Two infrared light gates a known distance apart start and stop a timer as the
shell's shadow crosses each beam. Velocity is simply gate spacing divided by the
measured Δt — this is the same principle real optical chronographs use. Because
drag is already slowing the shell between the gates, the measured value is an
average, not the instantaneous muzzle velocity.
v_measured = gate_spacing / Δt
Δt = t(gate2) − t(gate1)
v(x) decays from v₀ under drag: dv/dt = −k·v² (k from ballistic coefficient)
- Muzzle velocity — true instantaneous speed leaving the barrel.
- Ballistic coefficient — how well the shell resists drag; lower BC means faster velocity decay.
- Gate spacing — distance between the two chronograph sensors; wider spacing averages over more deceleration, adding a small systematic bias.
The error readout shows how far the two-gate average drifts from the true
instantaneous velocity at the first gate — the reason precision chronographs use
close gate spacing near the muzzle.