What Are The Best AR-15 Barrel Twist Rates?

Whether buying a new rifle, building one yourself or selecting the right ammunition for one you already own, it is important to understand barrel rifling twist rates and how they affect accuracy. Most .223 Remington, .223 Wylde and 5.56x45mm NATO barrels these days come with either a 1-7, 1-8, or 1-9 inch twist rate, however, there have been many iterations over the years. This has led to arguments over which is the correct choice and a common misconception regarding the driving factor behind choosing a proper twist rate. I’m going to address all of these topics in this article.  

What are barrel rifling twist rates?

Before we go any further, and to benefit the uninitiated, what exactly is a barrel rifling twist rate and what do these numbers mean? Rifling consists of helical grooves machined into the interior of the barrel. Rather than being machined straight, they are machined to impart a spin to the projectile to stabilize it in flight, thus increasing its accuracy. How fast the projectile spins depends on the twist rate of the rifling. This is measured by how many inches it takes for the rifling to complete one full revolution. For example, a barrel marked “1-7 inches” means the bullet is being spun by the rifling at a rate of one rotation per every 7 inches traveled. It can also be expressed as “one turn in 7 inches.” This indicates the lower the second number (“1” is its base) the faster the twist rate. For example a 1-7 inch twist rate is faster than a 1-12 inch twist rate. Without the stability imparted by the spin via the rifling, the projectile would wobble and yaw in flight without a predictable or repeatable trajectory.

Why have there been so many different twist rates over the years?

Over time, the .223 Remington and 5.56x45mm NATO cartridges have evolved with projectiles increasing in length and weight to meet certain requirements. These longer projectiles then required faster twist rates to properly stabilize them. A brief history lesson brings us back to the 1-14 inch twist rate of the original ArmaLite AR-15 barrel in the late 1950s. While made famous for its combat performance in DARPA field testing of the Colt Model 601 AR-15 in Vietnam, testing showed this slow twist rate would not effectively stabilize issue 55-grain ammunition in extreme cold conditions. The slower 1-14 inch twist barrels were changed to a slightly faster 1-12 inch twist on subsequent Colt Model 602 rifles and this would become the standard on XM16E1, M16 and M16A1 rifles. In 1983 the USMC adopted the M16A2 with its faster 1-7 inch twist, and the U.S. Army followed their lead in 1986. The faster twist was needed for the new, fairly long, 5.56x45mm NATO 64-grain M856 tracer ammunition, which was adopted along with the 62-grain M855 ball round. Since then the 1-7 inch twist rate has dominated U.S. military 5.56x45mm NATO applications, including the M4 carbine series, MK12 series and shorter variants like the MK18. The 1-7 inch twist rate is also present in the M249 Squad Automatic Weapon. 

The great misconception…

Ask most people about choosing a twist rate and you’ll likely head down a road talking about bullet weights. You might be headed in the right direction, but you wouldn’t know why. The truth is that bullet length is the variable by which twist rate is determined, with added weight being a symptom of the projectile getting longer in many, but not all cases. The most obvious example of this is a solid copper bullet will be substantially longer than a similar weight projectile with a lead core. You’ll see charts and graphs, read articles and watch videos that discuss twist rates in terms of bullet weight, but if you want to know what factor really calls the shots, know your projectile lengths. Pun intended. 

Okay, not too slow, but not too fast either, right?

Experience tells us that an under-stabilized projectile will severely degrade accuracy. Naturally, conventional wisdom suggests the same outcome from over-stabilized projectiles. While this sounds reasonable, modern ballisticians have debunked the over-stabilization theory as results differ in practice. Because of this, it is better to have too much twist than not enough, and I’ll explain why. 

Why heavier projectiles typically reign 

Longer and heavier projectiles require a faster twist rate and have greater sectional density. The heavier a bullet, the higher its sectional density, and the more momentum it will retain in flight and penetration. Don’t confuse sectional density, however, with ballistic coefficient, which is not just based on mass, diameter, and length, but also the shape of a projectile. Take different bullets of equal weight in the same caliber and they will always have an equal sectional density. The bullet design having a higher ballistic coefficient such as a sharper tip, steeper tapering of the ogive, and a boat tail, will slice through the atmosphere encountering less friction, and delivering the projectile with a higher retained velocity. Mass times speed equals momentum, which means better flight, a higher retained energy on impact and (depending upon projectile design) deeper penetration. This is how sectional density and ballistic coefficient work together to achieve terminal performance. Longer, heavier projectiles with a high ballistic coefficient will deflect less in the wind and maintain a more consistent trajectory in unfavorable conditions. This advantage becomes increasingly tangible at longer engagement distances. This is important with precision rifles like the MK12 series, carried by snipers and designated marksmen and meant to maximize range and precision while maintaining caliber, magazine and general parts compatibility with rifles carried by other teammates. Incidentally, typical 55-grain M193 style FMJ ammunition relies heavily on speed for fragmentation and thus terminal performance. This becomes a problem in shorter barrel lengths like the 10.3-inch barrel of the MK18. In contrast, the Sierra 77-grain OTM projectiles loaded in MK262 Mod 1 can reliably expand and deform at much lower velocities, increasing terminal performance in close-quarters when fired from short barrels. This has something to do with bullet design, but length and sectional density play a big role as the bullet's shank continues, providing deeper penetration and improving terminal performance.

Where do these twist rates fit in?

