Are You Spinning Your Bullets Fast Enough? Twist Rate Calculator Predicts Gyroscopic Stability
Berger Twist-Rate Stability Calculator
On the 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.
How to Use Berger’s Twist Rate Calculator
Using the Twist Rate Calculater is simple. Just enter the bullet DIAMETER (e.g. .264), bullet WEIGHT (in grains), and bullet overall LENGTH (in inches). On its website, Berger conveniently provides this info for all its bullet types. For other brands, we suggest you weigh three examples of your chosen bullet, and also measure the length on three samples. Then use the average weight and length of the three. To calculate bullet stability, simply enter your bullet data (along with observed Muzzle Velocity, outside Temperature, and Altitude) and click “Calculate SG”. Try different twist rate numbers (and recalculate) until you get an SG value of 1.4 (or higher).
Gyroscopic Stability (SG) and Twist Rate
Berger’s Twist Rate Calculator provides a predicted stability value called “SG” (for “Gyroscopic Stability”). This indicates the Gyroscopic Stability applied to the bullet by spin. This number is derived from the basic equation: SG = (rigidity of the spinning mass)/(overturning aerodynamic torque).
If you have an SG under 1.0, your bullet is predicted not to stabilize. If you have between 1.0 and 1.1 SG, your bullet may or may not stabilize. If you have an SG greater than 1.1, your bullet should stabilize under optimal conditions, but stabilization might not be adequate when temperature, altitude, or other variables are less-than-optimal. That’s why Berger normally recommends at least 1.5 SG to get out of the “Marginal Stability” zone.
In his book Applied Ballistics For Long-Range Shooting, Bryan Litz (Berger Ballistician) recommends at least a 1.4 SG rating when selecting a barrel twist for a particular bullet. This gives you a safety margin for shooting under various conditions, such as higher or lower altitudes or temperatures. Try changing the altitude and temperature in the calculator and you will see that the SG can increase or decrease when these environmental factors change. Under optimal circumstances you should aim for a 1.4, that way if you change circumstances you are still over 1.1.
Erik Dahlberg rifling illustration courtesy FireArmsID.com.
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This shows the 155 right on the edge but ive seen dozens of these barrels handle the 155 without issue as the 13 twist is the favored twist by far with the 155.
At what number is the stability factor to high? Is there a point that you can say no that is far to fast of a twist?
No, it shows stability at 1.42 at 59deg and sea level. This is not on any ‘edge’.
Folks should keep in mind though that this Miller stability calc is a rule of thumb only, so there are assumptions and it’s limited in application.
For example, it can’t account for a change in meplat diameters, plastic tips, nose shapes, or base shapes(flat/boat tail), etc.
That’s not to say it isn’t useful, rules of thumb are easier(like Greenhill). But you need to consider their limitations.
With a 1.42 and Berger recommending a 1.50 or higher and Bryan Litz recommends at least a 1.4 SG looks on the edge to me.
We’re learning a lot about twist rates, stability and BC thru recent testing. Something I can share is that bullets can shoot ‘just fine’ in a twist that produces marginal stability (SG of 1.1 to 1.5), meaning good groups. However, you might be leaving some BC on the table with these marginal twist rates.
In the old days, with the old breed of 155’s which were shorter and blunter, the 1:13″ twist was plenty. However, with the new breed of 155’s being longer, they are certainly more comfortable in a 1:12″ or faster. By ‘more comfortable’ I mean potentially better BC’s, especially in high density air (cold and near sea level).
As for over-spinning bullets, the downsides to this are not as bad as many fear. If the bullets are poorly balanced, then you might see some added dispersion due to the imbalance, but most bullets made these days are very well balanced. Another potential downside is bullet failure, but this is more of a problem for the long heavy bullets which already require somewhat radical twist rates: 6mm 115’s for example going from a 1:7″ to 1:6″ might cross the line.
In summary, there are benefits to spinning bullets faster than previously thought (SG of 1.4), and the downsides of ‘over spinning’ bullets are not as bad as many think. The specific test results are planned for publication later this summer.
-Bryan
Thanks for the update Bryan. Just put a Broughton 5C on my palma rifle, 13 twist but will try a 12 twist next time around, shooting the newer Sierra 155 with higher BC and Berger 155`s. Looking forward to the summer test results. GG