Barrel twist rates!
In this section, bullet stability is the name of the game! And stability is determined by the rate of twist in the barrel…well, it’s one of the determining factors, anyway.
In layman’s terms, the twist rate is the number of revolutions the rifling makes inside the barrel.
Think of a Slinky (younger folks, look it up). The spiraling of the toy is what it’d look like if you took away the outer metal of the barrel and only left the lands and grooves inside. You can stretch it out so that the spirals appear farther apart, or mash it together and the spirals will compact. The spirals in a Slinky are the revolutions we’re talking about.
The more times the spiral makes a full revolution, the lower the twist rate. For example, a 1:9 (to be read “one in nine”) twist rate means that, for every 9 inches, the rifling makes one complete revolution inside the barrel.
When it all comes down to it, we’re really talking about stabilizing bullets. That is, taking into account the air that the bullet must pass through and figuring out how long the bullet will remain stable.
The numbers of revolutions made inside a barrel determine the stability of the bullet. So, the lower the number, the more (and sometimes faster) the bullet is spinning, the more stable the bullet is.
Now, most AR shooters are familiar with the 1:7 and the 1:9 twist. In fact, most AR barrels can come with twist rates between 1:6 to 1:12.
For example, in our store, the:
– 7.5” 5.56 AR-15 pistol barrels come in a 1:7 twist
– 10.5” 5.56 AR-15 pistol barrels come in a 1:9 twist
– 16” 5.56 AR-15 rifle barrels come in a 1:8 twist
Note: when it comes to talking about barrel twist rates – BARREL LENGTH DOES NOT MATTER. I listed the examples above to show you that IN THIS INSTANCE (talking about twist rates), barrel length doesn’t determine anything.
A manufacturer could have a 2” barrel with a 1:12 twist rate if they wanted.
So why bring barrel lengths into the discussion? Because that’s one of the first things most shoppers look for when they pick up a barrel in our store. And if you are unsure about what you’re looking at, it may lead to confusion.
Speaking of confusion, we could also go into the various rates, which length of bullet is best with which twist rate, etc. But that doesn’t say a lot – especially if you’re a novice when it comes to AR-15 building (or just want to expand your knowledge).
For the beginning AR-15 builder, a table is a great down-and-dirty way to understand twist rates without a degree in astro-physics.
As with everything else in this series, it all boils down to what you’ll do with the gun.
If you’re plinking, a 1:9 twist is fine. In fact, this is the default twist that most AR manufacturers choose for the barrels on their off-the-shelf firearms. Most 55 grain 5.56/.223 ammo will shoot very well in a 1:9 twist barrel.
However, if you have any inkling of doing anything else with your AR (hunting, for example), then go with a mil-spec 1:7 barrel. That way, you can shoot some of the heavier hunting rounds without having to spend the money to purchase another barrel.
Some of the high-end barrels even start with a 1:12 at the base of the barrel (to prevent deformation of the bullet) and “walk” up to as much as a 1:6 twist to get the extra spin on the bullet as it leaves the barrel.
If you’re a precision shooter, or are looking to get into competition shooting, then choosing the right twist rate for your AR barrel comes from knowing what will be the longest bullet you’ll fire.
Take time to consider your reasons behind the barrel purchase (including long term ideals) and you’ll find yourself owning a barrel you can use and enjoy for many, many years to come.
But once you’ve narrowed down the twist rate, material, rifling, hardening, and finishing you want on your barrel – you’ve still got to figure out which chamber will be best for you!