Barrel Twist Rate Calculator
Find the optimal barrel twist rate for your caliber and bullet weight. Uses the Greenhill formula with Miller stability analysis to help you choose the right barrel for accurate, stable bullet flight.
Configure Your Bullet
Auto-filled from known 55 gr bullet data
Recommended Twist
Greenhill: 1:10.2
Stability Factor
> 1.5 optimal
Assessment
Well stabilized. Optimal for accuracy and consistency.
Stability Factor by Twist Rate
Shows the Miller stability factor for a 55 gr .223 Remington bullet across common twist rates. Green zone (above 1.5) indicates optimal stability.
.223 Remington Twist Rate Reference
| Bullet Weight | Recommended Twist |
|---|---|
| 40 to 55 gr | 1:9 or 1:12 |
| 55 to 69 gr | 1:8 or 1:9 |
| 69 to 77 gr | 1:7 or 1:8 |
How It Works: The Greenhill Formula
The Greenhill formula estimates the optimal barrel twist rate needed to stabilize a bullet in flight:
- T = twist rate (inches per revolution)
- C = 150 for supersonic loads, 180 for subsonic
- d = bullet diameter in inches (0.224" for .223 Remington)
- L = bullet length in inches
- SG = specific gravity of bullet material (10.9 lead-core)
The stability factor displayed uses the Miller stability formula, which accounts for bullet mass and length-to-diameter ratio. A factor above 1.5 indicates the bullet will be well stabilized with the given twist rate. Below 1.3, accuracy typically degrades as the bullet may not spin fast enough to maintain a stable flight path.