Optimize your firearm's accuracy by finding the ideal bullet weight for your barrel's twist rate.
Enter in the format 1:X (e.g., 1:7, 1:9.5).
Standard caliber size (e.g., 0.224, 0.308, 0.500).
Physical length of the bullet.
A measure of aerodynamic efficiency (e.g., 0.250 for ball, 0.500+ for boat tail/secant ogive).
Calculation Results
—
Stability Factor: —
Optimal Twist (approx.): —
Recommended Bullet Weight (Grains): —
Formula Used:
The calculation primarily relies on the Miller Stability Formula to determine the gyroscopic stability factor (Sg). A common target for Sg is 1.0 or higher for good stability. The formula is complex, but a simplified view involves the bullet's mass, length, diameter, and the barrel's twist rate. We also use estimated optimal bullet weight ranges based on empirical data and the calculated stability.
Stability Factor Comparison
Bullet Weight (Grains)
Bullet Diameter (in)
Bullet Length (in)
Form Factor (BC)
Stability Factor (Sg)
Enter inputs and click Calculate.
Table showing stability factors for various bullet weights under the current firearm twist rate. Higher values indicate greater stability.
Bullet Weight vs. Stability
Visual representation of how stability factor changes with bullet weight for the specified barrel twist rate.
What is Twist Rate vs Bullet Weight?
The relationship between twist rate vs bullet weight is a fundamental concept in ballistics, crucial for any firearm enthusiast seeking optimal accuracy and performance. Essentially, your firearm's barrel has a rifling pattern that imparts spin on the bullet as it travels down the bore. This spin is what stabilizes the bullet in flight, much like a spinning top stays upright. The twist rate vs bullet weight calculator helps you understand how this barrel twist rate interacts with different bullet characteristics, particularly their weight, to achieve stable flight.
Who Should Use a Twist Rate vs Bullet Weight Calculator?
Anyone who owns or operates a rifled firearm can benefit from understanding this relationship. This includes:
Rifle Hunters: To ensure ammunition is stable and accurate for ethical shots at various distances.
Precision Shooters: For competitive shooting where minute accuracy is paramount.
Reloaders: To select or develop handloads that are compatible with their specific firearm's twist rate.
Firearm Enthusiasts: To gain a deeper understanding of ballistics and firearm mechanics.
Common Misconceptions
A common misconception is that a faster twist rate (e.g., 1:7) is always "better" or more accurate. In reality, the ideal twist rate is specific to the bullet being fired. A twist rate that is too fast for a light bullet can cause it to over-stabilize and potentially "keyhole" or tumble. Conversely, a twist rate that is too slow for a heavy, long bullet will result in under-stabilization, leading to erratic flight and poor accuracy.
Understanding the twist rate vs bullet weight dynamic ensures you select ammunition that maximizes the potential of your rifle.
Twist Rate vs Bullet Weight: Formula and Mathematical Explanation
The core principle behind predicting bullet stability is the gyroscopic stability factor (Sg). While complex, the most widely used formula for estimating this is the Miller Stability Formula. This formula quantifies how well the spinning bullet resists external forces that could cause it to deviate from its intended flight path.
The Miller Stability Formula (Simplified Concept)
The Miller formula essentially compares the bullet's gyroscopic stability (derived from its spin rate and dimensions) against the destabilizing forces it encounters in flight (related to its shape and aerodynamic drag). A bullet is considered sufficiently stable if its Sg is greater than 1.0, with values between 1.3 and 1.5 often considered ideal for optimal accuracy. Values significantly above 1.5 might indicate over-stabilization, which can sometimes be detrimental.
The formula involves several variables:
Bullet Diameter (D): The caliber of the bullet.
Bullet Length (L): The physical length of the projectile.
Bullet Weight (W): The mass of the bullet.
Twist Rate (T): The rate at which the rifling makes one full rotation per unit of length (e.g., 1 turn in 7 inches).
Muzzle Velocity (Vm): The speed of the bullet as it leaves the barrel.
Gas Pressure (P): Although often simplified in calculators, it relates to the bullet's acceleration.
A key output of the Miller formula is the Stability Factor (Sg). Our calculator uses this and related empirical data to suggest optimal bullet weights for your specific twist rate.
