — downrange you still read flags and mirage.

Spin drift — the bullet's gyroscope

A bullet from a right-hand-twist barrel slowly walks to the right as it flies (left-twist walks left). It's a gyroscopic effect tied to how fast the bullet spins and how long it's in the air, so it only becomes noticeable past ~500–600 yards — often several inches at 1,000. AHL Sight uses the Litz approximation from your barrel twist and gyroscopic stability (Sg); entering bullet length sharpens it.

Coriolis — the spinning Earth

Over a long flight the Earth rotates under the bullet, nudging the impact. The horizontal part depends on your latitude; there's also a small vertical (Eötvös) part that depends on whether you're firing east or west. It's small — inches at 1,000 yards — but real for extreme long range. AHL Sight computes it from your latitude and shooting bearing; tap "Use current conditions" to fill your latitude from your location.

Transonic — where it all gets messy

As a bullet slows toward the speed of sound (about Mach 1.2, ~1,340 fps), groups open up and drop becomes less predictable. AHL Sight flags the range where your load goes transonic so you know where your dope stops being trustworthy.

Put it together

Turn on spin drift and Coriolis in the Advanced card, set your wind, and the DOPE table and reticle fold all of it into a single elevation and windage hold. True your muzzle velocity to your real drops first, and your long-range solution gets dramatically more honest.

AHL Sight is a planning aid. Always confirm your zero and dope with live fire and follow all firearm safety rules.