Stability Calculator

Whether your barrel’s twist will actually stabilise a bullet, worked out with Don Miller’s twist rule and corrected for the air you are shooting in.

Reached from Stability Calculator (drawer → Tools & System), or from the Open Stability Calculator link in the Stability section of a profile.

Reading the result

The big figure is the stability factor, Sg, in the conditions you entered:

• Below 1.00: Unstable. The bullet will not hold its nose forward and is likely to keyhole.
• 1.00 to 1.49: Marginally stable. It flies, but a colder or denser day can push it under, and accuracy often suffers.
• 1.50 and above: Comfortably stable. Miller’s recommended minimum, which holds across most temperatures and atmospheric changes.

Underneath:

• At standard atmosphere: the same load at sea level, 15 °C. This is the figure bullet makers and other twist calculators quote, so use it to compare bullets.
• Twist for Sg 1.50: the slowest twist that reaches 1.50 in your conditions. Any faster twist (a smaller number) also works.
• Longest bullet at this twist: the longest bullet of this weight that your twist holds at 1.50, assuming it weighs the same as the one you entered. A longer bullet of the same calibre is usually heavier, which raises Sg, so the real limit is a little longer than shown: the figure errs on the safe side.
• Bullet length: the length you entered, and how many calibres long it is.

Inputs

• Projectile Weight, Diameter and Length. Use Choose a projectile to fill all three from the library or your own projectiles; you can still type over any of them afterwards.
• Muzzle Velocity and Barrel Twist.
• Temperature, plus either Altitude or Station Pressure, picked with Work Air Density Out From. Altitude estimates the pressure from the standard atmosphere; a pressure reading from a weather meter is more accurate when you have one. It starts on pressure whenever your active location has conditions saved, so its figure matches the profile’s Stability section.

The button at the top of the screen names the rifle the figures came from, or reads Select Profile… when there is none. Tap it to load a different saved profile: its bullet, velocity and twist replace what is on screen, along with your active location’s conditions, and the result recalculates straight away. Picking a rifle here does not change your active profile anywhere else in the app.

Notes

• Profiles hide the Stability section for airgun pellets. Drag on the skirt, not spin, keeps a pellet nose-forward, so Miller’s rule does not apply to them.
• Humidity is not an input. It moves Sg by around 1% at most, and the calculator and the profile section both treat the air as dry so they always agree.
• Nothing here is saved, and nothing you type changes your profile. It is a scratchpad, like the Muzzle Energy Calculator.
• A projectile with no measured length cannot be worked out: Sg depends on length more than on anything else, so a guessed length would give a confidently wrong answer. Enter the length by hand if you know it.
• Miller’s rule is an estimate calibrated on boat-tail match bullets. It is less reliable for flat-based bullets, very long monolithic bullets and airgun pellets, whose shape it was never fitted to.