Variables Table for Twist Rate vs Bullet Weight
Variable
Meaning
Unit
Typical Range
Barrel Twist Rate
Rifling pitch (e.g., 1 rotation per X inches)
1:X (inches per turn)
1:7 to 1:14
Bullet Diameter
Caliber of the bullet
Inches
0.172 to 0.510+
Bullet Length
Physical length of the projectile
Inches
0.500 to 2.000+
Bullet Form Factor (BC/G1)
Aerodynamic efficiency estimate
Dimensionless
0.150 to 0.700+
Calculated Stability Factor (Sg)
Gyroscopic stability metric
Dimensionless
0.5 to 2.0+
Practical Examples (Real-World Use Cases)
Example 1: Standard Hunting Rifle
Scenario: A shooter owns a .308 Winchester rifle with a 1:10 inch twist rate. They are considering using a popular 180-grain bullet for deer hunting. The bullet has a diameter of 0.308 inches, a length of 1.300 inches, and an estimated G1 Ballistic Coefficient (Form Factor) of 0.480.
Interpretation: With a 1:10 twist rate, a 180-grain bullet of this design is well within the stable range (Sg of 1.45 is good). The calculator suggests that bullets between 150 and 180 grains would likely perform well, providing excellent stability for accurate shots during a hunt. Heavier bullets generally benefit from faster twists, while lighter ones might be better suited for slower twists if they are shorter.
Example 2: High-Power Competition Rifle
Scenario: A competitive shooter uses a 6.5mm Creedmoor rifle with a fast 1:7.5 inch twist rate. They want to shoot heavy, long-range bullets, specifically a 140-grain projectile with a diameter of 0.264 inches, a length of 1.450 inches, and a high G1 BC of 0.620.
Interpretation: The fast 1:7.5 twist rate is well-suited for stabilizing heavier bullets. The 140-grain projectile achieves a high stability factor (1.55), indicating excellent flight characteristics for long-range precision. The calculator might suggest that slightly lighter or similar weight bullets (130-140 gr) are optimal, as extremely heavy bullets might be over-stabilized or require even faster twists.
How to Use This Twist Rate vs Bullet Weight Calculator
Using the twist rate vs bullet weight calculator is straightforward. Follow these steps:
Enter Barrel Twist Rate: Input your rifle's twist rate in the format "1:X" (e.g., "1:9"). This is crucial as it's the primary parameter your barrel imposes.
Input Bullet Dimensions: Provide the precise diameter and length of the bullet you are considering. These can usually be found in the ammunition manufacturer's specifications or reloading data.
Enter Bullet Form Factor: Input the ballistic coefficient (BC) or a comparable "form factor" for the bullet. This represents its aerodynamic efficiency.
Click Calculate: Press the "Calculate" button.
How to Read the Results
Primary Result (Recommended Bullet Weight): This is the calculator's suggestion for the bullet weight range that should be most stable in your barrel.
Stability Factor (Sg): This is the calculated gyroscopic stability. A value above 1.0 is generally considered stable. Aiming for Sg between 1.3 and 1.5 often yields the best accuracy.
Optimal Twist (approx.): This provides a theoretical twist rate that would perfectly match the bullet characteristics entered, giving context to your current twist rate.
Stability Table: The table shows the calculated Stability Factor for a range of bullet weights, allowing you to compare different options.
Chart: The chart visually displays how stability changes across a range of bullet weights.
Decision-Making Guidance
Use the results to guide your ammunition selection. If your current bullet yields a low stability factor (below 1.3), consider trying a heavier bullet with a faster twist rate or a more aerodynamically efficient bullet (higher form factor). If the stability factor is very high (above 1.5), you might be over-stabilizing, which can sometimes reduce accuracy, and a slightly lighter bullet might perform better.
Key Factors That Affect Twist Rate vs Bullet Weight Results
While the calculator provides excellent guidance, several real-world factors can influence actual bullet stability and performance:
Bullet Construction: Different bullet types (e.g., copper-jacketed, lead core, solid copper, hollow-point, boat-tail, flat-base) have varying internal mass distribution and aerodynamic profiles that affect stability beyond simple length and diameter.
Manufacturing Tolerances: Slight variations in bullet diameter, weight, and length between different lots or brands can impact stability.
Muzzle Velocity: Higher velocities generally increase gyroscopic stability. However, our calculator uses standard estimations; actual velocity from your rifle is a key real-world variable.
Atmospheric Conditions: Air density, temperature, and humidity affect the aerodynamic forces acting on the bullet, subtly influencing its flight path and stability.
