How this calculator works
Greenhill estimates twist from diameter, length and a density adjustment with an inch-based empirical coefficient. Miller additionally uses mass and a target gyroscopic stability factor. Calculations convert SI inputs internally to inches, grains and feet per second for the published constants. Velocity correction is clamped below 1120 ft/s as in Miller's rule. Use measured local pressure and temperature, or a dry standard-troposphere altitude correction, without applying both. Altitude mode accepts geopotential heights from -500 to 11000 m and assumes the standard 6.5 K/km temperature lapse rate. These rules assume conventional projectiles and do not model hollow tips, plastic tips, dynamic stability or individual barrel behaviour.
Formula
Greenhill: T=C d²/L sqrt(SG/10.9), inch units. Miller: Sg=30 m/[t² d³ l(1+l²)] × fv × fatm; t=T/d, l=L/d, m in grains. fv=(max(v,1120)/2800)^(1/3); fatm=(TK/288.15)(Pref/P) or (1-0.0065 H/288.15)^(-4.25588) in the standard troposphere, H in metres.
Worked example
Greenhill with coefficient 150, specific gravity 10.9, diameter 0.300 in and length 1 in gives 13.5 in per turn, or 0.3429 m. With 0.200 in diameter and the same length, the estimate is 6 in per turn, or 0.1524 m.
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Frequently asked questions
What does a smaller twist-length number mean?
It means a faster spin for the same forward speed because the projectile completes a turn over less distance.
Is sea-level weather pressure the right input?
Use actual local absolute pressure. Weather reports often show pressure adjusted to sea level.
Can the Miller result predict accuracy?
No. It estimates gyroscopic stability under a simplified empirical model, not dispersion or dynamic flight behaviour.