Rifling Twist Explained: Why Your AR-15 Is Inaccurate

You bought your first or next rifle and gleefully head to the range for some much-needed RnR and, hopefully, get your new purchase sighted in. You brought along good quality ammunition and–from the bench–you are able to walk your shots into the bullseye, adjusting the sights accordingly. Then you proceed to go for groups only to find that your new AR-15 doesn’t shoot straight.

Maybe its the ammo? You switch to premium match grade ammunition but the results are the same. Instead of a golf-ball sized group, you get a grapefruit. This can happen for a few reasons, but the most obvious and easiest to remedy is to check your rifling twist rate and see if it is compatible with the grain weight of your ammunition. Here is a rundown on rifling twist rate, how it affects accuracy, and which rate may work best for you.

Rifling Twist: An Old Spin on Rifle Accuracy

Rifling is a series of spiral grooves inside the barrel that spin the bullet before it leaves the barrel. This spin stabilizes the bullet in flight through centripetal force. That spin also balances out minor imperfections in the bullet that would otherwise make it go astray. But how fast the rifling grooves turn inside the barrel has a direct impact on accuracy.

Depending on grain weight and profile of the bullet, a given rifling twist might be too slow to stabilize the projectile. In that situation, the bullet will not have spun up enough to fly true once it leaves the muzzle. In a worst-case scenario, the bullet will keyhole (tumble in flight) and strike a target lengthwise instead of on the nose–if it hits the target at all. At best, the bullet hits the target more or less nose first but with unreliable accuracy.

Twist rate: Know your numbers

With any rifle, it is possible to achieve a bullet and rifling combination that simply will not stabilize. For example, hunting rifles chambered for the .243 Winchester arrived from the factory with a 1:10 twist rate (or 1 revolution in 10 inches of barrel). This served well as the .243 was intended as a dual varmint/deer cartridge and could stabilize rounds ranging from 58-100 grains. Target shooters have come to love the .243 Winchester, but heavier, more aerodynamic projectiles are hard to stabilize. Those who like the .243 will use a custom barrel with 1:8.5 or faster twist rate 115 grain rounds.

Paradoxically, it is also possible to over stabilize a projectile. Fast twist muzzleloaders using 1:30 or faster rates work well with conical shaped bullets but not round balls, where a much slower twist rate (like 1:66) can achieve better accuracy.

The Changing Twists of the M16/AR-15 Platform

The original M16 platform as it debuted in late 1963 in the .223 Remington cartridge used a 20-inch barrel with a 1:14 twist rate. In these early days, a version with a 1:12 twist rate was fielded. Quickly, a 1:12 twist rate became standard with the M16 and M16A1 platforms. Both rifles used 55 grain bullet weights.

But the desire for extended range led to the development and adoption of the M16A2 in 1983. The platform uses the 5.56x45mm NATO cartridge and has a 1:7 twist rate. The 1:7 twist rate paid dividends when going beyond standard 55 grain M193 ball ammunition, as this faster rate could stabilize the new M855 and SS109 armor piercing and tracer ammunition, which used a 62 grain bullet. The same twist rate has endured the introduction of the M4 platform and the later introduction of the Mk 262 77 grain precision round in 2002.

AR-15 Rifling Twist Rates: Best Practices

On the civilian market, the AR platform is available in all the aforementioned rifling twists as well as a few more. 1:14 twist rate rifles are highly unusual to find. Many budget AR-15s come with a 1:8 or 1:9 twist, while 1:7 rifles run the price gamut. Most rifles have the rifling twist rate stamped on the barrel. If that is not present, you can swab the bore with a cleaning patch and cleaning rod. Count every revolution and take it into account with the barrel length of your rifle to find the twist rate. When all else fails, try a variety of ammunition and see which one your rifle agrees with most.

Although all 5.56 NATO and .223 Remington ammo will work in the AR, the twist rate does limit bullet selection for downrange accuracy. 1:9 twist rifles work well with lightweight 45-50 grain varmint loads and conventional 55 grain ammunition. In my own shooting using my Smith & Wesson M&P 15 with a 1:9 twist, I can achieve groups just over one inch at 100 yards using 55 grain FMJ boat-tail ammunition.

Switching to 62 grain ammunition widened my results to 3 inches. With 77 grain match loads, the group is measured in feet, not inches. Depending on what you are looking to shoot, this sort of accuracy will not suffice and 55 grain ammunition may be light for the task. 1:8 twist guns are not as common, but work well with those lighter-grained rounds up to 62 grain ammunition. 1:7 twist rifles are the Goldilocks option, since they will stabilize any conventional 5.56 or .223 Remington round to be found.

Which twist rate is best for an AR?

For a do-it all AR, which is why most of us own AR-15s, the 1:7 and 1:8 twist rates are common. The 1:8 is a standard for many manufacturers of entry-level barrels and rifles and works well with 55 grain bullets–the most common weight for AR ammo. Test out heavier 62 grain or 77 grain ammo.

The rule of thumb is this. Heavy rounds need faster twists. Lighter rounds need slower twists. What do you want your gun to do?

Once you know what you want (like a do-it all rifle, a rifle for long-range punch, a fast accurate varmint gun, a suppressed SBR…) you need to pick out a variety of ammuntion and see what works best. Start with boxes of different makes, grain weights, and bullet types. Find the right one, and then buy in bulk from someone that specializes in ammo like AmmoToGo.

Take A Spin with Rifling Twist

As with any rifle, it is easy to get bogged down with AR-15 upgrades and accessories. But before you slap on that new handguard or even loop in a sling, it bears knowing what kind of ammunition your rifle is likely to shoot well. While most rifles are more accurate than the one squeezing the trigger, equipment does matter. Understanding rifling twist is key to making sure you get your hands on the best possible ammunition and rifle for the tasks you anticipate.