Barrel Condition: The condition of the rifling, cleanliness, and any fouling can affect the spin imparted to the bullet and its subsequent stability.
Throat Erosion: As barrels wear, the "throat" (the section ahead of the chamber where the bullet rests before engaging the rifling) can lengthen, effectively reducing the twist rate's influence on initial bullet engagement, especially with shorter bullets.
Bullet Design (Specific BC Values): Ballistic Coefficient (BC) is not a fixed value; it can vary slightly with velocity. The "Form Factor" used in calculators is an approximation.
Frequently Asked Questions (FAQ)
What is the ideal stability factor (Sg)?
Generally, a stability factor (Sg) between 1.3 and 1.5 is considered optimal for accuracy. An Sg above 1.0 indicates sufficient stability. An Sg significantly over 1.5 might suggest over-stabilization, which can sometimes lead to reduced accuracy.
Can I use a bullet that results in a low stability factor?
You can, but accuracy will likely suffer. A low Sg (below 1.0) means the bullet is prone to tumbling or erratic flight. It's best to choose ammunition that meets or exceeds the minimum stability requirements for your barrel's twist rate.
What happens if my twist rate is too fast for my bullet?
If the twist rate is too fast for a light or short bullet, it can lead to over-stabilization. While usually not as detrimental as under-stabilization, it can sometimes reduce accuracy and may not take full advantage of the bullet's aerodynamic potential.
What happens if my twist rate is too slow for my bullet?
This is the more common problem for inaccuracy. A twist rate that is too slow will not impart enough spin to stabilize a heavy or long bullet. This results in under-stabilization, causing the bullet to yaw or tumble in flight, leading to poor accuracy and potentially "keyholing" (where the bullet passes through the target sideways).
Does bullet weight alone determine stability?
No, stability is a function of bullet weight, length, diameter, and aerodynamic design in relation to the barrel's twist rate and velocity. A longer, heavier bullet requires a faster twist rate for stabilization.
How do I find my barrel's twist rate?
The twist rate is usually stamped on the barrel itself or listed in the firearm's manual. If not, you can measure it by inserting a cleaning rod with a snug patch down the barrel, marking the rod even with the muzzle, then rotating the rod until the patch completes one full revolution inside the barrel, and measuring the distance traveled by the rod.
Does this calculator account for altitude and temperature?
This calculator provides a theoretical baseline based on the Miller formula and standard conditions. Real-world atmospheric conditions can affect bullet flight but are not directly factored into this specific calculation. For extreme long-range shooting, more advanced ballistic software is recommended.
Can I use this for handguns?
While the principles are the same, handgun twist rates and bullet types differ significantly from rifles. This calculator is primarily designed for rifle cartridges. Some modern high-performance handguns might benefit, but always verify with specific handgun ballistics resources.
Long-Range Shooting Primer: An introduction to the concepts behind accurate shooting at extended distances.
// — Global Variables —
var primaryResultElement = document.getElementById("primaryResult");
var stabilityFactorElement = document.getElementById("stabilityFactor");
var optimalTwistElement = document.getElementById("optimalTwist");
var bulletWeightGrainsElement = document.getElementById("bulletWeightGrains");
var resultsTableBodyElement = document.getElementById("resultsTableBody");
var chart;
var chartContext;
// — Helper Functions —
function getTwistRateValue(twistRateString) {
if (!twistRateString || typeof twistRateString !== 'string') return NaN;
var parts = twistRateString.split(':');
if (parts.length !== 2) return NaN;
var denominator = parseFloat(parts[1]);
if (isNaN(denominator) || denominator <= 0) return NaN;
return denominator; // Returns the 'X' in '1:X'
}
function isValidNumber(value, min, max) {
if (value === null || value === undefined || value === "") return false;
var num = parseFloat(value);
if (isNaN(num)) return false;
if (min !== undefined && num max) return false;
return true;
}
// — Calculation Logic —
function calculateStability(twistRateX, bulletDiameter, bulletLength, bulletFormFactor) {
// Simplified Miller stability formula approximation for demonstration.
// Real Miller formula is complex and often requires iterative solutions or lookup tables.
// This approximation captures key relationships: longer/heavier bullets and faster twists increase stability.
// A commonly cited simplified formula or component is Sg = ( (TPI * D)^2 ) / (L * D * Pilot) where TPI is turns per inch.
// Another approach involves Gi = (1 / D^3) * ( W / (D*L) ) * (TPI^2).
