Twist rate is the measurement of how much the lands and grooves of your barrel's rifling rotate. This determines the kinds of bullets you'll shoot. It's so important for accuracy that it's advertised in the titles of just about every AR-15 upper and build kit. Why? Twisted rifling gives spin and stability to a bullet as it travels downrange, like a Hail Mary thrown by a quarterback into the end zone. All rifles and handguns must have a twist rate expressed in their specifications. Smoothbore guns (most shotguns, save for rifled slug guns) have no rifling.
How Rates Are Measured
The rate is measured in inches per full rotation and is expressed as a ratio, written as 1:9, 1/9, or 1-in-9. This measures how many inches the rifling travels down the barrel before it completes one 360-degree rotation. For this example, that's one rotation every nine inches. The smaller the number, the more frequent the rifling twists.
Twist vs. Bullet Weight
Twist rate determines how heavy or light your bullets will be, which is usually advertised as a "grain" count or grain weight. The heavier the bullet (higher the grain count), the faster the twist rate must be to effectively spin up and stabilize that round before it leaves the barrel.
Twist vs. Barrel Length
Twist rate is not related to barrel length, and barrel length doesn't affect which twist rate is best for a weapon or cartridge. If the cartridge in question performs best with a 1:10 twist rate, that rule always applies, whether it's fired from a 7.5" barrel, 16" barrel, or a 24" barrel. Barrel length only affects velocity, not the spin of the bullet.
This chart provides a visual for which twist rates are best for most 5.56/.223 loads. The numbers in each cell measure gyroscopic stability. Faster rates produce higher factors. Factors between 1.5 and 2.0 are ideal for all bullets and calibers. Factors between 1.0 and 1.3 are marginally stable. Factors between 2.1 and 2.9 are considered fast, but still stable and highly accurate. Factors above 3.0 are suitable, but climbing above 4.0 may begin to cause over-stabilization, or too much spin. Based on this chart, the optimal twist rates for 5.56 and .223 bullets are:
- 45-gr Varminter: 1:12 twist
- 55-gr (M193): 1:12/1:9 twist
- 62-gr (M855): 1:8 twist
- 77-gr (Mk262): 1:8 twist
- 80-gr Sierra Match: 1:7 twist
- 90-gr Sierra Match: 1:7 twist
Overall, the 1:8 twist rate is best for AR owners who want to shoot all common .22 loads. The fast 1:7 rate's designed for match loads, while the 1:9 twist is optimized for the most commonly available ammo ( M855 and M193) at most sporting goods stores and general stores, like Walmart.
What does too much spin do?
A bullet in flight travels with six degrees of freedom: Forward, up and down, and left and right with yaw, roll, and pitch in motion, too.
- Roll affects the spin of the bullet perpendicular to its travel, like a thrown football.
- Yaw affects the horizontal rotation of the bullet, like a car turning left or right.
- Pitch affects the rise and fall of the bullet, like a plane taking off or landing.
The right amount of spin helps a bullet to resist changes to its yaw and roll, and it helps to stabilize its pitch as it arcs from the barrel to the target. Over-stabilization causes spin drift, which makes the bullet yaw in the direction its spinning. Too much drift will cause a bullet to yaw left or right and ultimately tumble, losing its "football"-esque roll and stable flightpath.
Bullet Spin vs. Ballistic Coefficient
Gyroscopic stability correlates directly with a bullet's ballistic coefficient (BC). BC measures a bullet's ability to power through air and wind without changing course. The higher the coefficient, the more accuracy is maintained at longer shooting distances. Generally, a round loses 3% of its BC for every 0.1 loss in gyroscopic stability below factors of 1.5.
Twist Rate FAQs
Q: Are marginally stable twist rates OK?
A: Yes. Even if you invest in a barrel with a twist rate that yields a gyroscopic stability factor of about 0.8 to 1.4, you can still likely shoot with accuracy. As distance increases, however, stability could falter. You may witness one or two keyhole rounds impacting your target as you approach max effective range of the round you're shooting. This indicates bullet spin and stability was lost in flight.
Q: What's a "keyhole" round?
A: A keyhole round is a bullet that impacted its target after it lost spin and stable flight, and began tumbling through the air. The "keyhole" moniker comes from the shape the round makes when it hits the target, resembling that of the keyhole in an old door's lock.
Q: What are the best rates for the AR's calibers, again?
A: Ideal rates for common loads are pulled from our charts above.
- 5.56 NATO, .223 Remington: 1:8 twist
- Supersonic 300 Blackout: 1:12 twist
- 308 Winchester, 7.62x51: 1:10 twist
- Subsonic 300 Blackout: 1:8 twist
- 9mm Parabellum: 1:10 twist
- 6.5 Creedmoor: 1:8 twist
Q: How do I calculate the ideal twist rate for any bullet?
A: The calculation is called the Miller Formula. The formula requires that you know the following data:
- M: Bullet mass in grains
- S: Gyroscopic stability factor (1.5 is optimal)
- D: Bullet diameter in inches
- L = Length in caliber (bullet length / bullet width)
- The equation is written as Twist Rate = Square Root of (30M / ((S * D * L (1 + L^2)).