The Guide to AR-15 Barrels and Twist Rates

This guide covers all the important aspects of the AR-15 barrel: Rifling, forging, and barrel steel, treatments and coatings, and the best barrel lengths and twist rates for the most popular cartridges.

Here's how to install the AR-15 barrel and gas system.

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AR-15 Barrel Types

There are three primary methods of barrel manufacturing in today's black rifle market: Cold hammer forging, cut rifling, and button rifling. Each type has its own pros and cons. These methods are used to finish carbon steel (like 4150) and stainless steel (like 416R) barrels.

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Cold Hammer Forged (CHF)

CHF barrels are rifled by taking an over-sized barrel blank and forcing a shaped tungsten-carbide mandrel through the bore. The mandrel has the shape of the rifling imprinted on it. With the mandrel inserted, the barrel blank is forged into its final shape by very large hammers. This process requires approximately 50 tons of force per hammer.

Cold hammer forging isn't actually done at "cold" temperatures, but rather at room temperature. The immense pressure of the forging process heats up the steel. The finished barrel must be stress-relieved with oven-baking at a few hundred degrees for 1 to 2 hours.

Pros

• CHF barrels typically have longest lifespan.
• Produces an incredibly strong, hear-resistant barrel.
• Creates a uniform grain structure that follows the rifling.
• Provides consistent rifling lands and grooves that are highly accurate.
• Creates incredibly smooth, uniform rifling free of small imperfections.
• CHF is favored by military groups; Often produced by defense contractors.

Cons

• High-end button and cut rifling has the potential to be more accurate.
• High overhead costs for forging machines tends to increase retail prices.

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Cut-Rifled Barrels

Although it's the oldest method of barrel finishing, cut rifling is used for most modern barrels. Cold hammer forging made mass production overall less expensive, but the forging equipment and overhead are costly. So, cut rifling is still favored by many manufacturers.

Unlike CHF barrels, cut-rifled barrels are already forged before the rifling is added. The smooth-bore barrels are attached to a rifling lathe after forging. The lathe pushes a single cutter, attached to a long rod, through the bore, carving each land and groove. Only a few thousands of an inch of steel are removed with each cutting pass. This process is time-consuming, but compared to CHF and button rifling, cut rifling produces the most accurate firearm.

Pros

• Cut rifling produces incredibly accurate barrels.
• Manufacturing process is honed to perfection.
• Top-tier, match-grade barrels are all cut-rifled.
• Creates incredibly consistent rifling profile.

Cons

• More expensive to produce than CHF.
• More time-consuming manufacturing process.

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Button-Rifled Barrels

Button rifling uses a hardened tungsten-carbide "button" attached to a long rod that is forced through the barrel's bore under immense pressure to form the rifling inside. Some makers pull the button through the barrel, while others use a large pneumatic rod to push it through. Because of advancements in cut rifling and cold hammer forging, button-rifled barrels have largely fallen out of favor, especially for the AR-15 market. Virtually all tactical rifle barrels are cold hammer forged or produced with cut rifling.

Pros

• Relatively fast and economical.
• Well-suited for mass production.
• Produces relatively consistent rifling.

Cons

• Button rifled barrels require stress-relieving treatments.
• Rifling button tools are expensive to make and maintain.

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CHF vs. Button vs. Cut Rifling: Which is Better?

First, ignore the 20-page-long arguments on AR-15 forums.

A CHF or cut-rifled barrel will serve you just as well as the other.

Both options provide more accuracy and capability than what the average shooter can achieve. If you want a barrel with the longest lifespan and heat resistant, and if you can afford the higher price, get a CHF barrel. 

If you want a barrel that's as accurate as possible, stick with a cut-rifled barrel. Even cut-rifled barrels, once properly treated with nitride or chrome lining, will withstand immense heat and rapid fire.

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Types of AR-15 Barrel Steels

 There are seven types of steel used to make AR-15 barrels, separated into carbon and stainless steel categories:

• Types of Carbon Barrels: 4140, 4150, and Chrome-Moly-Vanadium (CMV), also called 41V50.
• Types of Stainless Barrels: 410, 416, 416R, and 17-4 PH.

