Twist Rate Calculator – Calculate Bullet Stability
Want to know if your bullet is stable in your barrel? This free twist rate calculator determines the gyroscopic stability of your projectile in seconds. Enter your barrel data and bullet, and you instantly see whether your combination is stable, what the optimal twist rate would be and at what speed your bullet spins in RPM. The calculator includes a database of 394 bullets from Berger, Hornady, Sierra, Lapua, Nosler and Barnes.
This calculator is optimised for use on desktop and laptop. The mobile version is currently in development.
HOP's Twist Rate Calculator
Calculate the optimal twist rate for your barrel and determine the gyroscopic stability of your projectile using the Miller Twist Rule
Bullet Selection
inch
gr
inch
Rifle & Conditions
m/s
1: inch
°C
m
Results
Stability Factor (SG)
--
Gyroscopic Stability
Optimal Twist Rate
--
1: inch (for SG=1.5)
Minimum Twist Rate
--
1: inch (for SG=1.0)
Rotation Speed
--
RPM
Stable - Tight Group
Unstable - Keyholing
Stability Analysis
Unstable Marginal Stable Very Stable
Enter data and click "Calculate Stability"
Miller Twist Rule Formula (with velocity correction)
SG = (30 × m) / (t² × d³ × l × (1 + l²)) × (V/2800)^⅓ × (ρ₀ / ρ)
Where:
m = bullet weight (grains)
t = twist rate (calibers per turn)
d = caliber (inches)
l = bullet length / caliber
V = velocity (fps, ref. 2800 fps)
ρ = air density
Where:
m = bullet weight (grains)
t = twist rate (calibers per turn)
d = caliber (inches)
l = bullet length / caliber
V = velocity (fps, ref. 2800 fps)
ρ = air density