FREEBORE BALLISTICS

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Twist and stability

Will your barrel's twist stabilize this bullet? Enter the bullet, the rifle and the air, and see the stability factor and the slowest twist that works.

Bullet

Bullet diameter.

Measure the bullet tip to base, or use the maker's published length.

Rifle and load

1:12 is 12

Air

Cold, thin or high-altitude air changes stability. Check the worst conditions you shoot in.

Gyroscopic stability (Sg, Miller rule)

1.67

Stable. Sg of 1.5 or more is considered comfortably stable.

Slowest twist for Sg 1.3

1:13.6 in

A common minimum.

Slowest twist for Sg 1.5

1:12.6 in

Comfortably stable.

Bullet length

4.03 cal

Longer bullets need faster twist.

Uses the Miller twist rule, an empirical estimate that works best for conventional jacketed spitzer bullets. Solid, plastic-tipped or unusual bullets can differ, and the barrel's actual twist can vary slightly from its marking. A faster twist than needed is generally harmless to accuracy; too slow a twist is not. Freebore never suggests loads.