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Carbon Barrels

4140 Carbon Steel

4140 steel is a type of 4000-series carbon steel that contains 0.80% to 1.10% chromium and 0.15% to 0.25% molybdenum. These two elements make the steel much stronger and harder than regular carbon steel. The "41" in the number identifies these two elements. The "40" refers to the 0.40% carbon also added to the steel to make it even harder. This steel maintains a tensile strength of 655 mega-pascals, or 95,000 PSI. For reference, 5.56 cartridges are proofed 125% maximum pressure, or 77,958 PSI. This type of steel is the most affordable but least desirable.

4150 Carbon Steel

4150 steel is also 4000-series steel that contains chromium and molybdenum, like 4140. However, the "50" denotes that it contains 0.50% carbon, which is 0.10% more than 4140. The higher carbon content requires additional stress-relieving treatments. This makes 4150 more expensive and harder than 4140. These barrels have a higher tensile strength of 730 mega-pascals, or 105,880 PSI.

Chrome-Moly Vanadium (CMV)

Chrome-Moly Vanadium, or simply "CMV", is the official mil-spec gun steel developed by Colt Firearms for the military-issued M16 and M4. It contains the same elements as 4150 steel. It also contains Vanadium. Vanadium significantly increases the strength, hardness, and high-temperature stability of the steel. This makes CMV the most suitable steel for producing machinegun barrels for full-auto or rapid fire.

"4150 CMV" Steel 

The label "4150 CMV" is often used to advertise both CMV barrels, and barrels made from 4150 with Vanadium added. The differences between CMV steel and 4150 with Vanadium added is negligible, providing no difference in performance or quality.

What is MIL B-11595?

Some barrels are is even labeled "MIL B-11595." This is not a steel. It's used to identify a proper "CMV" barrel. MIL B-11595 is the military's specification document that lists the elements contained in mil-spec barrels. 

Looking at the spec sheet, you can compare 4150 with Vanadium added, to actual CMV alloy. Both contain generally the same percentages of carbon, manganese, phosphorus, sulfur, silicon, and chromium. CMV might contain 0.05% more molybdenum, though this is again negligible. 

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Stainless Barrels

410 Stainless

410 stainless steel is an incredibly hard alloy that produces sharp rifling and high accuracy. This alloy is more durable than 416 and 416R stainless, offering the longest rifling lifespan of the three. This steel risks developing brittle inclusions or microscopic cracks in extreme cold. Firing rounds through a 410 stainless barrel below freezing temperatures increases the risk of cracking the barrel.

416 Stainless

416 stainless yields the highest "machinability" of any stainless steel. This stainless can be easily cut like a carbon steel barrel, allowing for sharp rifling and consistent accuracy. Unfortunately, 416 stainless is so "workable" because it contains more sulphur than 410 stainless. This higher inclusion of sulphur reduces the corrosion resistance.

We recommend avoiding regular 416 stainless barrels.

416R Stainless

416R stainless is a newer alloy produced by Crucible Industries. 416R boasts high machinability and accuracy, like regular 416. It also also includes molybdenum, increasing its hardness and corrosion resistance. It contains less sulphur reducing brittleness. Crucible Industries says any barrel made from 416R stainless can be safely fired at temperatures as low as -40 degrees (F). 416R barrels are highly resistant to heat and are hardened to 28 RC.

17-4 PH Stainless

Precision shooters laud the advantages of 17-4 PH stainless over 416R and 410 stainless. The "PH" in 17-4 stainless stands for precipitation hardening. This involves heating the barrel, then introducing fine, solid additives, like magnesium, aluminum, titanium, and nickel to the steel's structure. This reduces deformations and warping and produces a finely honed, hardened barrel.

17-4 PH contains 17% chromium. This increases hardness and resistance to heat. It also creates a uniform, smooth finish inside the bore. The inclusion of precipitates allows this barrel to withstand extremely high temperatures without deformation. 17-4 barrels suffer less drift with temperature changes. They preserve accuracy as they heat up and cool down. 17-4 PH barrels are hardened to around 36 to 44 RC.

A 17-4 PH barrel is superior in every way to other stainless and carbon barrels. It offers sub-MOA accuracy out to hundreds of meters. These barrels typically cost 3 to 5 times more than a 416R or CMV barrel.

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Barrel Treatments & Coatings

The method of production and the type of steel used to make an AR-15 barrel are just parts of the total equation. Picking the right finishing treatment and/or coating is just as important, lest you suffer a short barrel life and poor accuracy.

