Finding the Ideal Twist Rate for Your Production Barrel Design
The last few decades have seen incredible growth in cartridge development across the firearm industry. Rapid advances in firearm technology have created niche load recipes ideally suited for a variety of different shooting disciplines and range conditions. Much of this has been driven by improvements in bullet design and performance, often delivered in the form of increased ballistic coefficient.
Most manufacturers reference SAAMI and mil-spec standards when selecting an optimal twist rate, but oftentimes these specifications are fairly arbitrary in nature. A classic example of these unusual specifications is the 1:11.25 twist rate requested for the M24 Sniper Weapon System. Was this unique twist rate uniquely well-suited a particular bullet design? Not at all! This particular standard was established due to the limitations created by the tooling, cam gear, and sine bar limitations of the cut rifling machines used at that time.
The Vortakt Barrel Works engineering team spends considerable time and resources in researching the appropriate chamber and rifling tolerances required to maximize the potential of countless bullet, rifle, and pistol designs. While those specifications are often pre-defined by our clients, several additional factors come into play when considering optimal twist rate.
Finding the Sweet Spot in Rifling Twist Rates
While the science behind rifling twist is extensive, the fundamental principles behind it are fairly straightforward. Without rifling, a bullet would not generate sufficient gyroscopic stability and would simply tumble downrange in an uncontrolled manner.
While many articles reference bullet weight as the determining factor in twist rate requirements, overall bullet profile and composition also play a role in defining the location of the center of mass (the point where the bullet balances its weight in flight), and the center of pressure (the point where aerodynamics act upon the bullet).
The further apart these two points lie from one another (this distance typically increases with longer bullet profiles), the more the bullet needs to spin in order to remain stable. While many of the classic bullet designs may remain stable with SAAMI twist specifications, newer, longer bullet designs with a higher ballistic coefficient may require a faster rate of twist.
We interviewed Mitchell Demand, a Ballistician at Sierra Bullets, to provide some additional insight regarding optimal twist selection:
1. How can rifle or pistol manufacturers future-proof their product line to accommodate new bullet designs?
“Most manufacturers will simply base that decision off of current market demand, but the easiest way to guarantee a twist rate is optimal for a particular bullet design is to create a bullet to match those unique production requirements. A good recent example includes the creation of a proprietary design for an OEM customer, where we designed a 131 grain .25 caliber projectile to meet the design parameters of the client. Another recent example was a collaboration between Winchester and Browning, where they both wanted 175 and 277 grain bullets designed to accommodate the new 6.8 Western cartridge.
One thing I can say is that when it comes to match rifle designs, is that a faster twist is probably a safer bet for future-proofing a rifle line to accommodate new bullet designs. With 6mm bullets we’ve recently seen the standard shift from slower twist rates (1:9 for 90+ grain bullets) to faster twist rates (1:7, 1:7.5, 1:8) to accommodate 100+ grain bullet designs.”
2. Do you have any recommendations regarding twist rate/bullet stability calculators and other related tools for rifle and pistol manufacturers?
“If you’re not reinventing the wheel with a new cartridge or bullet design, you can just give us a call and ask what the optimal twist rate is for the platform or existing bullet selection in question. We have a calculator that we use to determine bullet stability and optimal twist rate based off a list of variables.”
3. What are your thoughts on bullet overstabilization?
“There are a ton of myths perpetuated out there regarding overstabilization. For the most part it’s a non-issue if you’re using the right kind of bullet for your rifle or pistol build. Overstabilization may cause a bit of yaw (a differentiation between the projectile’s angle and its direction of travel) in the bullet as it travels downrange, but it isn’t until there’s a significant discrepancy between smaller bullet designs and the overall rate of twist that you see catastrophic results.
An extreme example could include the use of a lightweight .22 caliber 50gr. Blitzking bullet in a .22-250, with a 1-6.5 twist rate designed for a much longer and heavier 90 grain bullet design. If you’re zipping that little 50 grain projectile at 3,800 FPS with a twist rate that fast, the gyroscopic force imparted on that bullet will explode the projectile, disintegrating it before it hits the target.”
Twist rate standards will often change to find a “sweet spot” for commercially available projectile options. A perfect example includes the classic .223 Remington, 5.56×45 NATO, or .223 Wylde-chambered barrel designs. While 1:7 twist is the “mil-spec” standard, most manufacturers planning on optimizing their twist rate for a wide range of commercial bullet designs will gravitate toward a .223 Wylde chamber with a 1:8 twist rate.
Online Resources and Calculators Regarding Twist Rate Selection
- JBM Ballistics Twist Rate Calculator
- Berger Bullets Twist Rate Stability Calculator
- Berger Flat Based Bullet Twist Recommendations Calculating Bullet RPM
- Calculating Twist Rates (A Historical Reference)
Conclusion
Ultimately, rifle and pistol designs should incorporate twist rates optimal for the bullet designs best suited for the intended application of their products. Rifle and pistol manufacturers should factor in the possibility of future introduction of longer, heavier match bullet designs to future-proof their existing product line.
While we provide a complete list of standard twist rate options on our website, we also have the internal capability to produce custom tooling for any twist rate, including the option to produce barrels with gain twist rifling.
Contact Vortakt Barrel Works for best practices and recommendations if you have any additional questions regarding twist rate selection and barrel design for your next rifle or pistol production run.