1. What is Barrel Twist Rate?

The barrel twist rate refers to the distance a bullet must travel down the barrel to complete one full rotation. It's a critical factor in firearm accuracy and stability, measured in inches per turn (e.g., 1:10" means one rotation every 10 inches).

2. How Does the Calculator Work?

The calculator uses the Greenhill formula:

Where:

• \( T \) — Twist rate in inches per turn
• \( D \) — Bullet diameter in inches
• \( L \) — Bullet length in inches
• \( SG \) — Specific gravity of bullet material (10.9 for lead)

Explanation: The formula calculates the optimal twist rate needed to stabilize a bullet of given dimensions and material.

3. Importance of Twist Rate Calculation

Details: Proper twist rate ensures bullet stability in flight, affecting accuracy, precision, and terminal performance. Too slow causes instability, too fast increases wear and may over-stabilize.

4. Using the Calculator

Tips: Enter bullet diameter and length in inches, and specific gravity (10.9 for lead, 8.5 for copper). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What's the most common twist rate for .223/5.56mm?
A: Common rates are 1:7" to 1:9", with 1:7" stabilizing heavier bullets and 1:9" better for lighter ones.

Q2: How does twist rate affect accuracy?
A: Proper twist stabilizes bullets without over-spinning them. Mismatched twist/bullet can cause keyholing or excessive drift.

Q3: Can I use this for all bullet types?
A: The Greenhill formula works best for lead-core bullets. Modern long-range bullets may need Miller or other formulas.

Q4: What's the stability factor?
A: A value indicating bullet stability (1.3-2.0 is generally good). This calculator provides twist rate, not stability factor.

Q5: How does altitude affect twist requirements?
A: Thinner air at altitude requires faster twist rates for stability. Add 5-10% twist for high-altitude shooting.