Chrome Lining (Interior)

This age-old process involves coating the inside of the barrel and chamber with chromium. Chrome lining adds to the physical interior dimensions of the barrel, so the bore must be over-sized. Chrome lining creates a silvery finish and it measures a few thousands of an inch. It insulates the rifling and barrel, and it's incredibly effective at reducing the effects of rapid-fire and the immense heat created.

A chrome-lined barrel is easier to clean and it will last longer than a non-coated barrel, usually by a few thousand rounds. With a quality lining, the loss in accuracy usually amounts to just 0.25 MOA, or around 0.25" at 100 meters.

Phosphate/Parkerizing (Exterior)

Chrome lining only protects the inside the barrel, so most lined barrels also get an exterior manganese phosphate treatment, often branded as a Parkerized finish (named after the Parker Rust-Proof Phosphating Company). A chrome-lined-and-phosphate-coated treatment is the most common type of treatment applied to mil-spec barrels and most current, military-issued rifles. A phosphate finish is incredibly resistant to heat and the elements, but it is also slightly porous. It requires occasional oiling to effectively prevent corrosion.

Nitride/Melonite (Interior & Exterior)

Shooters who want the latest hotness should stick with a nitride finish instead. Also called a Melonite finish (which is a brand that performs this type of treatment), the nitride finish effectively coats both the exterior and interior of the barrel. Unlike chromium and manganese phosphate, nitride doesn't "coat" the barrel. Instead, it penetrates the steel structure itself, attaching nitrogen atoms to the atoms in the steel.

This process doesn't add any physical material to the rifling, so the barrel in question doesn't need to be specially fabricated to accommodate this treatment. Nitride is applied by quenching, polishing, and then quenching the barrel a final time in a liquid salt bath. The nitrogen thus penetrates the steel by a few thousands of an inch. A nitride treatment is technically more effective than a chrome-lined finish.

In fact, nitride-treated barrels can withstand direct exposure to water, heat, salt, and corrosive elements better than any other barrel treatment or coating. Nitride is also non-porous and doesn't require any oiling or extra coating to prevent corrosion. A barrel treated with nitride boasts a surface hardness of around 60 HRC, making it the same hardness as hardened tool steel.

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AR-15 Barrel Twist Rates

Twist rate describes how often the lands and grooves of your barrel's rifling rotate 360 degrees. Twist rate greatly affects accuracy. It also determines what loads you'll fire.

How Rates Are Measured

Twist rates are measured in inches per full rotation, expressed as ratios. They're written as "1:7" or "1:8". This describes how many inches the rifling travels down the barrel before it completes one full rotation. For example, "1:7" means the rifling rotates once every seven inches. The smaller the second number, the more often the rifling twists.

Twist and Bullet Weight

Twist rate determines how heavy or light your bullets will be. Loads are advertised by grain weight, like "55-grain" or "55-gr". The heavier the bullet, the higher the grain count. Heavier loads require faster twist rates, or more spin, to stabilize the round before it exits the barrel. Lighter loads require slower twist rates, or less spin.

Twist and Barrel Length

Twist rate is not related to barrel length. Barrel length doesn't determine which twist rate is best for any cartridge. If a cartridge performs best with a 1:7 or 1:10 twist, that rule always applies, even if those cartridges are fired from a 7.5" barrel, 16" barrel, or a 24" barrel. Barrel length only affects velocity, not the spin of the bullet.

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Twist Rate vs. Accuracy

Twist rate is important for accuracy. You will have to compromise on twist rate to achieve good accuracy with varying grain weights. A rate that's optimal for a lighter bullet may fail to stabilize a heavier load. And a heavier bullet's optimal twist rate could over-stabilize a lighter bullet, decreasing accuracy.

So, how do we determine the best twist rate?

Gyroscopic Stability

Gyroscopic stability factor (GSF) indicates what twist rate is best for a bullet's weight. GSFs show whether a rate is too fast or too slow for a particular load.

• GSFs below 1 are unstable, and might tumble.
• GSFs between 1.1 and 1.3 are marginally stable.
• GSFs of 1.4 to 2.0 are optimal for good accuracy.
• GSFs of 1.5+ are best for long-range shooting.
• GSFs of 2.1 to 3.5 may cause spin drift at long distances.
• GSFs of 3.6 and above may cause drift at shorter distances.
• GSFs greater than 6.0 may cause the bullet to tumble.

