G1 & G7 drag models with spin drift, Coriolis correction, and aerodynamic jump. Compute drop, wind drift, time of flight, remaining energy, and transonic thresholds.
This calculator uses a 4th-order Runge-Kutta numerical integrator to solve the point-mass equations of motion for a spin-stabilized projectile. It supports both the G1 (flat base) and G7 (boat tail / VLD) standard drag models using published BRL drag coefficient tables. The atmosphere model follows ICAO Doc 7488/3 with humidity correction via the Wobus polynomial for saturation vapor pressure.
Spin drift is the lateral deflection caused by the bullet's gyroscopic precession interacting with aerodynamic forces during flight. A right-hand twist barrel produces rightward drift. This calculator uses the Litz approximation: SD = 1.25 × (SG + 1.2) × TOF1.83, where SG is the Miller gyroscopic stability factor computed from bullet weight, diameter, length, twist rate, velocity, and temperature. Spin drift is small at short range but becomes significant past 1000 yards for most rifle cartridges.
The Earth's rotation causes two deflection components on long-range trajectories. The horizontal component (proportional to sin(latitude)) deflects the bullet right in the northern hemisphere and left in the southern hemisphere, regardless of firing direction. The vertical component (Eötvös effect, proportional to cos(latitude) × sin(azimuth)) causes the bullet to drop more when firing east and less when firing west. Both effects are negligible under 500 yards but become meaningful at ELR distances.
Aerodynamic jump is the vertical deflection caused by the interaction between crosswind and the bullet's spin (yaw of repose). A right-hand twist bullet experiencing a crosswind from the right will impact slightly high. The effect is small (typically under 0.2 MOA) but is included here as an estimate based on the stability factor and crosswind component. The actual aerodynamic jump for a specific bullet depends on detailed aerodynamic coefficients (CMa, CLa) that require wind tunnel or Doppler radar data.
This is a point-mass solver. It does not model 6-DOF (six degree of freedom) dynamics, epicyclic sway, Magnus effect, or dynamic stability changes through the transonic regime. Results in the transonic zone (Mach 0.9-1.2) should be treated with caution, as real-world bullet behavior in this regime is highly dependent on individual bullet geometry and cannot be accurately captured by a point-mass model with standard G1/G7 drag curves. For the most accurate results in the transonic regime, use a custom drag model (CDM) from Applied Ballistics or equivalent Doppler-measured data.