What Is Twist Rate?
Twist rate refers to the rate at which the rifling inside your barrel spins the bullet as it travels down the bore. Expressed as a ratio like 1:7 or 1:9, the numbers indicate that the bullet makes one complete rotation in that many inches of barrel. A 1:7 twist means one revolution every 7 inches, while a 1:9 twist completes one revolution every 9 inches. The lower the second number, the faster the twist.
This spin is what stabilizes the bullet in flight, much like a football spiral. Without adequate spin, bullets tumble through the air, destroying accuracy. Too much spin can also cause problems, though this is less common in practice.
Why Twist Rate Matters
The relationship between twist rate and bullet weight is fundamental to rifle accuracy. Heavier, longer bullets require faster twist rates to achieve stability. Lighter, shorter bullets can be stabilized with slower twist rates. Getting this relationship wrong means your rifle will never shoot to its potential, regardless of how well it’s built.
For the 5.56 NATO and .223 Remington cartridges used in AR-15 rifles, common twist rates range from 1:7 to 1:12. The original M16 used a 1:14 twist optimized for the 55-grain M193 ball ammunition. When the military transitioned to the heavier 62-grain M855 round, the twist rate was changed to 1:7 to properly stabilize the longer bullet.
Common AR-15 Twist Rates and Their Applications
1:7 Twist: The current military standard, this fast twist stabilizes bullets from 55 grains up to 80+ grains. It’s the most versatile choice and excels with heavier match-grade ammunition like the 77-grain Sierra MatchKing. If you plan to shoot a variety of bullet weights or focus on longer-range precision, 1:7 is an excellent choice. All G2 Precision rifles feature carefully selected twist rates matched to their intended purpose.
1:8 Twist: Often considered the best all-around twist rate for .223/5.56, the 1:8 handles everything from 55-grain to 77-grain bullets well. It offers a slight accuracy edge with lighter bullets compared to the 1:7 while still stabilizing most heavy match bullets.
1:9 Twist: Best suited for bullets in the 55 to 69-grain range. This was the most common twist rate in civilian AR-15s for decades and remains popular for shooters who primarily use standard 55-grain and 62-grain ammunition.
The Greenhill Formula and Stability
The Greenhill formula, developed by British mathematician Sir Alfred Greenhill in 1879, provides a mathematical foundation for determining the twist rate needed to stabilize a given bullet. While modern ballistic software has largely replaced hand calculations, understanding the principle helps shooters make informed decisions about barrel selection.
The key factors are bullet length, diameter, and the density of the bullet material. Lead-core bullets and solid copper bullets of the same weight will have different lengths and thus require different twist rates. This is particularly relevant as more states mandate lead-free ammunition for hunting.
Signs of Over-Stabilization and Under-Stabilization
An under-stabilized bullet will keyhole, hitting the target sideways and leaving elongated or oval-shaped holes rather than round ones. This is the most obvious sign that your twist rate is too slow for your chosen ammunition. An over-stabilized bullet may not transition properly from the flat trajectory phase to the nose-down descent at longer ranges, potentially reducing accuracy at extended distances. However, over-stabilization is rarely a practical concern for most shooters.
Choosing the Right Twist Rate
For most AR-15 shooters, a 1:7 or 1:8 twist rate provides the best balance of versatility and performance. If you know you’ll exclusively shoot lighter varmint bullets, a 1:9 may offer a marginal accuracy improvement. For dedicated long-range precision builds shooting 77-grain and heavier bullets, a 1:7 twist is the clear choice. At G2 Precision, we select twist rates based on each rifle’s intended application, ensuring optimal performance from the factory.