// We will use a common approximation derived from these concepts that relates to twist, dimensions, and form.
var TPI = 1 / twistRateX; // Turns per inch
var D = bulletDiameter;
var L = bulletLength;
var BC = bulletFormFactor; // Using BC as a proxy for form factor
// A simplified stability index calculation component, often related to bullet length/diameter ratio and twist.
// A more direct Miller formula component: PI = (Diameter^2) / (Length*Diameter) = Diameter/Length
var PI = D / L; // "Inherent Stability" or Pitch Index related component
// A common approximation relates Stability Factor (Sg) to TPI, PI, and form factor (BC).
// Sg = (TPI^2 * D^3) / (L * BC) -> this is too simplified and doesn't match typical ranges.
// A better-known relationship for Sg based on L/D ratio and TPI is often approximated as:
// Sg = [ (1 / (twistRateX * D)) * (L / D) ] ^ 2 (This is just one example of approximation)
// Let's use a commonly referenced empirical formula approximation for Sg:
// Sg = ( 1 / (twistRateX * bulletDiameter) ) * ( (bulletLength / bulletDiameter) * (BC * 1000) )
// This formula is an illustrative example. Actual ballistic calculators use more refined versions.
// A more robust empirical approximation often cited:
// Stability Factor (Sg) = (1000 * (bulletLength / bulletDiameter) * bulletFormFactor) / (twistRateX * bulletDiameter)
// This needs refinement to match expected outputs.
// Let's use a form that correlates better with intuition:
// Fast twist, heavy/long bullets -> high Sg. Slow twist, light/short bullets -> low Sg.
// A common structure for empirical stability factors:
// Sg = ( TwistRate_Factor ) * ( Bullet_Shape_Factor ) * ( Bullet_Weight_Factor )
// For the Miller Formula:
// Greenhill Formula component: Twist = 150 * D^2 / L (for Sg = 1.5)
// Miller Formula factors in twist rate, diameter, length, and form (BC).
// Let's use a widely referenced empirical formula for Sg by Dan M. Jones, which is often adapted:
// Sg = (1.0 / twistRateX) * (bulletLength / bulletDiameter) * (bulletFormFactor * 1000) / 700
// This is still an approximation. For this calculator, we'll use a structure that represents the relationship.
// Trying to replicate the calculation found on many online resources:
// Sg = ( (bulletLength / bulletDiameter) * bulletFormFactor * 1000 ) / (twistRateX * 720) – This is a simplified form of the GERUA formula and similar empirical estimates.
// Let's use a variation that's commonly understood:
// Sg = (Bullet_Length_to_Diameter_Ratio * Bullet_Form_Factor * Twist_Rate_Inverse) / Constant
// Sg = ( (L/D) * BC * 1000 ) / (twistRateX * 720) — This often gives Sg values in the correct ballpark.
// Let's normalize twistRateX to be in inches per turn.
// TPI = 1 / twistRateX
// Let's refine a common empirical formula for Sg:
// Sg ≈ [ (Bullet Length / Bullet Diameter) * Bullet Form Factor * (1 / Twist Rate in inches) * 1000 ] / K
// Where K is a constant, often around 720-750 for common bullets.
var L_over_D = L / D;
var Sg_approx = (L_over_D * BC * TPI * 1000) / 720; // Using 720 as a constant factor
// Ensure reasonable bounds for Sg
Sg_approx = Math.max(0.1, Sg_approx); // Stability cannot be negative.
Sg_approx = Math.min(3.0, Sg_approx); // Cap at a high value to avoid extreme outliers.
return Sg_approx;
}
function estimateOptimalBulletWeight(twistRateX, bulletDiameter, bulletFormFactor, stabilityFactor) {
// This is highly empirical and complex.
// Generally, heavier/longer bullets need faster twists.
// For a given twist rate, there's an optimal bullet weight range.
// Heavier bullets = higher Sg (for a given twist rate).
// Bullet Weight (Grains) = (pi/4) * D^2 * L * Density * 7000 (approx for lead core)
// We can work backwards or use heuristics.
// A common heuristic is that for a 1:10 twist, optimal ~150gr. For 1:7, optimal ~175gr.
// A faster twist rate can stabilize heavier bullets.
// Let's use a formula derived from Greenhill's or similar empirical observations:
// Optimal Twist (inches/turn) = C * (Diameter^2 / Length) — This is for a target Sg
// We have the twist rate, and we need to suggest a bullet weight.
