Sierra Twist Rate Calculator

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Sierra Twist Rate & Bullet Stability Calculator

Stability Factor (Sg): 0.00

How to Use the Sierra Twist Rate Calculator

Choosing the correct barrel twist rate is critical for long-range precision and bullet terminal performance. This calculator uses the Miller Stability Formula to determine the gyroscopic stability factor (Sg) of a bullet based on its physical characteristics and velocity.

Understanding the Results

  • Sg < 1.0: Unstable. The bullet will likely tumble or "keyhole" upon exiting the muzzle.
  • Sg 1.0 to 1.3: Marginally Stable. The bullet may stabilize in warm air but lose stability in cold, dense air. Accuracy is typically poor.
  • Sg 1.3 to 2.0: Optimally Stable. This is the "sweet spot" for most target shooters and hunters. Sierra often recommends an Sg of at least 1.3 for consistent groups.
  • Sg > 2.0: Over-stabilized. While accurate, excessive spin can amplify small bullet imbalances and cause "point-of-impact" shifts at extreme ranges.

Key Inputs Explained

To get an accurate calculation, you need precise measurements of your bullet. Do not guess the length; use a caliper to measure from the tip to the base (including the boat-tail).

Bullet Length: This is the most critical variable in the formula. Longer bullets require a faster twist rate (a lower number, like 1:7 vs 1:12) to stay stable.

Atmospheric Conditions: Cold air is denser than warm air. A bullet that is stable at 90°F in the summer might become unstable at 20°F in the winter. Our calculator accounts for temperature to ensure your load remains stable year-round.

Example Calculation

If you are shooting a .308 caliber bullet weighing 175 grains with a length of 1.24 inches out of a 1:10 twist barrel at 2,600 FPS, the stability factor is approximately 1.58. This is considered highly stable and ideal for precision long-range shooting.

function calculateStability() { var d = parseFloat(document.getElementById('bulletDiameter').value); var w = parseFloat(document.getElementById('bulletWeight').value); var l = parseFloat(document.getElementById('bulletLength').value); var t = parseFloat(document.getElementById('barrelTwist').value); var v = parseFloat(document.getElementById('muzzleVelocity').value); var temp = parseFloat(document.getElementById('airTemp').value); var resultDiv = document.getElementById('stabilityResult'); var sgSpan = document.getElementById('sgValue'); var sgDesc = document.getElementById('sgDescription'); var sgAdvice = document.getElementById('sgAdvice'); if (isNaN(d) || isNaN(w) || isNaN(l) || isNaN(t) || isNaN(v) || d <= 0 || t <= 0) { alert("Please enter valid positive numbers for all fields."); return; } // Miller Stability Formula // Sg = (30 * m) / (t^2 * d^3 * L * (1 + L^2)) // m = mass in grains // v = velocity in fps // d = diameter in inches // l = length in inches // t = twist in calibers per turn (Twist / d) var twistInCalibers = t / d; var lengthInCalibers = l / d; var sg = (30 * w) / (Math.pow(twistInCalibers, 2) * Math.pow(d, 3) * lengthInCalibers * (1 + Math.pow(lengthInCalibers, 2))); // Velocity correction (Miller uses 2800 fps as base) var velocityFactor = Math.pow(v / 2800, 1/3); sg = sg * velocityFactor; // Temperature correction (Approximate air density effect) // Standard temp is 59F. Stability increases as air density decreases (hotter air). var tempFactor = (temp + 460) / (59 + 460); sg = sg * tempFactor; resultDiv.style.display = 'block'; sgSpan.innerHTML = sg.toFixed(2); if (sg = 1.0 && sg = 1.3 && sg <= 2.0) { resultDiv.style.backgroundColor = '#e8f5e9'; sgDesc.style.color = '#2e7d32'; sgDesc.innerHTML = "OPTIMALLY STABLE"; sgAdvice.innerHTML = "Excellent stability for precision shooting. This is the ideal range for most Sierra bullets."; } else { resultDiv.style.backgroundColor = '#e3f2fd'; sgDesc.style.color = '#1565c0'; sgDesc.innerHTML = "OVER-STABILIZED"; sgAdvice.innerHTML = "The bullet is spinning very fast. While stable, any minor bullet imperfections may be magnified at long range."; } }

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