- Drop
- How far below your line of sight the bullet is at a given distance. Gravity starts pulling the moment it leaves the barrel; the sights are tilted so it crosses your aim point at the zero range, then falls away beyond it.
- Zero
- The distance where the bullet hits exactly where you aim. Set with the Zero range slider; shown as the green dot on the flight view and the highlighted row in the range card.
- Wind drift
- How far a side wind pushes the bullet off line. It grows quickly with distance because the bullet slows down and spends longer in the wind.
- MOA and MIL
- The two units scope turrets click in. 1 MOA is about 1 inch at 100 yd (1.047 in). 1 MIL is 3.6 in at 100 yd (10 cm at 100 m). Both scale with distance, so a 10 MOA correction is 10 in at 100 yd and 50 in at 500 yd.
- Ballistic coefficient (BC)
- A single number for how well a bullet cuts through the air. Higher means it slows down less, drops less and drifts less. A BC is always quoted against a drag model, G1 or G7.
- G1 and G7
- Two standard bullet shapes used as yardsticks. G1 is short with a flat base; G7 is long with a tapered boat tail. Modern long-range bullets behave like G7, so a G7 BC predicts them more reliably across the whole flight.
- Muzzle velocity and energy
- Speed as the bullet leaves the barrel, and the energy it carries (½ × mass × speed²). Energy is what a bullet delivers on impact; hunters often look for a minimum at the target distance.
- Supersonic, transonic, subsonic
- Above about Mach 1.2 the bullet flies cleanly. Passing through the speed of sound (the transonic band, shaded on the charts) it becomes unsteady and the drag model is least accurate. Below Mach 1 it is subsonic.
- Sight height
- How far the scope sits above the barrel. Because the bullet starts that far below the line of sight, it crosses the line twice: once close in and once at the zero.
- Twist
- How fast the rifling spins the bullet: 1:8 means one turn every 8 inches. Faster twist stabilises longer, heavier bullets. Shown for information; it does not change these numbers.
- Maximum ordinate
- The highest point of the arc above the line of sight, and where it happens. With a long zero the bullet can be several inches high mid-way, which matters for small targets.
- Range card
- A table of the corrections you need at each distance. Shooters print one, tape it to the stock, and dial or hold whatever it says for the range they are shooting.
Point-mass trajectory
The bullet is a point with mass, velocity and a drag coefficient that depends on Mach number. Each step integrates gravity and drag with a fourth-order Runge-Kutta scheme at 0.5 ms. Drag deceleration is a = 2.0855e-4 · (ρ/ρ₀) · Cd(M) · v² / BC in ft/s², the same form used by JBM and most commercial solvers.
Zeroing is solved first: the bore angle is found by secant iteration so the path crosses the line of sight at the zero range on flat ground with no wind. That angle is then fired on the chosen incline with wind.
G1 and G7
A ballistic coefficient compares the bullet to a standard projectile. G1 is a short flat-base shape, G7 a long boat-tail. A G7 BC stays nearly constant across the speed range for modern match and hunting bullets, so the solver uses G7 whenever the maker publishes one. Rimfire, round-nose and flat-base bullets are modelled on G1, which fits them better.
The Cd-versus-Mach tables are the public reference tables (as reproduced in the open-source py-ballisticcalc project).
Barrel length
Every catalogue velocity is quoted from a test barrel. The slider rescales it with the cartridge's measured fps-per-inch, curving the loss for short barrels (each inch cut from an already short barrel costs more) and damping gains beyond the reference length. Rimfire and pistol rounds that burn all their powder early (.22 LR peaks near 18 in, 9mm near 16 in) stay flat or slowly lose speed in longer barrels. Real rifles vary by ±50 fps from these figures.
Wind, weather and angle
Wind direction is where the wind comes from: 0° is a headwind, 90° is from the shooter's right (3 o'clock), 180° a tailwind. Drag acts on the bullet's velocity relative to the moving air, so head and tail winds also change drop slightly.
Air density is computed from temperature, pressure and humidity. Leave pressure blank to use the standard pressure for the altitude. BCs are quoted at ICAO standard (59 °F, 29.92 inHg, sea level), and thin air at altitude lowers drag.
Up or downhill shots use the full vector solution, so the bullet strikes high in both directions, as on the range.
Not modelled
- Spin drift and the Coriolis effect (a few inches at 1000 yd for a .308).
- Gyroscopic stability. Twist rates are shown but bullet length is not in the dataset.
- Aerodynamic jump from crosswind, and vertical wind.
- Velocity spread between shots.
The dataset is for education and comparison, not load development. Velocities and BCs are manufacturer figures.
Sources
Hornady, Federal, Winchester, Remington, Sierra, Nosler and Lapua ballistics tables; SAAMI and CIP pressure standards; US Army TM 43-0001-27 and Russian GRAU data for military ball; Ballistics by the Inch and Rifleshooter.com barrel cut-down studies; manufacturer rifle spec sheets.