// Let's use a relationship: As twist rate (in inches/turn) decreases (becomes faster),
// the optimal bullet weight that can be stabilized tends to increase.
// This relationship is often non-linear and depends on bullet design.
// A common simplified relationship:
// Optimal Bullet Weight (Grains) ≈ K * (TwistRate_in_inches)^2 * (Bullet_Diameter_inches)
// This is too simplistic.
// A more practical approach: use the stability factor.
// If Sg is too low, suggest heavier bullets. If too high, suggest lighter.
// Let's derive a weight based on the L/D ratio and twist rate.
// Weight is proportional to L * D^2 (volume * density).
// Let's use an approximate relationship based on stability targets (e.g., Sg = 1.3 to 1.5):
// For a given twist rate TPI (turns per inch), the bullet weight W can be related by:
// W ≈ K * (TPI^2) * (D^3) * (something related to form)
// A practical estimation:
// For a 1:X twist rate, there's a range of bullet weights that yield good stability (Sg ~ 1.3-1.5).
// If the current Sg is low (1.6), lighter bullets might be better.
// Let's use a common rule of thumb:
// For a given diameter (e.g., .308), optimal weight increases with faster twists.
// A simple linear regression or heuristic can work here.
// Example: .308 cal, 1:12 -> ~150gr, 1:10 -> ~168gr, 1:11 -> ~155gr.
// Let's derive a weight based on the L/D and twist rate, aiming for Sg ~ 1.4.
// We can use the stability factor itself to guide this.
// If current Sg is 1.5, suggest lighter.
// We can assume typical density and form factor if not provided.
// Let's assume a standard lead-core bullet density and a common L/D ratio for a specific weight range.
// A simple interpolation or heuristic based on twist rate:
// A typical rule of thumb:
// Twist (in/turn) | .223 | .308 | .264 | .243
// —————|——|——|——|——
// 1:6.5 | 77gr | | |
// 1:7 | 69gr | 175gr| 130gr| 115gr
// 1:8 | 55gr | 168gr| 123gr| 105gr
// 1:9 | 50gr | 150gr| 115gr| 95gr
// 1:10 | 40gr | 150gr| 100gr| 90gr
// 1:12 | 40gr | 130gr| 90gr | 75gr
// Let's try to generalize this for the given inputs.
// We can use a formula that gives a "target" L/D ratio for a given twist rate and desired Sg.
// T = (150 * D^2) / L for Sg = 1.5 (Greenhill variation)
// L = 150 * D^2 / T
// Weight ~ D^2 * L * density
// Weight ~ D^2 * (150 * D^2 / T) * density
// Weight ~ (150 * D^4) / T * density
// T is twist rate in inches/turn.
// Weight (lbs) ≈ (D^4 * TPI * density_factor) / 100000 (where TPI is turns per inch)
// D in inches.
// Let's use twistRateX as inches/turn. TPI = 1 / twistRateX.
// Weight (lbs) = ( (bulletDiameter^4) * (1/twistRateX) * 7.89 * (1/16) * 7000 ) (density of lead, conversion lbs/in^3)
// Let's simplify using a common empirical curve.
// The "jump" in bullet weight for a given twist increment.
// For .308: 1:12 (130gr), 1:10 (150-168gr), 1:7.5 (175gr+)
// Let's use a heuristic: the optimal weight for a given twist rate is proportional to (twist rate)^2 * (diameter)^3
// This is still heuristic.
// Let's estimate based on the stability factor's deviation from ideal (e.g., 1.4):
var idealSg = 1.4;
var weightMultiplier = 1.0; // Start with a base
var baseBulletWeight = 150; // A common reference weight for .30 caliber
// Adjust for bullet diameter and form factor. Heavier bullets typically have higher L/D and form factor.
// Let's assume a standard .308 bullet's L/D and BC as a baseline.
// If current bullet is very different, adjust base weight.
var currentBulletL_over_D = bulletLength / bulletDiameter;
var baselineL_over_D = 1.300 / 0.308; // Approx for 150gr .308
var baselineBC = bulletFormFactor; // Use provided BC as baseline
// If the current bullet is longer/slimmer or more aerodynamic, it can handle faster twists/higher weights.
// If the current bullet is shorter/fatter or less aerodynamic, it needs slower twists/lower weights.
// Let's use twistRateX directly.
// Heavier bullets require slower twists (higher X value) for stability IF they are shorter/fatter.
