This calculator utilizes the Miller Stability Formula, which is the industry standard for determining if a specific rifle barrel twist rate will stabilize a bullet of a given length, weight, and diameter. While caliber (.223 Rem / 5.56 NATO) is important, the physical length of the projectile is the primary factor dictating the required twist rate.
How to Interpret Your Results
The output provides a Gyroscopic Stability Factor (Sg). Here is what the numbers mean for your shooting accuracy:
Sg < 1.0 (Unstable): The bullet will tumble in flight. Expect "keyholing" on targets and zero accuracy. Do not use this combination.
1.0 < Sg < 1.4 (Marginally Stable): The bullet may fly straight in warm weather or high altitude but could become unstable in cold, dense air. Accuracy is compromised.
1.4 < Sg < 2.0 (Optimal): This is the "Sweet Spot". The bullet is perfectly stabilized for maximum accuracy and ballistic coefficient (BC) realization.
Sg > 2.0 (Over-Stabilized): Generally safe to shoot. While extreme over-stabilization can magnify bullet imbalances, modern match bullets handle high twist rates well. You may see a negligible loss in velocity.
Common .223/5.56 Twist Rates & Recommendations
Twist Rate
Common Usage
Optimized For (Approx.)
1:12
Varmint / Bolt Action
Light bullets (40gr – 52gr)
1:9
Standard Sporter
Mid-range bullets (55gr – 69gr)
1:8
Versatile / Modern
Wide range (55gr – 77gr)
1:7
Mil-Spec / Long Range
Heavy bullets (62gr – 85gr+)
Why Bullet Length Matters More Than Weight
While most ammo boxes list bullet weight (e.g., 55 grain, 62 grain), the length of the bullet is what actually resists the gyroscopic forces. A 62-grain lead core bullet is shorter than a 62-grain solid copper bullet or a tracer round. Therefore, the copper or tracer round requires a faster twist rate (lower number, e.g., 1:7) to stabilize, even though the weight is the same.
Use this tool to ensure your barrel twist (typically 1:7, 1:8, or 1:9 for AR-15 platforms) is sufficient for your chosen projectile before heading to the range.
function calculateStability() {
// 1. Get Inputs
var diamStr = document.getElementById('bulletDiameter').value;
var weightStr = document.getElementById('bulletWeight').value;
var lengthStr = document.getElementById('bulletLength').value;
var twistStr = document.getElementById('barrelTwist').value;
// 2. Validate Inputs
if (!diamStr || !weightStr || !lengthStr || !twistStr) {
alert("Please fill in all fields (Diameter, Weight, Length, and Twist Rate).");
return;
}
var d = parseFloat(diamStr); // Diameter in inches
var m = parseFloat(weightStr); // Mass in grains
var l = parseFloat(lengthStr); // Length in inches
var t = parseFloat(twistStr); // Twist (1 turn in t inches)
if (d <= 0 || m <= 0 || l <= 0 || t <= 0) {
alert("Please enter positive values greater than zero.");
return;
}
// 3. Logic: Miller Twist Rule Formula
// Formula: Sg = (30 * m) / (t^2 * d^3 * (l/d) * (1 + (l/d)^2))
var l_cal = l / d; // Length in calibers
// Breaking down the denominator for safety
var term1 = Math.pow(t, 2);
var term2 = Math.pow(d, 3);
var term3 = l_cal * (1 + Math.pow(l_cal, 2));
var sg = (30 * m) / (term1 * term2 * term3);
// 4. Update UI
var resultBox = document.getElementById('resultBox');
var sgValueEl = document.getElementById('sgValue');
var sgStatusEl = document.getElementById('sgStatus');
var indicator = document.getElementById('statusIndicator');
resultBox.style.display = "block";
sgValueEl.innerHTML = sg.toFixed(2);
// Determine Status and Color
var color = "#333";
var statusText = "";
if (sg = 1.0 && sg = 1.4 && sg 100) percentage = 100;
if (percentage < 0) percentage = 0;
indicator.style.left = percentage + "%";
}