A bullet is stabilised by spin. The rifling’s twist rate is how far the bullet travels along the bore for one full rotation — expressed as 1:12" (one turn in 12 inches) or, metrically, 1:305 mm. A faster twist is a smaller number.
Note the direction of the convention: 1:7” is faster than 1:12”. European barrels are often quoted in millimetres; 1:12” = 1:305 mm, 1:8” = 1:203 mm.
Spin rate follows from twist and velocity
The bullet’s rotational speed is set entirely by muzzle velocity and twist:
spin (rev/s) = muzzle velocity / twist length
A 5.56 bullet at 990 m/s from a 1:7” (0.1778 m) barrel spins at about 5570 revolutions per second — 334 000 rpm. That is a useful number to have in mind: it is why jacket integrity matters at high velocity, and why very light varmint bullets occasionally come apart in flight from fast-twist barrels.
The spin decays much more slowly than the forward velocity. Downrange the bullet is therefore relatively more stabilised than at the muzzle, not less — a point that surprises people.
What actually needs stabilising
The aerodynamic force on a bullet acts at its centre of pressure, which for a supersonic bullet sits ahead of its centre of gravity. That is statically unstable: any small angle between the bullet’s axis and its flight path produces a moment that increases the angle. Left alone, the bullet tumbles.
Spin converts that overturning moment into gyroscopic precession — the nose traces a small circle around the flight path instead of flipping. Enough spin, and the motion is bounded and damps out.
What determines “enough” is mostly the bullet’s length, not its weight. Length is what sets the overturning moment arm. Weight correlates with length within a caliber, which is why twist requirements are usually quoted per bullet weight, but for a monolithic copper bullet — longer than a lead bullet of the same weight — the weight-based rule of thumb under-predicts what is needed.
The Miller stability rule
The standard quick estimate is Miller’s formula for the gyroscopic stability factor Sg:
Sg = 30 m / (t² d³ l (1 + l²))
with m in grains, t the twist in calibers per turn, d the diameter in inches, and l the bullet length in calibers. A velocity and air-density correction is applied on top; Miller’s rule is defined at 2800 fps and standard conditions.
The interpretation:
| Sg | Meaning |
|---|---|
| below 1.0 | unstable — the bullet will not fly point-first |
| 1.0 – 1.4 | marginal; group size and BC both suffer |
| 1.4 – 2.0 | the target window |
| above 2.0 | over-stabilised; small practical penalty |
Two corrections matter in the field. Cold, dense air lowers Sg, so a load that is marginal at +25 °C can destabilise at −25 °C — a real consideration in Finland. And a slower muzzle velocity lowers Sg, so a short barrel can turn a marginal combination into an unstable one.
Under- and over-stabilisation
Under-stabilised bullets show up unmistakably: elongated or keyholed holes in paper, groups that are wild rather than merely large, and a BC well below the published figure because the bullet is flying at a persistent yaw angle.
Over-stabilised bullets fly nose-first, but too rigidly. A stable bullet should slowly pitch its nose down to follow the descending trajectory; an over-spun one holds its original attitude and meets the air at a small angle late in flight, costing a little BC. The effect is small — it is not a reason to choose a slower twist if you might ever shoot heavier bullets. Marginal stability costs far more than excess stability.
Common twist rates
| Cartridge | Twist | Suits |
|---|---|---|
| .223 Rem / 5.56 | 1:12” | 40–55 gr |
| 1:9” | 55–69 gr | |
| 1:7” | 62–80 gr (NATO standard, needed for tracer length) | |
| .308 Win / 7.62×51 | 1:12” | 150–175 gr |
| 1:11” – 1:10” | up to 185–200 gr | |
| 1:8” | heavy subsonic, 200–230 gr |
The 5.56 NATO 1:7” specification is a good illustration that twist follows length: it was chosen not for the 62 gr M855 ball round but for the much longer M856 tracer that has to be stabilised by the same barrel.
Subsonic .308 loads are the other clear case. A 200–230 gr bullet is long, and it is launched at a third of the usual velocity, so it gets a third of the usual spin. Both effects push Sg down, which is why dedicated subsonic rifles use faster twists than standard .308 barrels.