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Twist rate refers to the rate of spin in the rifle barrel, and is represented in inches per turn. It’s important that your barrel has an adequate twist rate to stabilize the bullets you’re shooting. Sometimes ignored or overlooked by inexperienced shooters, this number plays a critical role when determining what ammo to use to achieve optimum performance.
For those new to firearms (or who simply need a refresher), inside the barrel of most centerfire and rimfire rifles and pistols (and all AR-15s), you’ll find a series of lands and grooves cut into the barrel. This is called rifling, responsible for spinning the projectile in order to stabilize it as it travels. Think of how throwing a football with a perfect spiral makes it travel far and fast.
As projectiles come in different weights denoted in “grains” – 55-grain, 62-grain, 77-grain, etc. – bullets of varying weights require different amounts of spin to stabilize it in flight. Finding the right balance of spin for the projectile you’re shooting is where twist rate comes into play.
If a bullet spins too slowly, it won’t stabilize enough to achieve optimum velocity, will yaw in its flight path and be far less accurate. If a bullet is over-stabilized, it could actually self-destruct and fragment in flight (though modern ballistics experts maintain it’s better to have too much twist rather than too little.)
A good rule of thumb to remember is, “the smaller the number, the faster the twist.”
A barrel that is a 1:10” twist means that the rifling will spin the bullet one revolution in 10 inches. The lower the number of the twist, the faster the twist rate or the faster the bullet will spin. For example, a 1:8 twist will spin the bullet one revolution in 8 inches, whereas a 1:10” twist will spin a bullet one revolution in 10 inches. If you were shooting a bullet in both barrels at a velocity of 2800 feet per second, the 1:8” twist barrel (252,000 RPM) will spin a bullet much faster than a 1:10” twist barrel (201,600 RPM). So a 1:10” twist is slower than a 1:8” twist, a 1:12” twist is slower than a 1:10” twist, and so on. Generally, fast twist barrels are used for longer bullets while slower twist barrels are used for shorter bullets.
If a bullet has a twist recommendation of 1:10”, it will be stable when fired from any rifle having a 1:10” or faster. So a 1:9” would work fine, but a 1:11” may not. The recommended twist rate will ensure adequate stability in all conditions, but there are some cases when the bullet may be stable from a slower twist (high altitude for example). Use the twist calculator on this page to calculate a stability factor for any bullet in your conditions to determine the actual twist requirement.
For those new to firearms (or who simply need a refresher), inside the barrel of most centerfire and rimfire rifles and pistols (and all AR-15s), you’ll find a series of lands and grooves cut into the barrel. This is called rifling, responsible for spinning the projectile in order to stabilize it as it travels. Think of how throwing a football with a perfect spiral makes it travel far and fast.
How you do this with bullets is by rifling the barrel. Rifling was invented in Germany at the end of the 15th century, although it really didn’t catch on until the 1800s. However, in America, before the American Revolution, there were a relatively large number of German immigrants who were quality barrel makers and gunsmiths and knew the true value of WHY to rifle a barrel and, more importantly, HOW to rifle a barrel.
Back then, when they started rifling the barrel of a musket, the musket became known as a “rifle.” This led to the American “long rifle” (more commonly called the “Kentucky Rifle”).
The Kentucky Rifle had a devastating effect on the British in the American Revolution because it allowed the Americans to shoot the British at distances far beyond the effective accurate range of the British Brown Bess rifles, which were only accurate to about 80 yards.
Barrels vary greatly between manufacturers and even models in some cases. One of the biggest variables is the AR15 barrel twist, also known as the “rate of twist.”
I get a great number of questions about this from people who are AR shopping, such as:
Common AR-15 barrel twist rates vary between 1:7 and 1:12, but why should you even care what your rate of twist is in the first place? The reason is that different bullet weights perform better with certain rates of twist.
So, if you are just going to go plinking and do short- to medium-range target shooting, you may be happy with 40-grain bullet weights, which would mean a 1:12 rate of twist would be ideal for you. The 1:12 is perfect for lightweight 55-grain bullets, which constitute the majority of practice and plinking 223 ammunition. However, most modern carbine barrels shorter than 20 inches feature faster twist rates, so shooters wanting a 1:12 should stick to the old-school 20-inch rifle barrel.
Most modern AR-15 barrels will come in three twist rates: 1:9, 1:8, and 1:7. The 1:9 barrel is best for stabilizing lighter and mid-weight bullets between 45 and 77 grains. If you are shooting long range, or want more penetration, you want a heavier bullet in the AR, such as a 62- or (preferably) a 77-grain. While 62-grain bullets prefer a rate of twist around 1:8, 77-grain bullets weights favor a 1:7 twist rate.
With AR-15 ammo, it is likely going to be the most common bullet weight, which is 55-grain (a 1:9 twist rate). Ideal rates of twist for a particular bullet weight are somewhat subjective, however, opinions will not vary greatly among the experts.
The M16A2 comes with a 1:7, and the military typically shoot bullet weights from 52 grains up to 77 grains, with 62 grains being the most common in combat. The 1:7 barrel can stabilize up to a 90-grain bullet, and handles 70- to 77-grain projectiles across a wide spectrum of velocities. However, shooters will likely notice a serious deterioration of performance trying to run lighter 55-grain ammo through longer 1:7 barrels, as the light projectile becomes over-stabilized.
Most experts would agree that the best all-around rate of AR-15 barrel twist would be something in the middle such as a 1:8 or 1:9. The 1:8 twist barrel is the most versatile of the bunch, the perfect option for16-inch carbine AR-15 (the most common configuration on the market.) Keith Wood of Guns and Ammo said this barrel can “comfortably stabilize” up to an 80-grain projectile while still providing enough flexibility for shooters to use lighter ammo for matches like 3-Gun.
A downside to this is that it is generally believed that the faster rate of twist means the faster you wear out your barrel. That being said, you should ask yourself what kind of shooting you envision yourself doing and pick a rate of twist accordingly.
Lastly, most manufacturers do not offer a different rate of twist options within a particular AR model. Typically, each model line is going to have a given rate of twist and that’s it.
So, you should know what your intention is with your prospective new AR purchase beforehand, and select a model that has a barrel with a rate of twist that is consistent with your primary goals for the rifle..
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