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Calcimator

Barrel Twist Rate Calculator

Calculate optimal barrel twist rate from bullet length and diameter using the Greenhill formula. Accounts for bullet material density.

Inputs

in
in

Results

Recommended Twist (1:X)

11.5"

Greenhill C=15011.5"
Greenhill C=180 (>2800fps)13.8"
Length/Diameter Ratio4.03
Recommended Twist11.48
How to Use This Calculator
  1. Enter Bullet Length, Bullet Diameter, and Bullet Specific Gravity.
  2. Review the Recommended Twist (1:X) () result.
  3. Use Greenhill C=150 () and Greenhill C=180 (>2800fps) () to inform your decision.
  4. Use the chart to visualize the results and explore different scenarios by adjusting inputs.

How the result changes with Bullet Diameter

Bullet DiameterRecommended Twist (1:X)
0.236.3"
0.3716.8"
0.5536.2"
0.6957.9"

What each input means

Bullet Length
Total bullet length in inches.
Bullet Diameter
Bullet diameter (caliber) in inches.
Bullet Specific Gravity
Bullet material density. Lead core = 10.9, copper solid = 8.9, brass = 8.5.

What each result means

Recommended Twist (1:X)
1 turn per X inches of barrel

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    Bullet Length = 1.24, Bullet Diameter = 0.308, Bullet Specific Gravity = 10.9 = 3 input(s) provided
  2. Calculate Recommended Twist
    Recommended Twist
    11.5 = 11.5
  3. Calculate Greenhill C=150
    Greenhill C=150
    11.5 = 11.5
  4. Calculate Greenhill C=180
    Greenhill C=180
    13.8 = 13.8

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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