Twist Rate Calculator (Miller stability formula, 1 in X barrel twist)

Understanding the twist rate calculator — concept and principle

Rifling spins a bullet so that it flies point-first. Too little spin and a long bullet can wobble or tumble. This calculator uses the Miller twist rule to estimate the gyroscopic stability factor (Sg) of a bullet in your barrel and the slowest twist rate that still reaches a target stability, with corrections for velocity and air conditions.

A 1-in-10 barrel spins this bullet to Sg 2.4, comfortably stable.

How it is calculated

Sg = 30 m ÷ (t² d³ L (1 + L²)) × (V ÷ 2800)^(1/3) × (T + 460) ÷ 519 × 29.92 ÷ P

Slowest twist (in per turn) = d × √(30 m × fv × fa ÷ (Sg,target × d³ L (1 + L²)))

m bullet weight (grains), d diameter (in), L length in calibers (length ÷ diameter), t twist in calibers per turn (twist ÷ diameter).
V muzzle velocity (fps), T air temperature (°F), P air pressure (inHg).

The principle

A spinning bullet behaves like a gyroscope: its spin resists the aerodynamic force that tries to turn its nose sideways. Longer bullets have a larger overturning moment relative to their spin inertia, so they need faster spin. Don Miller fitted a simple formula to this physics for conventional lead-core bullets. Faster muzzle velocity adds spin and stability, and thinner air (high altitude, warm weather) reduces the overturning force, also improving stability. Sg above 1 is stable in theory; shooters usually aim for 1.4–2.0 to allow for real-world imperfections.

Worked example

Default inputs: 175 grains, 0.308 in diameter, 1.240 in long, 2,600 fps, 1-in-10 twist, target Sg 1.5, 59 °F, 29.92 inHg.

Length = 1.240 ÷ 0.308 = 4.03 calibers; twist = 10 ÷ 0.308 = 32.5 calibers per turn.
Velocity factor = (2,600 ÷ 2,800)^(1/3) = 0.976; air factor = 1 at standard conditions.
Sg = 30 × 175 ÷ (32.47² × 0.308³ × 4.03 × (1 + 4.03²)) × 0.976 = 2.4: Stable (Sg 1.4 or more).
Slowest twist for Sg 1.5 = 12.6 in per turn = 321 mm.

Each point is the calculator re-run with only “Bullet length (in)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Bullet length (in)	Gyroscopic stability (Sg) in your barrel
0.31	82.08
0.62	16.35
0.93	5.44
1.24	2.4
1.55	1.26
1.86	0.73
2.48	0.31

How to read the result

Where does your result fall?

Any twist faster than the slowest value (a smaller "1 in X") also stabilises the bullet. An Sg well above 2 is not usually harmful for accuracy but may increase spin drift and stress on thin-jacketed bullets. If Sg is marginal, a shorter or lighter bullet, more velocity or a faster-twist barrel can help. Cold, dense air lowers Sg, so check stability for the coldest conditions you will shoot in.

Common mistakes

Using bullet weight instead of bullet length; length matters most.
Writing twist as turns per inch instead of inches per turn.

Keep in mind: The Miller rule is an estimate fitted to conventional bullets. Confirm with the bullet maker's twist recommendations, and follow safe handling practice and local laws.

Frequently asked questions

What does a 1-in-10 twist mean?

The rifling makes one full turn every 10 inches of barrel.

What Sg is recommended?

About 1.4 to 2.0 is the commonly recommended range.

Why do longer bullets need faster twist?

Their greater length increases the overturning force relative to their spin stability.

Sources: D. Miller, "A New Rule for Estimating Rifling Twist: An Aid to Choosing Bullets and Rifles", Precision Shooting, March 2005 · M. W. Courtney & D. G. Miller, "A Stability Formula for Plastic-Tipped Bullets, Part 1", Precision Shooting, January 2012 (arXiv:1410.5340)

Checked 9 Nov 2026. D. Miller, “A New Rule for Estimating Rifling Twist: An Aid to Choosing Bullets and Rifles”, Precision Shooting, March 2005, pp. 43–48, as given (eq. 1, velocity and atmospheric corrections, 1.4–2.0 range) in M. W. Courtney & D. G. Miller, “A Stability Formula for Plastic-Tipped Bullets, Part 1”, Precision Shooting, Jan 2012 (arXiv:1410.5340).

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