Bullet Stability Calculator

If you’ve ever wondered why some bullets fly straight while others tumble, the answer is stability. Bullet stability depends on weight, length, twist rate, speed, and air density. We built an online Bullet Stability Calculator so you can check this in seconds. It’s quick, accurate, and beginner-friendly.

What is Bullet Stability?

Bullet stability means how steady a bullet stays in flight. When a bullet is fired, it spins due to the barrel’s twist. This spin keeps it pointed forward. If the spin is too slow, the bullet wobbles. If it’s just right, it flies true.

This spin effect is called gyroscopic stability. Think of a spinning top. The faster it spins, the longer it stands. A bullet works the same way in the air.

The Formula for Bullet Stability

Our calculator uses the Miller Twist Rule. It’s one of the most trusted ways to predict bullet stability. The formula is:

Sg = (30 × m) ÷ (t² × d³ × l × (1 + l²))

Where:

• m = bullet mass in grains
• d = bullet diameter in inches
• L = bullet length in inches
• l = L ÷ d (length in calibers)
• T = twist rate in inches per turn
• t = T ÷ d (twist in calibers per turn)

Corrections:

• Velocity factor = (v ÷ 2800)^(1/3)
• Altitude factor = e^(3.158 × 10^-5 × h)

Final corrected Sg = Base Sg × velocity factor × altitude factor.

What is the Optimal Bullet Stability Factor?

The stability factor (Sg) shows how stable a bullet is.

• Sg < 1.0 → unstable, bullet may tumble.
• Sg between 1.0 and 1.4 → marginal, not good for accuracy.
• Sg between 1.4 and 2.5 → ideal stability, best for precision.
• Sg above 2.5 → over-stable, stable but may drift more.

For most shooters, an Sg around 1.5–2.0 is the sweet spot.

At What Speed Does a Bullet Become Unstable?

There’s no single number because it depends on twist, bullet size, and air density. But here’s the rule: if speed drops too low, spin slows, and Sg falls under 1.0. That’s when instability starts.

Example: A .308 bullet stable at 2800 fps may wobble if it slows below 1800 fps at long range.

How to Use the Bullet Stability Calculator Online

1. Enter bullet mass, length, diameter, and twist rate.
2. Add muzzle velocity and altitude if you know them.
3. Click calculate.
4. The tool shows base Sg, corrected Sg, and stability status.

It also tells you if your bullet is unstable, marginal, stable, or over-stable. You’ll see extra info like length-to-diameter ratio and twist-to-diameter ratio.

Final Verdict

Bullet stability is the key to accuracy. With our Bullet Stability Calculator online, you don’t need complex math. Just enter your data and get instant results. It helps hunters, sport shooters, and reloaders pick the right bullet and barrel twist for the job.

Stable bullets mean tighter groups and cleaner shots. Unstable bullets waste ammo and hurt precision. Try the calculator and check your setup today.

FAQs

What is the stability of a bullet?

It’s how steady a bullet flies without tumbling.

At what speed does a bullet become unstable?

When its gyroscopic stability factor (Sg) drops below 1.0, usually at lower speeds.

What is the optimal bullet stability factor?

Between 1.4 and 2.0 for most rifles and conditions.

What is the gyroscopic stability of a bullet?

It’s the spin effect that keeps a bullet pointed forward in flight.

Can a bullet be over-stable?

Yes. Over 2.5 Sg, it’s still stable, but spin drift may increase.