Overview
The modern rifle is named for the spiral grooves cut in the barrel which impart a spin to the bullet. Spinning imparts gyroscopic stability, which is what keeps the bullet flying point forward. Longer bullets need to spin faster than shorter bullets to achieve stability.
For many years, the relative proportions of bullets haven’t changed much, and so the twist rate of riflings has sort of stagnated around standard values. However in recent times, technology is advancing to the point where higher performance (longer) bullets can be made with greater precision, and so the thoughtful consideration of twist rate and stability is becoming important. Many of the modern high BC bullet offerings are too long to fully stabilize in conventional twist rates. In order to maximize the benefits of these high performance bullets, shooters need to have a basic understanding of stability to select a suitable twist rate.
Basic Gyroscopic Stability
Stability is 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. SG’s between these two extremes represent bullets flying in ‘the gray area’ of marginal stability.
Bullets flying with SG’s 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.
How do you know how fast a bullet needs to spin?
Traditionally, bullet manufacturers indicate a bullets twist requirement on the bullet box label, in brochures, and on websites. The twist requirement is usually a single number, and looks something like this:
Recommended Twist Rate:
1:12” or faster
The recommended twist rate of 1:12” is stated as: one in twelve, and means the bullet needs to complete at least one full rotation in 12” of travel. A lower number means faster spin and higher stability.
This basic level of information tells the shooter if a given bullet will be stabilized by his rifle or not. For many decades, this basic level of black and white information has been adequate, but the modern bullet designs that are available today are often in the gray area when fired from conventional twist rate barrels, and require a higher level of information and understanding by the shooter to ensure maximum performance.
For this reason, Berger Bullets is elaborating on the twist requirements of the longer bullets with the following format:
Recommended Twist Rate:
Minimum: 1:13”
Optimal: 1:12.1” or faster
The Minimum twist is calculated for an SG of 1.2, which is enough to shoot the bullet with good accuracy, but the BC will be compromised. The Optimal twist rate corresponds to an SG of 1.5, which is the level of stability you need to get all of the BC out of the bullet. Note that these calculations are based on 2800 fps, and standard sea level conditions. Velocity and atmospherics both play a role in stability, and we’ll get to that in a moment.
These twist recommendations will be included on the bullet box labels, as well as the Berger Bullets Quick Reference Sheet (QRS). You can view the QRS HERE (twist rate updates coming soon). Offering this additional information on twist rates will enable shooters to make better informed decisions about what bullets will work best for their application. For example, if you have at least the minimum recommended twist rate; that means you can shoot a bullet with good accuracy which is sufficient for a short range application. However if you want to use that bullet for long range, then you may want a faster twist to maximize its flight performance at distance.
Common Questions
Q: Why are there two twist rates and what do they mean?
A: The minimal twist rate is the bare minimum you need for the bullet to fly straight. The optimal twist rate is what you need to achieve full stability, and maximize BC.
Q: If you say the optimal twist is 1:12.4” and I have a 1:12”, will that work?
A: Yes. Your 1:12” is faster than the optimal 1:12.4”. It’s always OK to spin a bullet faster than recommended, but not slower. One caveat is if the bullet is on the verge of failure, a much faster twist rate may put it over the edge but this is a rare problem.
Q: What conditions are the minimum and optimal twist rates based on?
A: The minimum and optimal twist rates are based on achieving stability factors (SG’s) of 1.2 and 1.5 in standard sea level conditions with a muzzle velocity of 2800 fps.
Q: What if I’m shooting the bullet at a velocity or atmospheric condition that’s different from what the twist requirements are based on?
A: Visit the Berger Twist Rate and Stability calculator online where you can enter the details of your specific bullet and environment. The calculator will tell you what the stability of your bullet is in your specific conditions.
The Berger Twist Rate Calculator
Providing a range of twist rates (minimal and optimal) addresses the issue of marginal stability, but there are still variables related to your specific environment and the muzzle velocity you’re shooting the bullet at. The online Berger Stability calculator allows you to explore all of these variables, so you can truly understand the unique stability requirements of your bullet in any environment and at any muzzle velocity.
The Berger Twist Rate calculator tells you what the SG is for a bullet in any conditions, and tells you if and how much the BC may be reduced in this condition.
The math behind the Berger Stability calculator is based on live fire testing conducted in the Applied Ballistics Laboratory, and published in Modern Advancements in Long Range Shooting. This book is recommended for those interested in further reading on the effects of twist rate including: twist rate effects on muzzle velocity, effects of altitude on stability, spin rate decay at long range, effects of stability on BC in supersonic and subsonic flight, and more.
Conclusion
The modern age of rifle shooting (especially long range rifle shooting) is seeing a great deal of advancement which challenges our conventional tools and knowledge. To fully benefit from the advanced bullet designs that are becoming available in modern times, manufacturers need to communicate the stability requirements of those bullets at an appropriate level of detail so that shooters can get the most out of the advancements.
Bryan Litz