Twist Rate & Bullet Stability
Miller stability formula: check whether a barrel twist will stabilize a given bullet.
How this works
A spinning bullet stays point-forward through gyroscopic stability, the same principle that keeps a thrown football spiral straight. Miller's formula — a widely published approximation of the more complex rigorous stability calculations — estimates that stability factor (SG) from your barrel's twist rate and the bullet's own dimensions and speed.
Reading the SG number
An SG below 1.0 means the bullet is undergunned for that twist and will likely keyhole (strike targets sideways) rather than fly true. Most shooters aim for at least 1.4 for reliable accuracy, with target and long-range shooters often preferring values in the 1.5–2.0 range. Very high SG (well above 2) rarely causes real problems — it just means the barrel has more twist than that particular bullet strictly needs.
Frequently asked questions
What twist rate do I need for heavy, long-range bullets?
Longer, heavier-for-caliber bullets generally need a faster twist (a smaller number, like 1:8) to stay stable, while shorter, lighter bullets can often stabilize with a slower twist (a larger number, like 1:12). Check this calculator with your specific bullet's length and weight rather than assuming from caliber alone.
What happens if my twist rate is too slow?
An understabilized bullet can keyhole — strike the target sideways instead of point-first — which badly hurts accuracy and can be dangerous downrange. This shows up as a stability factor (SG) below about 1.0 in the calculator.
Can a barrel have too much twist?
Yes, though it's rarely a practical problem — overstabilization (a very high SG) is usually harmless for hunting purposes and mostly just means the barrel has more twist than that particular bullet strictly needs.
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