Table of Contents
What Is Twist Rate?
Twist rate refers to the rate of rifling twist in a gun barrel, expressed as one full revolution in a given length of barrel (e.g., 1:10" means one complete rotation in 10 inches). The rifling imparts spin to the projectile, gyroscopically stabilizing it in flight and ensuring it travels point-forward.
The required twist rate depends primarily on the bullet's length-to-diameter ratio and its density. Longer, heavier bullets require faster twist rates (shorter twist lengths) for adequate stabilization. An improperly stabilized projectile will yaw, tumble, or keyhole, drastically reducing accuracy.
Greenhill Formula
Where d is the bullet diameter in inches, L is the bullet length in inches, and SG is the specific gravity of the bullet material (10.9 for lead-core bullets). The constant 150 applies to muzzle velocities around 2800 fps; use 180 for velocities above 2800 fps.
Common Twist Rates
| Caliber | Common Twist | Bullet Weight Range |
|---|---|---|
| .223 Rem / 5.56mm | 1:7" to 1:12" | 40-77 grains |
| .308 Win / 7.62mm | 1:10" to 1:12" | 147-175 grains |
| 6.5 Creedmoor | 1:8" | 120-147 grains |
| .338 Lapua | 1:9" to 1:10" | 250-300 grains |
Frequently Asked Questions
What happens with too slow a twist rate?
An understabilized bullet will yaw in flight and may tumble, producing elongated or "keyhole" impacts on the target. Accuracy degrades dramatically, and the bullet cannot follow its intended trajectory.
Can a twist rate be too fast?
Yes, over-spinning lightweight or frangible bullets can cause them to come apart from centrifugal force. However, moderate overstabilization typically has minimal effect on accuracy. Very high spin rates can also cause greater drift in crosswinds due to gyroscopic precession.
What is the stability factor?
The gyroscopic stability factor (Sg) is a dimensionless number. A value of 1.0 is marginally stable, and values above 1.3-1.5 are considered adequately stable. Most well-matched bullet/barrel combinations have Sg between 1.5 and 3.0.