.twist-calc-container {
max-width: 800px;
margin: 0 auto;
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
background: #ffffff;
border: 1px solid #e0e0e0;
border-radius: 8px;
box-shadow: 0 4px 12px rgba(0,0,0,0.05);
padding: 2rem;
}
.twist-calc-header {
text-align: center;
margin-bottom: 2rem;
border-bottom: 2px solid #b31b1b; /* Hornady Red-ish */
padding-bottom: 1rem;
}
.twist-calc-header h2 {
margin: 0;
color: #333;
font-size: 24px;
}
.twist-calc-grid {
display: grid;
grid-template-columns: 1fr 1fr;
gap: 20px;
}
@media (max-width: 600px) {
.twist-calc-grid {
grid-template-columns: 1fr;
}
}
.form-group {
margin-bottom: 15px;
}
.form-group label {
display: block;
margin-bottom: 5px;
font-weight: 600;
font-size: 14px;
color: #444;
}
.form-group input {
width: 100%;
padding: 10px;
border: 1px solid #ccc;
border-radius: 4px;
font-size: 16px;
box-sizing: border-box;
}
.form-group .help-text {
font-size: 12px;
color: #666;
margin-top: 4px;
}
.calc-btn {
grid-column: 1 / -1;
background-color: #b31b1b;
color: white;
border: none;
padding: 15px;
font-size: 18px;
font-weight: bold;
border-radius: 4px;
cursor: pointer;
transition: background-color 0.2s;
width: 100%;
margin-top: 10px;
}
.calc-btn:hover {
background-color: #8a1515;
}
.results-section {
grid-column: 1 / -1;
margin-top: 25px;
padding: 20px;
background-color: #f9f9f9;
border-radius: 6px;
display: none; /* Hidden by default */
}
.result-row {
display: flex;
justify-content: space-between;
align-items: center;
margin-bottom: 15px;
padding-bottom: 15px;
border-bottom: 1px solid #ddd;
}
.result-row:last-child {
border-bottom: none;
margin-bottom: 0;
padding-bottom: 0;
}
.result-label {
font-weight: 600;
color: #333;
}
.result-value {
font-weight: 700;
font-size: 20px;
}
.status-badge {
display: inline-block;
padding: 5px 10px;
border-radius: 4px;
color: white;
font-size: 14px;
font-weight: bold;
}
.status-stable { background-color: #28a745; }
.status-marginal { background-color: #ffc107; color: #333; }
.status-unstable { background-color: #dc3545; }
.article-content {
margin-top: 40px;
line-height: 1.6;
color: #333;
}
.article-content h3 {
color: #b31b1b;
margin-top: 1.5rem;
}
.article-content p {
margin-bottom: 1rem;
}
.article-content ul {
margin-bottom: 1rem;
padding-left: 20px;
}
function calculateStability() {
// 1. Get Input Values
var diam = parseFloat(document.getElementById('bullet_diam').value);
var weight = parseFloat(document.getElementById('bullet_weight').value);
var length = parseFloat(document.getElementById('bullet_length').value);
var twist = parseFloat(document.getElementById('barrel_twist').value);
var velocity = parseFloat(document.getElementById('muzzle_velocity').value);
// 2. Validation
if (!diam || !weight || !length || !twist) {
alert("Please fill in all bullet dimension and twist fields to calculate.");
return;
}
if (!velocity) velocity = 2800; // Default if left empty
// 3. Calculation Logic (Miller Twist Rule)
// S = (30 * m) / (T^2 * d * l_cal * (1 + l_cal^2))
// Where T is inches/turn, but formula requires derivation correction.
// Let's use the specific implementation for T in inches:
// SG = 30 * weight / ( (twist^2) * diameter * (length/diameter) * (1 + (length/diameter)^2) )
// Wait, standard Miller derivation:
// SG = 30 * W / (T^2 * D^3 * L_cal * (1 + L_cal^2)) is incorrect for T in inches.
// Correct implementation for T in INCHES:
// L_cal (Length in calibers) = Length / Diameter
var l_cal = length / diam;
// Miller Formula Constant = 30
// Denominator components:
var t_sq = Math.pow(twist, 2);
var d_val = diam;
// The formula for T in inches is: SG = (30 * m) / (T^2 * d * l_cal * (1 + l_cal^2))
// Let's verify units.
