Question 1

What is gyroscopic stability factor (Sg)?

Accepted Answer

Sg is a number describing how stable your bullet is in flight. Below 1.0 means the bullet will tumble. 1.0–1.4 is marginal — it might fly straight at sea level but get unstable in colder/denser air. 1.4 and above is what you want for accuracy. Above 2.5 the bullet is over-stable, which is fine for accuracy but slightly inefficient.

Question 2

What twist rate do I need for [bullet]?

Accepted Answer

Plug in the bullet's weight, length, and diameter, then enter the twist rate from your barrel and target velocity. If the result is below 1.4, you need a faster (lower number) twist — like 1:8 instead of 1:9. Common choices: 1:8 for 6.5 Creedmoor heavy match bullets, 1:7 for .223 with 77gr+ bullets, 1:9 for standard .223 55–69gr loads.

Question 3

Why does temperature affect twist stability?

Accepted Answer

Cold, dense air requires more spin to keep a bullet stable than warm, thin air. Miller's formula corrects to standard atmosphere (59°F, sea level). If you shoot in extreme cold or at high altitude, the actual Sg differs slightly — but for most rifle scenarios the standard-atmosphere number is conservative enough.

Question 4

How do I measure bullet length?

Accepted Answer

Use a caliper to measure overall bullet length from base to tip. Bullet manufacturers like Berger, Hornady, and Sierra publish exact lengths in their reloading data — those numbers are more accurate than your caliper. Plastic-tipped bullets like Hornady ELD-X or A-Tip count the full length including the tip.

Question 5

What units does the calculator expect?

Accepted Answer

Bullet weight in grains, bullet length in inches, bullet diameter in inches (e.g. 0.264 for 6.5mm), twist rate as the rate-of-twist denominator (e.g. 8 for a 1:8 barrel), and muzzle velocity in feet per second.

Twist Rate Stability Calculator