A Look at Twist Rates and Ballistic Perfromance

Looking at Faster Twist Rates, Specifically in 8.6 Blackout

Twist rate is an exciting and apparently controversial point of discussion these days. While many shooters are used to twist rates in the 1:7 to 1:10 range, 8.6 Blackout introduces an opportunity to rethink that entirely. The unique nature of this cartridge—heavy bullets, often solid copper, frequently subsonic, and commonly shot from short barrels—means traditional stability models and design logic need to be revisited.

What Twist Rate Does

Twist rate refers to the distance a bullet must travel down the barrel to complete one full rotation. A 1:3 twist rate means the bullet makes one full rotation every 3 inches of barrel travel. Faster twist rates (e.g., 1:3) spin the bullet more aggressively than slower ones (e.g., 1:7).

Understanding Bullet Stability: The Math

To determine if a bullet will stabilize in a given twist rate, we often turn to gyroscopic stability models, such as the Miller Stability Formula or the Sharps Bros subsonic formula. These formulas aim to calculate a stability factor (Sg), where values above 1.4 are considered stable.

Miller Stability Formula (simplified):

Sg = (30 * m) / (d^3 * l * (1 + (l^2 / d^2))) * (t / 10.9)^2

Where:

• m = bullet weight (grains)

• d = bullet diameter (inches)

• l = bullet length (inches)

• t = twist rate (inches per turn)

Sharps Bros Subsonic Formula:

Sg = ((WT * (20.62 * DIA^2.25)^2) / ((TWIST * BL^2.25)^2)) / (1 - (V / 5705))

Where:

• WT = bullet weight

• DIA = bullet diameter

• BL = bullet length

• TWIST = twist rate

• V = velocity (fps)

The Miller formula is a helpful tool, but it has shortcomings:

• It was designed for lead-core bullets, not long solid copper designs.

• It may underestimate the rotational inertia of very long projectiles.

• It tends to give unrealistically high or low Sg values in extreme edge cases.

For example, using Miller's formula, a prototype 410-grain .338 solid copper projectile measuring 2.3" long and fired at a subsonic velocity predicts stability factors well into the hundreds in 1:3 twist barrels—clearly not a realistic interpretation. The Sharps Bros formula offers more practical numbers, predicting:

• Sg = 3.78 at 1:3 twist

• Sg = 2.13 at 1:4 twist

• Sg = 1.36 at 1:5 twist

• Sg = 0.69 at 1:7 twist

This suggests that 1:5 is marginal and 1:4 or faster is preferable for subsonic stability.

Real-World Check: 240gr .308 Solids

Let’s compare that with very well tested and known performance in a 240-grain .308 solid copper projectile, 2.21" long, tested and sometimes used in 300 Blackout. Despite predictions from the Sharps Bros formula suggesting a twist faster than 1:3.4 is required for stability, these bullets are reliably stabilized in 1:5 and even 1:6 twist barrels in real-world use.

This highlights that empirical formulas, while useful, can be overly conservative. Bullets may be more stable in practice than theory suggests, particularly when dealing with monolithic copper projectiles with favorable mass distribution, but that is not always the case. These formulas generally serve as a way to gut check what we see in real life. For example, the results from the Sharps Bros formula are tracking very much along the lines of what we are seeing in reality with prototype 8.6 Blackout projectiles in terms of requiring a 1:3 or 1:4 twist rate to achieve proper stability while 1:5 seems to be marginal at least in early testing with these specific, 400 gr + projectiles.

Application of the 1:3 Twist Rate

Subsonic Applications:

While a 1:5 twist may provide marginal stabilization for some of the heaviest possible 8.6 projectiles at subsonic speeds, a 1:3 twist ensures full stabilization and contributes to more reliable terminal performance (more on that later). Greater rotational velocity can enhance the expansion of solid copper bullets at low impact velocities which are inherent in subsonic ammunition.

In our development of the Revenant 8.6 line, we've shot out multiple barrels in pursuit of the optimal balance between stability, expansion, and accuracy. We've observed that 1:3 twist rates support more consistent performance in purpose-built subsonic 400+ grain projectiles, particularly in expansion and downrange terminal effect.

Supersonic Applications:

For supersonic loads, 1:3 twist also offers benefits by helping maintain terminal performance from smaller, shorter weapon systems that are producing lower muzzle velocities. Even though muzzle velocity may be lower due to the shorter barrel length, the high spin rate helps preserve the bullet's expansion characteristics at distance.

The above image is something we got from Q’s instagram. What we are seeing here is actually the same Barnes projectile, believe it or not, on both the top and bottom row. This projectile was originally designed for .338 Lapua Magnum and is a supersonic projectile that we use in 8.6 blackout right now (although it’s design is far from ideal, we will remedy that shortly). Both sets of these projectiles were shot into barrels of water at identical muzzle velocities. The top row is from a firearm with a 1:10 twist and the bottom is from a firearm with a 1:3 twist. What we are seeing here is that with the same bullet, both traveling at identical linear velocities, we see increased expansion with the faster twist rate.

Here is a link to a video by RMGC that explores faster twist rates in 5.56, he puts together a great comparison between a barrel with a 1:5 twist and a barrel with a 1:7 twist. NOTE - Ballistic gel is not a 1:1 medium for understanding what projectiles would do in organic material (like you and I). It is however an appropriate material for comparing two different projectiles under the same set of conditions (as long as you understand that it may or may not have meaningful real world implications.) or in this case comparing the same projectile fired from barrels with different twist rates.

