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.
When you think of the lands and grooves of a barrel, this configuration is typically the option that comes immediately to mind. Common in “mil-spec” rifling designs, traditional lands and grooves are joined at a 90-degree angle (or at angle that intersects the centerline of the bore), deforming a slightly oversized bullet as it engages with the rifling.
While the overall count of lands and grooves may vary, quite often the change in groove count will have no appreciable impact on barrel performance. A prime example of this phenomenon involves tests carried out by Remington during WWII with 2- and 4-groove 1903A3 barrel production. After extensive testing and evaluation, no significant difference in performance was detected between the two, with velocity and pressure numbers falling within the margin of error of measurement at the time.
There are perceived downsides to traditional rifling in the realm of production and maintenance of these barrels, as they may be slightly more difficult to pull a rifle button consistently, and may take slightly longer to clean.
If you’re browsing through forums and come across discussion regarding polygonal rifling, you may be inclined to think it’s the best thing since sliced bread. Formed through the cold hammer forging process, polygonal rifling eschews traditional lands and grooves, replacing them with a more gradual “hills and valleys” rifling geometry.
There have been many claims over the years regarding polygonal rifling, oftentimes detailing reduced bullet deformity, improved gas seal, and increased velocities. Recent scientific testing has proven these perceived benefits to be overstated, with both polygonal and traditional rifling designs offering roughly equivalent performance across the board.
One downside to polygonal rifling in pistol designs is that it does not lend itself well to the use of cast lead bullets, as it has a tendency to rapidly foul and create dangerous pressure spikes. Another downside to polygonal rifling is its reliance on cold hammer forging for its production process, a manufacturing technique that often places a heavy reliance on the stress relief process to ensure material uniformity. For this reason, traditional and hybrid rifling (delivered through single-point cut and button rifling manufacturing techniques) continues to be the norm for most competition applications.
Hybrid and 5R Rifling
Occasionally manufacturers will market polygonal rifling as a feature on precision rifles, but it often features elements of a traditional rifling design. This “hybrid” rifling design features lands and grooves with a trapezoidal sloping geometry with the promoted intention of minimizing jacket deformation while ensuring consistent rifling engagement by the projectile.
One popular form of this hybrid rifling concept is the 5R rifling design, a pattern designed by Boots Obermeyer in the 1970’s that incorporates five slightly offset lands of a specified slope angle. Proponents of this design believe that this rifling design allow for easier cleaning (since patches can engage the sloped angles easier than the sharp corners of traditional rifling), but claims of enhanced accuracy and barrel life have not been proven scientifically by any major manufacturers.
While some barrel manufacturers may promote one unique rifling pattern over another, the cost per barrel, cycle times, barrel performance, and overall production process are roughly equivalent across the board. Vortakt Barrel Works can accommodate a wide range of rifling patterns, as all of our button rifling tooling is custom-produced to meet unique client specifications.
If you have a unique preference on rifling design for your next OEM barrel project, contact the Vortakt sales team today!