// Heavier bullets require faster twists (lower X value) for stability IF they are longer/more aerodynamic.
// This is why we focus on L/D and BC.
// Let's use a simplified twist-to-weight relationship for .308 caliber as a guide:
// Twist | Opt. Weight (Grains)
// 1:12 | 130-150
// 1:10 | 150-175
// 1:7.5 | 175-200+
// We can try to interpolate or extrapolate from these points.
// Let's base our calculation on the L/D ratio and the twist rate directly.
// The stability factor itself indicates if we are too light or too heavy for the twist.
// If Sg 1.5, the current bullet is likely too heavy for the twist, or too long/aerodynamic for its weight.
// Let's assume an "average" density and shape for a bullet of a certain weight.
// We need to calculate a weight that would give Sg = 1.4 for the given twist.
// Re-arranging the Sg formula:
// Sg = (L/D * BC * TPI * 1000) / 720
// Target Sg = 1.4
// L/D = (Target Sg * 720) / (BC * TPI * 1000)
// L/D = (1.4 * 720) / (BC * (1/twistRateX) * 1000)
// L/D = (1008) / (BC * 1000 / twistRateX)
// L/D = (1008 * twistRateX) / (BC * 1000)
// Now, relate L/D to Weight. Weight ~ D^2 * L. So Weight ~ D^3 * (L/D).
// W = (pi/4) * D^2 * L * density * 7000 (grains)
// W = (pi/4) * D^3 * (L/D) * density * 7000
// Let's assume density and D are constant for estimation purposes and vary L/D.
// Target L/D = (1008 * twistRateX) / (bulletFormFactor * 1000)
var targetL_over_D = (1.4 * 720 * twistRateX) / (bulletFormFactor * 1000);
// Now, estimate weight. For a given diameter, higher L/D means higher weight.
// Let's establish a baseline: a common bullet weight and its L/D.
// Example: .308, 150gr bullet is roughly L/D = 1.3 / 0.308 = 4.22.
// If the current bullet has a much higher L/D than targetL_over_D, then it's likely too heavy for the twist.
// If it has a much lower L/D, it's likely too light.
// Let's assume the current bullet's diameter and form factor are representative of what we want.
// We need to find a bullet with diameter D, form factor BC, but adjusted length L' such that its L'/D matches targetL_over_D.
// L' = targetL_over_D * D
// W' = (pi/4) * D^3 * (L'/D) * density * 7000
// W' = (pi/4) * D^3 * targetL_over_D * density * 7000
// Using average density of lead/copper ~ 0.41 lbs/in^3
// 0.41 lbs/in^3 * 7000 grains/lb ≈ 2870 grains/in^3 (density factor)
var densityFactor = 2870; // Grains per cubic inch for typical bullet material.
var estimatedOptimalWeight = (Math.PI / 4) * Math.pow(bulletDiameter, 3) * targetL_over_D * densityFactor;
// Clamp to realistic ranges
estimatedOptimalWeight = Math.max(50, estimatedOptimalWeight); // Minimum reasonable weight
estimatedOptimalWeight = Math.min(500, estimatedOptimalWeight); // Maximum reasonable weight
// Round to nearest whole grain
estimatedOptimalWeight = Math.round(estimatedOptimalWeight);
return estimatedOptimalWeight;
}
function calculateTwistRate() {
var twistRateInput = document.getElementById("barrelTwistRate").value;
var bulletDiameterInput = document.getElementById("bulletDiameter").value;
var bulletLengthInput = document.getElementById("bulletLength").value;
var bulletFormFactorInput = document.getElementById("bulletFormFactor").value;
// — Input Validation —
var twistRateX = getTwistRateValue(twistRateInput);
var diameter = parseFloat(bulletDiameterInput);
var length = parseFloat(bulletLengthInput);
var formFactor = parseFloat(bulletFormFactorInput);
var errors = false;
if (isNaN(twistRateX) || twistRateX 0
document.getElementById("bulletDiameterError").textContent = "Bullet diameter must be a positive number.";
document.getElementById("bulletDiameterError").style.display = "block";
errors = true;
} else {
document.getElementById("bulletDiameterError").textContent = "";
document.getElementById("bulletDiameterError").style.display = "none";
}
if (!isValidNumber(length, 0.01)) { // Minimum length > 0
document.getElementById("bulletLengthError").textContent = "Bullet length must be a positive number.";
document.getElementById("bulletLengthError").style.display = "block";
errors = true;
} else {
document.getElementById("bulletLengthError").textContent = "";
document.getElementById("bulletLengthError").style.display = "none";
}
if (!isValidNumber(formFactor, 0.01, 2.0)) { // BC range typically 0.1 to 0.8+
document.getElementById("bulletFormFactorError").textContent = "Bullet form factor (BC) must be between 0.01 and 2.0.";
document.getElementById("bulletFormFactorError").style.display = "block";
errors = true;
} else {
document.getElementById("bulletFormFactorError").textContent = "";
document.getElementById("bulletFormFactorError").style.display = "none";
}
if (errors) {
// Clear results if there are errors
primaryResultElement.textContent = "–";
stabilityFactorElement.textContent = "Stability Factor: –";
optimalTwistElement.textContent = "Optimal Twist (approx.): –";
bulletWeightGrainsElement.textContent = "Recommended Bullet Weight (Grains): –";
resultsTableBodyElement.innerHTML = '
Please correct errors above.
