Rifle Twist Rate Calculator

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Rifle Twist Rate & Stability Calculator

Standard caliber (e.g., .224, .264, .308)
Total mass of the projectile
Tip to base (not including cartridge)
Distance for 1 full revolution
Standard correction at 2800 fps
Gyroscopic Stability Factor (SG)

Understanding Rifle Twist Rate and Stability

A rifle's twist rate is the distance the rifling takes to complete one full revolution inside the barrel. This rotation is what stabilizes the bullet in flight. If the twist is too slow for the bullet's length, the bullet will tumble (unstable). If it is too fast, you may experience excessive centrifugal force, though "over-stabilization" is rarely a problem compared to instability.

The Miller Stability Formula

This calculator uses the Miller Stability Formula, which is the industry standard for determining if a specific bullet will fly straight from your barrel. The key factors are Bullet Length and Twist Rate. Interestingly, length affects stability far more than weight does.

SG Value Stability Status
Below 1.0 Unstable
1.0 to 1.3 Marginally Stable
1.3 to 2.0 Stable (Optimal)
Above 2.0 Over-Stabilized

Common Twist Rate Examples

  • .223 Remington: 1:7″ or 1:8″ for heavy 77gr bullets; 1:12″ for light 40-55gr bullets.
  • .308 Winchester: 1:10″ is standard for 168gr to 175gr projectiles.
  • 6.5 Creedmoor: 1:8″ is common to stabilize long, high-BC bullets.
function calculateStability() { var diameter = parseFloat(document.getElementById('bulletDiameter').value); var weight = parseFloat(document.getElementById('bulletWeight').value); var length = parseFloat(document.getElementById('bulletLength').value); var twist = parseFloat(document.getElementById('twistRate').value); var velocity = parseFloat(document.getElementById('muzzleVelocity').value); var display = document.getElementById('resultDisplay'); var sgValueEl = document.getElementById('sgValue'); var sgStatusEl = document.getElementById('sgStatus'); if (!diameter || !weight || !length || !twist || !velocity) { alert('Please fill in all fields with valid numbers.'); return; } // Miller Formula Logic // SG = (30 * m) / (t^2 * d^3 * l * (1 + l^2)) // m = weight in grains // d = diameter in inches // l = length in calibers (L / d) // t = twist in calibers (T / d) var lCalibers = length / diameter; var tCalibers = twist / diameter; var denominator = Math.pow(tCalibers, 2) * Math.pow(diameter, 3) * lCalibers * (1 + Math.pow(lCalibers, 2)); var sg = (30 * weight) / denominator; // Velocity adjustment (Miller uses 2800 fps as base) var velCorrection = Math.pow(velocity / 2800, 1/3); var finalSG = sg * velCorrection; display.style.display = 'block'; sgValueEl.innerText = finalSG.toFixed(3); if (finalSG = 1.0 && finalSG = 1.3 && finalSG <= 2.0) { sgStatusEl.innerText = 'STABLE (Optimal)'; sgStatusEl.style.backgroundColor = '#e8f5e9'; sgStatusEl.style.color = '#2e7d32'; display.style.backgroundColor = '#f1fbf1'; } else { sgStatusEl.innerText = 'OVER-STABILIZED'; sgStatusEl.style.backgroundColor = '#e3f2fd'; sgStatusEl.style.color = '#1565c0'; display.style.backgroundColor = '#f0f7ff'; } }

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