Twist rate stability
Before you buy 500 rounds of a heavy-for-caliber match bullet, find out whether your barrel will actually spin it fast enough. This runs the Miller stability rule with velocity and air-density corrections, and tells you the slowest twist that would still work.
Bullet and barrel
Bullet length drives stability far more than weight does, and it is the number nobody publishes on the box. Measure a round with calipers, or find it on the bullet maker's technical page. A guess here makes the answer a guess.
Result
How this is calculated
The Miller twist rule: Sg = 30m / (t² · d³ · l · (1 + l²)), where m is bullet mass in grains, d is diameter in inches, t is twist expressed in calibers per turn, and l is bullet length in calibers.
That result is then corrected for velocity by (V / 2800)^(1/3) and for air density by ((T + 460) / 519) × (29.92 / P), using station pressure derived from your altitude. Cold, dense air is harder to stabilise in — which is why a load that shot fine in August can keyhole in January.
Reading the number
- Below 1.0 — not stabilised. Keyholing.
- 1.0 to 1.4 — marginal. Works in favourable air, loses effective BC, and is not a match load.
- 1.4 to 2.0 — the target band.
- Above 2.0 — more spin than needed. Harmless in practice, but the barrel would shoot a heavier bullet well.
Miller's rule is an approximation built for conventional jacketed bullets. Very long monolithic or low-drag designs sit outside its assumptions — treat a marginal result on those as a reason to ask the bullet maker directly.