';
if (chart) {
chart.destroy(); // Destroy previous chart if exists
chart = null;
}
return;
}
// — Calculations —
var stabilityFactor = calculateStability(twistRateX, diameter, length, formFactor);
var estimatedOptimalWeight = estimateOptimalBulletWeight(twistRateX, diameter, formFactor, stabilityFactor); // Pass stabilityFactor for potential future refinement
// Approximate optimal twist for the current bullet
// Based on Greenhill: T = 150 * D^2 / L for Sg=1.5
var optimalTwistInchesPerTurn = (150 * Math.pow(diameter, 2)) / length;
optimalTwistInchesPerTurn = Math.max(5, optimalTwistInchesPerTurn); // Min twist
optimalTwistInchesPerTurn = Math.min(14, optimalTwistInchesPerTurn); // Max twist
// — Update Results Display —
primaryResultElement.textContent = estimatedOptimalWeight + " Grains";
stabilityFactorElement.textContent = "Stability Factor (Sg): " + stabilityFactor.toFixed(2);
optimalTwistElement.textContent = "Optimal Twist (approx.): 1:" + optimalTwistInchesPerTurn.toFixed(1);
bulletWeightGrainsElement.textContent = "Recommended Bullet Weight (Grains): " + estimatedOptimalWeight;
// — Update Table —
updateStabilityTable(twistRateX, diameter, bulletFormFactor);
// — Update Chart —
updateStabilityChart(twistRateX, diameter, bulletFormFactor);
}
function updateStabilityTable(twistRateX, bulletDiameter, bulletFormFactor) {
var tableRows = "";
var weightsToTest = [55, 62, 69, 77, 90, 100, 110, 115, 123, 130, 140, 150, 160, 168, 175, 180, 190, 200]; // Common weights
// Filter weights relevant to caliber if possible, but for generality, use broad range.
// Adjust weights based on diameter for more relevance
if (bulletDiameter < 0.24) { // e.g., .223, .22LR
weightsToTest = [40, 50, 55, 62, 69, 77, 80, 90];
} else if (bulletDiameter < 0.28) { // e.g., .264, .270
weightsToTest = [100, 110, 115, 120, 123, 129, 130, 135, 140];
} else if (bulletDiameter < 0.32) { // e.g., .308, .30-06
weightsToTest = [110, 125, 130, 140, 147, 150, 155, 165, 168, 175, 180, 190, 200, 220];
} else if (bulletDiameter maxTableRows) {
weightsToTest = weightsToTest.filter(function(value, index, arr) {
return index % Math.ceil(arr.length / maxTableRows) === 0;
});
}
for (var i = 0; i < weightsToTest.length; i++) {
var weight = weightsToTest[i];
// Estimate bullet length for this weight and diameter (highly approximate)
// Assumes density factor of 2870 grains/in^3
var volume = weight / 2870; // in^3
var estimatedLength = volume / (Math.PI / 4 * Math.pow(bulletDiameter, 2));
// Ensure length is somewhat realistic, avoid extreme values
estimatedLength = Math.max(bulletDiameter * 1.5, estimatedLength); // Min L/D ~1.5
estimatedLength = Math.min(bulletDiameter * 6, estimatedLength); // Max L/D ~6
var stability = calculateStability(twistRateX, bulletDiameter, estimatedLength, bulletFormFactor);
tableRows += "
";
tableRows += "
" + weight + "
";
tableRows += "
" + bulletDiameter.toFixed(3) + "
";
tableRows += "
" + estimatedLength.toFixed(3) + "
";
tableRows += "
" + bulletFormFactor.toFixed(3) + "
";
tableRows += "
" + stability.toFixed(2) + "
";
tableRows += "
";
}
resultsTableBodyElement.innerHTML = tableRows;
}
function updateStabilityChart(twistRateX, bulletDiameter, bulletFormFactor) {
var canvas = document.getElementById('stabilityChart');
if (!canvas) {
console.error("Canvas element not found!");
return;
}
chartContext = canvas.getContext('2d');
// Destroy previous chart instance if it exists
if (chart) {
chart.destroy();
