How 247Range calculates

The method behind the numbers · November 2026

The flight path

The app steps the bullet forward every half a millisecond of flight (a fourth-order Runge–Kutta integration, accurate to well under a millimetre at 2 km). At every step it works out three forces.

Drag, from the bullet's BC

The ballistic coefficient scales a standard drag curve: G7 for long boat-tail match bullets, G1 for flat-base hunting bullets. Drag is read against the bullet's speed compared with the speed of sound, so it slows correctly as it gets close to subsonic. Where makers publish speed-banded BCs (multi-BC), the app changes the BC as the bullet slows. The built-in library of 1,200+ bullets uses each maker's own published BC, G7 where it exists.

Gravity

Pulls the bullet down for its whole time of flight, which is why slower and heavier-for-calibre bullets need more elevation.

The air you're shooting through

Temperature, station pressure and humidity give the true density of the air: thinner air means less drag and less drop. Humid air is slightly thinner than dry air, and the app counts that too. Temperature also gives the speed of sound, which decides where the bullet goes transonic. These come from the phone's barometer, the weather for where you stand, a Bluetooth weather meter, or what you type in. Station pressure is used as measured, not corrected to sea level.

Wind

Wind from any clock direction. A crosswind pushes the bullet sideways; a head or tail wind changes its speed through the air, so it moves the elevation slightly too. Optionally a second wind zone further out, for a different wind across a valley.

Added on top of the path

• Zero and sight height: the angle that puts the bullet on your zero with the scope above the bore.
• Slope: the true look angle up or down hill, not a simple cosine shortcut.
• Spin drift: Bryan Litz's published formula, driven by the bullet's gyroscopic stability, which is calculated with the Miller formula from weight, length, calibre, twist rate and the air.
• Coriolis: the earth's rotation, from your latitude and shot direction. Small, and only worth having past about 800 m.
• Aerodynamic jump (optional): the small up-or-down kick a crosswind gives the bullet as it leaves the muzzle, also from Litz's work.

Truing to your rifle

Box velocities and published BCs are a starting point. After a real shot at distance, tell the app what elevation actually hit and it adjusts the BC or the muzzle velocity so its answer matches your rifle. With chronograph data (Garmin Xero or LabRadar, in Pro) the muzzle velocity is your own measured average, and its spread feeds the hit chance.

First-shot hit chance

The same solver runs 10,000 times with small random errors in the range, the wind speed and direction, the muzzle velocity and your rifle's own group size. It counts how many of those shots land on the target, and tells you which error matters most, so you know whether to range again, read the wind better, or move closer.

How accurate is it?

The solver has been checked against an independent open-source solver and agrees to within a few hundredths of a MOA. Like every calculator, it can only be as good as what you put in: a real chronograph speed, the right BC, today's air and a confirmed zero matter far more than the method.

What it doesn't do: the 4DOF and Doppler-radar solvers from Hornady and Applied Ballistics use drag curves measured for each individual bullet. Past about 1,000 m, and through the transonic zone, those can keep a small edge. For hunting and most competition distances, a point-mass solver with a good G7 BC, trued to your rifle, is what most shooters use.