If you are looking for an ideal twist rate for a specific round you intend to shoot through the life of your barrel, perhaps a tailored approach might be the way to go, however, most shooters are likely to shoot multiple types of ammunition throughout the life of their rifle and may be looking for a best “do it all” twist rate instead. The good news is that there are answers for both of these scenarios. The “old school” 1-12 inch twist rate fielded on the M16A1 and on many older bolt action sporting rifles works well with the original-style 55-grain bullets they were designed for. Typically 45, 52 and 55-grain bullets prove quite accurate. Many AR-15 owners only shoot economical 55-grain ball, and the 1-12 inch twist is ideal for this. If you want to shoot longer and heavier projectiles, though, you are going to need a faster twist. In the “old days” the 1-9 inch twist rate was viewed as a universal twist by manufacturers. It was faster than the old 1-12 inch twist and allowed the use of more modern bullet designs. Heavier 68-grain Hornady HPBTs and 69-grain Sierra MatchKings will typically shoot well from this twist rate. These more efficient projectile designs with higher ballistic coefficients allowed rifleman to reach farther with their AR-15s in competition. This twist rate will stabilize and provide good accuracy with 55-grain M193 FMJ ammunition with its 0.745-inch projectile length and will shoot many heavier and longer bullets as well, including longer 62-grain M855 ball which, with its steel core penetrator measures in with a projectile length of 0.907 inch. 

Will 1-9 inch twist barrels accurately shoot heavier 75 and 77-grain bullets? Typically no, they will not stabilize, and accuracy will be poor. To quote Firearms News Field Editor Michelle Hamilton, “Ultimately, every barrel is unique, but rule of thumb, a 1-9 inch twist barrel is best with 50 to 69-grain bullets, typically.” However, it is possible to come across a 1-9 inch twist AR-15 which will shoot 75s or even 77s accurately. Firearms News Executive Editor David Fortier experienced that first-hand while shooting NRA High Power, witnessing a 1-9 inch twist 20-inch AR-15 which shot very well at 600 yards with Hornady’s 75-grain OTM load. Typically though, I would not expect a 1-9 inch twist barrel to perform well with a bullet with a length much longer than the 0.907-inch long 62-grain M855. However, many riflemen never shoot anything but 55 and 62-grain ammunition from their AR-15, and a 1-9 inch twist works just fine for this. The 1-8 inch twist rate barrels can be versatile, stabilizing M193 and M855 ammunition as well as the various 68 and 69 grain loads. It also has enough spin to stabilize more modern longer and heavier 73, 75 and 77-grain projectiles like Sierra’s famous 77-grain MatchKing measuring 0.994 inch in length.

This barrel twist rate became popular among many competitive shooters and David Fortier competed in NRA High Power with a 1-8 inch twist Les Baer Ultimate NRA Match rifle using 80-grain Sierra MatchKings and 80-grain Bergers. These were loaded long and had to be single-fed during the Slow Fire event at 600 yards, but he says they shot very well and beat the 77s in the wind. This could seem like a clear winner, but remember what I said about over-stabilization. 1-7 inch twist rate barrels are the U.S. military standard and will stabilize MK 262 Mod 1 77-grain projectiles easily, and based on what we already know, will do fine with shorter-length projectiles like the older 55-grain M193 and 62-grain M855 ball rounds as well as the current issue 62-grain M855A1. Plus, it will stabilize 68s, 69s, 73s, 75s, 77s and even the 80 grain bullets. In addition, a 1-7 inch twist rate is enough to stabilize Sierra’s 90-grain projectiles with their 1.171-inch bullet length. In fact, the only 0.224 caliber Sierra projectile I could find that’s not optimal with a 1-7 inch twist is the 95-grain HPBT Matchking, with a length closer to that of some 0.308 inch projectiles at 1.310 inches. 

Economy of Scale

Another factor to consider is the types of barrels available with different twist rates. For example, while you may find a rifle with a 1-8 inch twist rate barrel marketed for precision applications, you are far less likely to encounter one with a cold hammer-forged barrel designed for longevity and durability. This is because 1-8 inch twist barrels are typically marketed where accuracy rather than durability is the deciding factor. You will have different options to choose from such as economical broach cut barrels, expensive match grade barrels with cut rifling and very durable cold hammer-forged barrels. Then you have different bore coatings, such as Melonite or hard chrome, to consider.

Which do I choose?

All of my 5.56x45mm NATO chambered rifles are of the 1-7 inch twist variety. Most of this has to do with the rifle manufacturer. For example, I own a couple of FN SCAR 16s, originally introduced as a part of the military Special Operations Forces Combat Assault Rifle program. As these were geared toward military applications, a 1-7 inch twist cold hammer-forged barrel is the only factory option whether you choose to run a 10, 14 or 16 inch variation. The same can be said for my BCM rifle which is built to military specifications and features a 1-7 inch twist cold hammer-forged barrel. Even if I were to build a custom rifle, I would preserve the ability to maximize accuracy and stability using 77-grain match-grade ammunition, opting for a 1-7 inch twist barrel. This setup provides the most flexibility within the common commercial range of ammunition without sacrificing performance where I demand it the most.  A 1-7 inch twist is a wise choice for maximizing accuracy out of those more expensive heavier rounds where performance is more often scrutinized. At the same time, it is perfectly suitable for lighter bullets including the common and less pricy 55-grain M193 and 62-grain M855 ammunition, where expectations are more tempered and any effect from too much spin is likely to go unnoticed within an acceptable margin of error.

Final Thoughts

While matching the twist rate of your rifle or carbine to the appropriate ammunition is only part of the puzzle and does not guarantee accuracy, ensuring your rounds are properly stabilized is key to achieving optimum performance. Most barrels have their own particular character and may like one type of projectile more than another, with ultimate accuracy nodes able to be established through detailed load development, if you’re into that sort of thing. Accuracy is ultimately a matter of multiple factors and disciplines, but I hope the information in this article now has you thinking about twist rates in terms of projectile length (not just weight), and that it will help you decide which twist rate best suits your application.