Barrel twist rate and bullet stability (the Sg number)
Barrel twist rate spins a bullet to keep it stable in flight. This explains the twist-rate notation, the Sg stability factor, how bullet length and speed factor in, and what under- and over-stabilisation actually do.
Why bullets need spin
A bullet flying nose-first is not naturally stable — air pressure on the nose tries to tumble it. Spinning it fast enough turns it into a gyroscope that resists that overturning torque and keeps the nose pointed into the wind of its own flight. The barrel's rifling twist rate sets how fast it spins.
Reading twist-rate notation
Twist is written as 1:n — one full rotation of the bullet per n inches of barrel travel. 1:7 is faster (more spin) than 1:12. Spin rate in rpm is roughly:
rpm ≈ muzzle velocity (fps) × 720 ÷ twist (in)
A 2,800 fps load from a 1:8 barrel spins about 252,000 rpm. That figure barely changes downrange even as velocity bleeds off, which is why stability is usually worst right at the muzzle in cold, dense air — and why it is quoted at the muzzle.
The stability factor, Sg
Gyroscopic stability is summarised by Sg, a dimensionless number from the Miller stability formula. It depends mostly on:
- Twist rate — faster twist raises Sg.
- Bullet length (in calibres) — longer bullets are harder to stabilise. Length, not weight, is the real driver; heavier bullets just tend to be longer.
- Muzzle velocity — faster helps, but only weakly.
- Air density — cold, high-pressure, sea-level air is the hardest to stabilise in; altitude and heat make it easier.
| Sg < 1.0 | Unstable. The bullet yaws or tumbles; groups are wild and holes are keyholed (oval). |
|---|---|
| Sg 1.0–1.4 | Marginal. It may shoot acceptably in warm weather but lose accuracy and BC in cold, dense air. Not enough margin for serious work. |
| Sg ≥ 1.5 | The usual target. Full stability with margin for cold days and lot-to-lot variation, and the bullet achieves its advertised BC. |
Enter your projectile length in the calculator's Ammunition section and, with the barrel twist from the Firearm section, Ballistic Buddy estimates Sg and spin drift for you.
Under- vs over-stabilised
Under-stabilised (Sg too low) is the real problem: lost accuracy, reduced effective BC, keyholing, and it gets worse as the bullet slows at long range.
Over-stabilised (very fast twist for a short bullet) is mostly harmless. The often-repeated idea that a fast twist “blows up” ordinary bullets only applies to frangible varmint projectiles at extreme rpm. A slight downside: an over-stabilised bullet stays pointed at its original angle rather than tracking its arc, so it flies very slightly nose-high downrange, costing a tiny bit of BC. When in doubt, choose the faster twist — it will shoot short bullets fine and leaves the door open to heavier ones.