Greenhill Formula
Definition
Alfred Greenhill's 1879 empirical formula for the minimum barrel-twist rate to gyroscopically stabilise a bullet of given length and calibre. Uses a single constant (150 for lead-core bullets). Reliable for blunt ogive projectiles; less accurate for modern spitzer bullets.
- Originator
- Alfred Greenhill, 1879
- Purpose
- Minimum barrel-twist rate for gyroscopic stability
- Constant used
- 150, for lead-core bullets
- Limitation
- Less accurate for modern spitzer bullets
Common questions
Why does the Greenhill formula work less well for modern spitzer bullets?+
The formula assumes a fairly blunt, uniform bullet shape typical of Greenhill's era, whereas modern pointed spitzer bullets have a longer, more elongated ogive and different mass distribution, so the simple single-constant relationship underestimates the twist rate actually needed to stabilise them.
What practical decision does the Greenhill formula inform in firearms design and examination?+
It tells a barrel maker or examiner the fastest twist rate needed for a bullet of a given length and calibre to remain gyroscopically stable in flight, which matters when correlating rifling characteristics recovered from a bullet against the range of firearms capable of stabilising it.
Related terms
- Heydenreich Model
- A parameterised internal-ballistic model incorporating propellant specific force, burn-rate coefficients, and chamber volume. More physically grounded than Le Duc; the conceptual ancestor...
- IBHVG2
- Interior Ballistics of High Velocity Guns version 2; a US Army Research Laboratory code implementing the Lagrange pressure-gradient model for large-calibre and...
- LabRadar
- A Doppler radar chronograph measuring bullet velocity continuously from muzzle to several hundred metres, producing a velocity-distance profile. No physical contact with...
- Le Duc Model
- An empirical algebraic formula (1873) relating muzzle velocity to peak chamber pressure via two calibration constants. Produces velocity estimates within 3-5% of...
- Miller Stability Formula
- A dimensionless stability factor (Sg) formulation published by Donald Miller in 2005, calibrated for modern spitzer bullets. Sg greater than or equal...
- Oehler Model 35P
- A dual-screen optical chronograph (plus proof channel) widely used in US forensic and manufacturer testing. Accurate to within 0.2% of reading. The...
- PRODAS
- Projectile / Rocket Design and Analysis System; a US Army Research Laboratory / Arrow Tech Associates lumped-parameter code integrating internal, external, and...