That said, we've also found that many current projectiles used in 8.6 Blackout exhibit better accuracy in 1:5 twist barrels than in 1:3. This is likely because many of these bullets are scaled-up 300 Blackout designs, originally engineered for slower twist rates.

The work we’ve done in the development of our Revenant 8.6 line tells a different story. We've fired tens of thousands of rounds in pursuit of the ideal projectile for this cartridge. Our latest projectile designs are engineered specifically around the 1:3 twist rate—not retrofits or adaptations, but purpose-built from the ground up to take full advantage of fast twist stabilization and enhanced terminal effects. These designs have shown exceptional accuracy and consistency in real-world testing, reinforcing the idea that projectiles tailored to 1:3 twist unlock the true potential of this platform.

Barrel Length, Spin Drift, and Twist Rate Tradeoffs

Faster twist rates do increase spin drift—the lateral movement of a bullet caused by its rotation interacting with air resistance. Spin drift increases with velocity and spin rate, so longer barrels (e.g., 12") firing supersonic loads may experience more noticeable lateral deviation.

However, spin drift is predictable and correctable, just like elevation compensation. You simply hold for it.

In shorter barrels, faster twist rates almost always make sense. They help stabilize the heaviest possible subsonic projectiles, and they aid in expansion even when linear velocity is reduced. For performance out of barrels 8" or shorter, a 1:3 twist rate is hard to argue with in our collective opinion.

With longer barrels, the tradeoff becomes more nuanced and preference-driven. If your goal is to reduce spin drift and you're okay with sacrificing some potential terminal expansion capability at longer range—or forgoing the ability to shoot the heaviest 400-grain projectiles—then a slower twist like 1:5 or 1:6 may be appropriate.

That said, the increase in spin drift associated with 1:3 is a sacrifice we're generally willing to make, considering what we gain in stability and terminal performance. Faster twist enhances expansion potential:

• Expansion is more reliable, especially for monolithic copper bullets. If we agree that faster twist rates provide better expansion it would follow that the increase in expansion would create more resistance in tissue which translates to better transfer of kinetic energy in a given target.

• Rotational velocity decays more slowly than linear velocity, so spin continues to support expansion downrange.

While a 1:3 twist does contribute to an increase in total kinetic energy—potentially between 3% and 11% more than a 1:8 twist depending on the bullet's moment of inertia and other variables—it's important to note that rotational kinetic energy is not a separate type of energy but a component of the total kinetic energy of a spinning mass. At the muzzle, rotational velocity represents a relatively small portion of that total energy. However, because linear velocity decays more rapidly than rotational velocity, the contribution of spin becomes more meaningful as the bullet travels downrange.

More importantly, it’s not necessarily the added energy that imparts terminal effect, but rather the mechanical advantage that increased spin provides. The higher rotational velocity aids in projectile expansion, which directly translates to greater terminal performance. In addition to improved expansion, faster twist rates can also increase penetration and the ability to defeat intermediate barriers like bone or thick hide.

There is extensive testing—especially with 300 Blackout—that shows 1:5 twist barrels achieving better penetration than 1:7 and 1:8 twists… maybe. Most of that testing is done in ballistic gel and as we covered earlier, that’s not a perfect analogy for living tissue like pigs or bad guys… or really anything other than gel actually

This concept isn't limited to firearms either. A parallel can be seen in archery: single bevel broadheads use increased rotational velocity to split bone and penetrate deeper. There's a reason drill bits spin—the rotational force aids in cutting through resistant material. The same principle applies here. Greater spin can enhance both expansion and penetration, making faster twist rates a valuable tool for terminal effectiveness.

Daddy Phantom took the above shot with a bow this past archery season in the beautiful mountains of Utah. We believe that being successful on this particular hunt was in large part due to the arrow setup, and specifically the single bevel iron will broadhead that was used. (Phantom broadheads anyone? A discussion for another time.) This bull was shot at what most people would consider extended range for archery, while he was quartering to the shooter. This setup was able to defeat the bulls front shoulder and exited near the back hip as seen above. Again, is it a perfect analogy? Absolutely not, but we see it as a valid data point in examining how rotational velocity contributes to terminal performance.

Conclusion: It's About Intent

Choosing a twist rate is about what you want the platform to do. If your goal is maximum accuracy with existing commercial bullets, 1:5 is a solid middle ground. But if you’re aiming to push the limits of subsonic terminal performance from a compact platform, 1:3 gives you a more aggressive, specialized tool.

8.6 Blackout is exciting for the same reason 300 Blackout is: Versatility!

It's important to acknowledge that we obviously have a financial interest in the success of the 8.6 blackout cartridge. Ultimately though, we care about high performance and we don't care if the medium for that performance is 8.6 blackout, 300 blackout, 338 arc, or something else. It is a cartridge that we like a lot and have made extremely heavy investments in because we recognize so much potential. That being said we are not beholden to anybody and if we thought 1:5 or 1:7 was better suited to this specific cartridge we'd promote the hell out of that or we would spin up and sell barrels ourselves.

Ultimately our bias is in looking for the highest possible performance and passing that performance on to the end users of our products. The purpose of this article is to introduce our audience to our thought process and opinions around twist rates and the real world applications for faster twist rates. It doesn't mean that there is no room for anything that is not "Fast Twist". We welcome healthy discussion around the topic and even push back on our opinions. As always we reserve the right to change our minds as we learn, continue to test, and develop our products with the goal of bringing the best possible products to our customers.