Rifling spins the bullet so it flies point-first. Whether your barrel spins a given bullet fast enough is decided at manufacture — and the target tells you before the ammo box does.
A 1:8 twist barrel spins the bullet one full turn every 8 inches of travel. That spin is gyroscopic stabilisation — the same physics as a football spiral — and without enough of it a bullet tumbles within yards. Smaller number, faster twist, more stability for long bullets.
Fast twists (1:7, 1:8) stabilise long, heavy-for-calibre and high-BC match bullets, and are the modern default. Slow twists (1:12, 1:14) suit short, light bullets and put less spin stress on thin varmint jackets. A fast twist shoots light bullets fine; a slow twist cannot shoot long ones at all.
Modern calculators score stability as SG (gyroscopic stability factor, from the Miller formula). Below 1.0 the bullet tumbles; 1.0–1.4 is marginal — fine on a warm day, unstable in cold dense air; 1.5–2.0 is the target band; far above that is over-stabilised, which matters mostly to fragile varmint bullets. SG moves with conditions: thin air at altitude adds stability, cold sea-level air subtracts it — a marginal load can cross the line in winter.
Greenhill predates boat-tails, polymer tips and monolithics, so treat it as a starting point. Bullet makers publish a minimum twist per bullet (“1:10 or faster”), and online SG calculators take exact dimensions and your conditions.
| CALIBRE | COMMON TWISTS | BULLET RANGE |
|---|---|---|
| .223 / 5.56 | 1:7, 1:8, 1:9 | 55–90 gr |
| 6.5 Creedmoor | 1:8 | 120–147 gr |
| .308 Win | 1:10, 1:11, 1:12 | 150–185 gr |
| .30-06 | 1:10 | 150–220 gr |
| .22-250 | 1:12, 1:14 | 40–60 gr |
Shoot a group and run it through the analyzer — round holes and a tight cluster, or the oblong tears of a bullet flying sideways.