// Actually, the most robust Miller implementation is:
// SG = (30 * Weight) / (Twist^2 * Diameter * L_cal * (1 + L_cal^2)) ??
// No, let's use the T calculation and reverse it.
// T = sqrt( (30 * m) / (SG * d^3 * l_cal * (1+l_cal^2)) ) *usually* assumes T in calibers.
// Let's perform the calculation treating T as inches directly via the corrected formula:
// SG = (30 * Weight) / (Twist^2 * Diameter * (Length/Diameter) * (1 + (Length/Diameter)^2))
// This is the version that works for standard imperial inputs.
var term1 = 30 * weight;
var term2 = Math.pow(twist, 2); // T^2
var term3 = diam; // d
var term4 = l_cal; // l/d
var term5 = 1 + Math.pow(l_cal, 2); // 1 + (l/d)^2
var sg_static = term1 / (term2 * term3 * term4 * term5);
// Velocity Correction
// SG_dynamic = SG_static * (V / 2800)^(1/3)
var velocity_factor = Math.pow((velocity / 2800), 0.3333);
var sg_final = sg_static * velocity_factor;
// 4. Calculate Optimal Twist for SG = 1.5
// Rearranging the formula for T:
// T^2 = (30 * Weight * velocity_factor) / (1.5 * Diameter * L_cal * (1 + L_cal^2))
var optimal_t_sq = (30 * weight * velocity_factor) / (1.5 * diam * l_cal * (1 + Math.pow(l_cal, 2)));
var optimal_twist = Math.sqrt(optimal_t_sq);
// 5. Update UI
var resultBox = document.getElementById('results_box');
var sgDisplay = document.getElementById('sg_result');
var statusBadge = document.getElementById('status_badge');
var optimalDisplay = document.getElementById('optimal_twist');
resultBox.style.display = 'block';
sgDisplay.innerHTML = sg_final.toFixed(2);
optimalDisplay.innerHTML = "1:" + optimal_twist.toFixed(1) + '"';
// Set Status
statusBadge.className = 'status-badge'; // reset
if (sg_final = 1.0 && sg_final < 1.4) {
statusBadge.innerHTML = "Marginal";
statusBadge.classList.add('status-marginal');
} else {
statusBadge.innerHTML = "Stable";
statusBadge.classList.add('status-stable');
}
}
Twist Rate Calculator (Miller Stability)
Based on the Miller Twist Rule for Ballistic Stability
Ex: .224, .264, .308, .338
Ex: 55, 140, 168, 200
Total length including polymer tip
Ex: 10 for a 1:10 twist barrel
Standard correction base is 2800 fps
Gyroscopic Stability (S.G.):
0.00
Stability Status:
Unknown
Optimal Twist (for S.G. 1.5):
1:00″
*Calculation uses the Miller Stability Formula. An S.G. of 1.4 or higher is recommended for consistent long-range accuracy.
Understanding Ballistic Stability
When selecting ammunition for your rifle, particularly modern high-BC bullets like the Hornady ELD-X or ELD-M, matching the bullet length to your barrel's twist rate is critical for accuracy. This Twist Rate Calculator utilizes the Don Miller Twist Rule, which is widely regarded as more accurate for modern boat-tail and polymer-tipped bullets than the older Greenhill formula.
How to Read Your Results
- S.G. < 1.0 (Unstable): The bullet will likely tumble immediately upon exiting the muzzle. It will not hit the target accurately.
- S.G. 1.0 – 1.4 (Marginally Stable): The bullet may fly point-forward, but accuracy will degrade at longer ranges or in colder, denser air.
- S.G. 1.4+ (Stable): The ideal range for precision shooting. A stability factor of 1.5 provides a safety buffer for various atmospheric conditions.
The Miller Formula Explained
The Miller formula calculates the Gyroscopic Stability factor (S.G.) based on the bullet's geometry and mass. While velocity plays a role, the physical dimensions of the bullet—specifically the length relative to the diameter—are the primary drivers of stability requirements.
Note: Long, sleek bullets with high Ballistic Coefficients (BC) require faster twist rates (lower numbers, e.g., 1:8″ vs 1:12″) to remain stable. If this calculator shows an "Unstable" result, you must either switch to a lighter/shorter bullet or use a barrel with a faster twist rate.