1. What is the Greenhill Formula?

The Greenhill formula is a mathematical approximation for calculating the optimal twist rate for stabilizing a bullet of given dimensions. It's widely used in firearms and ballistics to determine the proper rifling twist for a barrel.

2. How Does the Calculator Work?

The calculator uses the Greenhill formula:

Where:

• \( d \) — Bullet diameter (inches)
• \( l \) — Bullet length (inches)
• 150 — Empirical constant for standard conditions

Explanation: The formula calculates the required twist rate (in inches per turn) needed to stabilize a bullet of given dimensions. The result indicates how many inches of barrel length are required for one complete rotation of the bullet.

3. Importance of Twist Rate Calculation

Details: Proper twist rate is crucial for bullet stability and accuracy. Too slow twist may fail to stabilize the bullet, while too fast twist can cause excessive wear on the barrel and may over-stabilize the bullet.

4. Using the Calculator

Tips: Enter bullet diameter and length in inches. Both values must be positive numbers. For best results, use precise measurements of your specific bullet.

5. Frequently Asked Questions (FAQ)

Q1: What is the 150 constant in the formula?
A: The 150 is an empirical constant that works well for most standard conditions. For higher velocity rounds (over 2800 fps), some use 180 instead.

Q2: What are typical twist rates for common calibers?
A: For example: .223 Remington often uses 1:7" to 1:9", .308 Winchester typically 1:10", and 9mm Luger usually 1:10".

Q3: When should I use a different formula?
A: For very long, heavy bullets or extreme velocities, more complex formulas like the Miller twist rule might be more accurate.

Q4: Does this work for all bullet shapes?
A: The formula works best for traditional bullet shapes. Very unconventional designs may require different calculations.

Q5: How precise does this need to be?
A: For most practical purposes, rounding to the nearest whole number is sufficient (e.g., 1:10" twist).