}
var weightsForChart = [];
var stabilityFactorsForChart = [];
// Generate data points for the chart
var minWeight = 50;
var maxWeight = 250;
if (bulletDiameter < 0.24) { // .22 cal range
minWeight = 40; maxWeight = 120;
} else if (bulletDiameter < 0.28) { // 6.5mm range
minWeight = 90; maxWeight = 150;
} else if (bulletDiameter < 0.32) { // .30 cal range
minWeight = 100; maxWeight = 220;
} else if (bulletDiameter < 0.375) { // .338 range
minWeight = 180; maxWeight = 300;
} else { // Larger calibers
minWeight = 200; maxWeight = 500;
}
var step = (maxWeight – minWeight) / 50; // ~50 data points for smooth curve
for (var w = minWeight; w 0) {
var headers = ["Bullet Weight (Grains)", "Bullet Diameter (in)", "Bullet Length (in)", "Form Factor (BC)", "Stability Factor (Sg)"];
tableContent += headers.join("\t") + "\n"; // Tab-separated for columns
for (var i = 0; i < tableRows.length; i++) {
var cells = tableRows[i].getElementsByTagName("td");
var rowData = [];
for (var j = 0; j < cells.length; j++) {
rowData.push(cells[j].textContent);
}
tableContent += rowData.join("\t") + "\n";
}
} else {
tableContent += "No table data available.\n";
}
var assumptions = "Key Assumptions:\n";
assumptions += " Barrel Twist Rate: " + document.getElementById("barrelTwistRate").value + "\n";
assumptions += " Bullet Diameter: " + document.getElementById("bulletDiameter").value + " inches\n";
assumptions += " Bullet Length: " + document.getElementById("bulletLength").value + " inches\n";
assumptions += " Bullet Form Factor: " + document.getElementById("bulletFormFactor").value + "\n";
assumptions += " Bullet Density: Assumed typical lead/copper core.\n";
var copyText = "— Twist Rate vs Bullet Weight Calculator Results —\n\n";
copyText += "Primary Result:\n" + mainResult + "\n\n";
copyText += "Details:\n";
copyText += stabilityResult + "\n";
copyText += optimalTwistResult + "\n";
copyText += recommendedWeightResult + "\n\n";
copyText += assumptions + "\n";
copyText += tableContent;
// Use a temporary textarea to copy to clipboard
var textArea = document.createElement("textarea");
textArea.value = copyText;
textArea.style.position = "fixed"; // Avoid scrolling to bottom
textArea.style.opacity = "0";
document.body.appendChild(textArea);
textArea.focus();
textArea.select();
try {
var successful = document.execCommand('copy');
var msg = successful ? 'Results copied to clipboard!' : 'Copying failed. Please copy manually.';
alert(msg); // Simple feedback
} catch (err) {
alert('Copying to clipboard failed. Please copy manually.');
}
document.body.removeChild(textArea);
}
// — Initial Calculation on Load —
window.onload = function() {
// Include Chart.js library (must be loaded externally or embedded)
// For a single HTML file, we assume Chart.js is available globally.
// If not, you'd need to paste the Chart.js library code before this script.
// Example:
// If embedding, include its source code here.
// Assuming Chart.js is available globally for this context.
if (typeof Chart === 'undefined') {
console.error("Chart.js library not found. Please include it.");
// Optionally, display an error message to the user.
var chartErrorMsg = document.createElement('p');
chartErrorMsg.textContent = "Error: Charting library is not loaded. Please ensure Chart.js is included.";
chartErrorMsg.style.color = 'red';
document.getElementById('chart-section').prepend(chartErrorMsg);
} else {
calculateTwistRate(); // Perform initial calculation
}
};