The design of the AR 15 is brilliant. If you’ve ever assembled one, I’m sure you’ve stopped and thought about just how awesome the system is and how well it works as a rifle. Every single part is important and designed to ensure the gun runs reliably and shoots accurately. One particular part that’s changed more and more as the gun has evolved is the barrel. Make no mistake, it’s changed inside and out. The profile and weight have changed, lengths have changed, and even inside the bore has seen major changes.

The first M16s the military adopted were fielded with 1 in 14-inch twist rates, but this lasted for almost no time at all and they soon adopted the 1 in 12-inch twist rate. Funny enough, that didn’t last very long either. These twist rates are measurements that refer to the rifling of the bore. Rifling refers to the lands and grooves that are driven into the bore via tools. Rifling is spiraled in nature and its purpose is to stabilize the projectile by engaging the projectiles and causing them to spiral.

A spiraling projectile is a stabilized projectile. The best way to visualize it is to picture a football being thrown correctly. Due to its shape, it has to spiral, or it will fly lopsided, inaccurately, and it won't fly for long. Rifling is present in all bores, but not all rifling is the same. It’s measured in turns per inches. A 1 in 14-inch twist rate means the projectile makes one complete rotation in the bore for every 14 inches. This is a slow twist rate, at least when it comes to 5.56. The higher the second number is the slower the twist rate is.

The modern AR 15 market is populated by guns with barrels of all different types. You may be left wondering what twist rate you should go with when choosing a barrel or an upper receiver. It’s a question you must answer based on several factors. These factors include how heavy you want your projectile to be, the velocity the round flies at, as well as the bullet’s construction characteristics. When it comes to all rifles you also must consider bore diameter, but since we are talking 5.56, we already have that established.

The biggest factor is going to be bullet weight. The general rule of thumb is that the heavier the bullet the faster the twist rate. If the bullet is rotating too slow, you’ll see accuracy issues and keyholing due to yaw. If the spiral is too fast the round can rip itself apart. Rounds coming apart are rare though, and only happens when you pair a very light round with a very fast twist rate.

You’ll have to go out of your way to do so. For example, a 35-grain projectile in a rifle with a 1:7-inch twist rate can cause the round to break apart in the air. When it comes to your standard weight 5.56 and 223 rounds it's better to over-stabilize than under stabilize.

Barrel length is also a consideration, but not a major one. The market regulates itself when it comes to stupidity, but in theory, a slow twist in an ultra-long barrel would have the same effect as a fast twist rate in a short barrel. When I say the market does a good job of self-regulating what I mean is you won’t see something crazy like a 7.5-inch barrel with a 1:12-inch twist rate.

At this point, you may be wondering what twist rate to choose, and I’ll make it easy on you. Here are the more common twist rates and what they are typically used for.

This is one you won’t see outside of a custom shop. It’s super slow and is best used to stabilize very lightweight rounds. Remember those 35-grain rounds I mentioned earlier? Well here you go, this is the AR barrel twist rate for them. They can stabilize 55-grain projectiles as well, but that isn’t their niche. This is great for really ratcheting up velocity and getting every foot per second you can.

This is another slow and uncommon twist rate. You’ll still find the barrels for sale, and they are popular with shooters who want to replicate the M16A1 as closely as possible. They are best in their 20-inch configuration when used with 55 to 62-grain projectiles. The 20-inch barrel length is a must have for those 62-grain loads. You’re unlikely to find barrels much shorter than 20 inches with a 1 in 12-inch twist rate.

While 1 in 10-inch barrels exist for 223 barrels, it's not readily available for ARs. This would have to be a custom job most likely. If you went this route, you'll be served by rounds that range from 55 to 69-grain projectiles in a barrel shorter than 20 inches. 16 to 18 inches would be a sweet spot.

1 in 9 is a ‘do it all’ twist rate in moderate length barrels. It can stabilize rounds as light as 40 grains and as heavy as 77 grain. It will stabilize them and allow you to hit the target, but you won't be seeing 1 MOA accuracy. It’s a jack of all trades but master of none. You can safely shoot lighter and heavier rounds just fine, but you’ll see the best accuracy with rounds 55 to 60 grains in most mid-length barrels.

1 in 8 inches is another twist rate trying to do it all in short barrels. From 14.5 to 18 inches the 1 in 8 will get you there with most 50 to 80-grain rounds. It’s fast, but not exceptionally so. A 1 in 8 will be perfect for shooting 62-grain rounds from a slightly shorter than average barrel. It still can stabilize those 55-grain rounds and can even step up to heavier 80-grain loads. However, most 80 grain rounds are too long to feed in an AR magazine.

1 in 7 is where we get the ability to use heavy rounds at various barrel lengths. The sweet spot for this fast rate of twist is 62 grains and up. If you are running a short barrel, like a 10.3, you’ll want this rifling to use the effective 70 grain and up projectiles. This is the current twist rate the military uses in the M4, the M16A4, and the HK 416. It’s perfect for heavy rounds of any velocity and can stabilize heavy rounds extremely well.

Credit: Tactical Life

Stabilization is the name of the game. A stabilized round is not only an accurate round but an effective one. Stabilized projectiles transfer the most energy and ensure it can reach its max effective range. Most common twist types are 1:9, 1:8, and 1:7. More than likely any of these rates will scratch your itch with common rounds ranging from 50 to 77-grain rounds. The best way to decide what rate of twist to pick is to decide what ammo you plan to shoot most often and go from there. It’s not too tricky, and with a hair of research you’ll end up on the right path.