## Ballistics Tip: Understanding Bullet Stability (Twist Rate and MV)

Bryan Litz has produced an informative new video on the subject of bullet stability. The video explains how stability is related to spin rate (or RPM), and how RPM, in turn, is determined by barrel twist rate and velocity. For long-range shooting, it is important that a barrel have a fast-enough twist rate to stabilize the bullet over its entire trajectory.

**Detailed Bullet Stability Article**

To complement the above video, Bryan has authored a detailed article that explains the key concepts involved in bullet stabilization. Bryan explains: “Bullet stability can be quantified by the gyroscopic stability factor, SG. A bullet that is fired with inadequate spin will have an SG less than 1.0 and will tumble right out of the barrel. If you spin the bullet fast enough to achieve an **SG of 1.5 or higher**, it will fly point forward with accuracy and minimal drag.”

## CLICK HERE to READ Full Bullet Stability Article by Bryan Litz

There is a “gray zone” of marginal stability. Bryan notes: “Bullets flying with SGs between 1.0 and 1.5 are marginally stabilized and will fly with some amount of pitching and yawing. This induces extra drag, and reduces the bullets’ effective BC. Bullets in this marginal stability condition can fly with good accuracy and precision, even though the BC is reduced. For short range applications, marginal stability isn’t really an issue. However, shooters who are interested in maximizing performance at long range will need to select a twist rate that will fully stabilize the bullet, and produce an SG of 1.5 or higher.”

**Berger Twist-Rate Stability Calculator**

On the updated Berger Bullets website you’ll find a handy Twist-Rate Stability Calculator that predicts your gyroscopic stability factor (SG) based on mulitiple variables: velocity, bullet length, bullet weight, barrel twist rate, ambient temperature, and altitude. This very cool tool tells you if your chosen bullet will really stabilize in your barrel.

**LIVE DEMO BELOW**— Just enter values in the data boxes and click “Calculate SG”.

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Tags: Berger Bullets, Bryan Litz, MV, SG Factor, Stability, Twist Rate, Velocity

I see the calculations start with BC. Since NC varies with velocity wouldn’t it be better to ask for the drag factor? I could be way off on that one but isn’t a constant more desirable than a variable in most cases?

NC=BC Goony finger at work . . .

Hm…Seems whether you select G1 or G7 the SG come out the same.

I input a set of data including .970 for the BC and got a result. I then noted that the button was set for G7 when the .970 BC value was a G1. I cleared the screen back to the base .308 projectile, re-input the values to match the prior setup but with the BC value set for a G1 and reran the number. Same SG.

Really?

This article implies way more(and different) than Bryan said.

Forget RPMs..

Stability does not come from turns per time, it comes from displacement per turn minus other moments to overcome.

Displacement being air density/drag.

Drag is tied to velocity, but this has nothing to do with RPMs. So any RPM correlations to stability will fail tests.

BC is not a factor in determining stability (SG). The reason for inputting BC is so you can see what the BC degrades to if the bullet is not fully stabilized. If your parameters result in full stability (SG>1.5) then the BC you input will be unchanged. But if the SG is below 1.5, the BC begins to be affected and the Berger Stability calculator shows you how much.

Stability calculations and the discussions surrounding them can get very complicated very fast. Typically, you want to consider inches per turn (or turns per some distance) because that measure is most closely related to stability (although it’s not directly proportional due to aerodynamics at different Mach numbers…) However, if you talk about RPM’s at a certain fps (both time based rates) then you can do some converstions and get to Revs per foot, and revs per inch/inches per rev. They’re just different ways to do the math; one is more direct and intuitive but the other (RPM’s and fps) isn’t ‘wrong’, just less direct.

For those interested in an in-depth exploration of stability and its effects on BC in both supersonic and transonic flight, check out: Modern Advancements in Long Range Shooting-Volume 1. This book presents all the live fire data that supports the link between stability and BC.

-Bryan

As Bryan explained, RPM is a meaningful measure of bullet rotation. We used this term because the acronym is familiar to most readers. There is a simple formula to calculate Bullet RPM with any MV and twist rate:

MV x (12/twist rate in inches) x 60 = Bullet RPM

Quick Version: MV X 720/Twist Rate = RPM

Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM.

If you imagine this is a Dasher bullet launched at 3000 fps, then it spins at 180K rpm in a 1:12″ twist barrel vs. 270K rpm in a 1:8″ twist barrel. That should be understandable.

So stability factor changes as velocity decreases as the bullet travels down range? You mention the 1.5 sf but is that at the muzzle or is that entering the transonic zone?