Twist Rate Chart Example (5.56 NATO, .223 Remington)

Based on the chart above, we see the optimal twist rates for various 5.56 and .223 loads:

• 45-gr: 1:12
• 55-gr: 1:9
• 62-gr: 1:8
• 77-gr: 1:8
• 80-gr: 1:7
• 90-gr: 1:7

Let's look at the best twist rates and barrel lengths for the most popular AR cartridges.

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AR Cartridges: Best Twist Rates & Barrel Lengths

5.56 NATO / .223 Remington

We already covered twist rate for these cartridges above, so let's focus on barrel length.

Best Barrel Length

A 16" barrel is best for 5.56 and .223. It provides the most effective velocity in the shortest length possible. Adding any more barrel past 16" provides little extra velocity, but plenty of extra weight. This is why U.S. Military went with a 14.5" barrel when switching from the M16 and its 20" barrel.

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300 AAC Blackout (Supers & Subs)

The 300 BLK is affected by barrel length and twist rate more than any other AR-15 load. That's because it actually provides two cartridges in one casing: Light supersonic loads, and heavy subsonic loads.

Best Barrel Length

Both supersonic and subsonic loads do well with a 9" to 11" barrel. This is the shortest length you can install while achieving nearly maximum velocity. Any length beyond 11" will only marginally improve velocity.

Best Twist Rate

We can see the challenge of picking a good twist rate for 300 BLK: Supersonic loads demand a slow twist rate that can't stabilize subsonics. Subsonic loads need fast twist rates that can over-stabilize supersonics.

The 1:8 twist rate works best if you want to shoot heavier (150-gr) supers and subs with good stability. The 1:8 rate also works best for a subsonics-only gun.

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6.5 Grendel

Best Barrel Length

The 129-grain Hornady SST, used to produce the velocities in this chart, reports an optimal muzzle velocity of 2,238 FPS, and an effective range of 800 yards.

Between all Grendel barrel lengths, these are its actual muzzle velocities:

• 14.5" Barrel: 2,238 FPS
• 16" Barrel: 2,310 FPS
• 20" Barrel: 2,391 FPS
• 24: Barrel: 2,470 FPS

The optimal muzzle velocity is achieved with the shortest tested barrel, 14.5". The 24" barrel provides just 10% more velocity. Between all lengths, the differences in bullet drop are also insignificant out to 800 yards. Obtaining an NFA tax stamp just to install a 14.5" barrel on a Grendel-chambered AR seems pointless. So, we recommend sticking with a 16" barrel. You should only consider a 20" or 24" barrel if you're trying to reach 1,000 yards.

Optimal Twist Rate

One twist rate is effective for generally all 6.5 Grendel loads: 1:8. This rate provides the best gyroscopic factors for the heaviest and most accurate loads (130 grains) without over-stabilizing the lightest (90-grain) loads.

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7.62x51 NATO / .308 Winchester

Consider .308 Winchester and 7.62 NATO identical for the sake of barrel length and twist rate. They use the same bullet weights and powder.

Best Barrel Length

7.62 NATO and .308 Win achieves maximum effective velocity with a 20" barrel. Increasing barrel length to 24" provides less than 3% more velocity.

The Best Twist Rate

The gyroscopic factors show that a single twist rate works or all loads, which is 1:12. This provides the best stability for light 147-grain loads and heavy 180-grain+ loads.

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6.5 Creedmoor

The 6.5 Creedmoor AR is a long-range cartridge chambered in the AR's .308 platform. The 6.5 Creedmoor requires the longest barrel of any AR cartridge. So, finding out the shortest possible length is critical if you don't want a rifle that's as long as you are tall.

Optimal Barrel Length

Thanks to RifleShooter for this data.

Ballistic data shows the Creedmoor cartridge begins to slow down if you go with a barrel that's too long. That happens at 25" and continues up to 27". So, a 24" barrel is best.

Optimal Twist Rate

The Creedmoor cartridge requires the same 1:8 twist rate for all its popular loads.

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9mm

Barrel length plays a critical role in in the AR9. It determines whether your loads will remain subsonic, or achieve supersonic velocity. For a cartridge to remain subsonic, it must exit the muzzle below 1,125 FPS.

Best Barrel Length

Our 9mm velocity chart identifies optimal velocities with the shortest barre length, highlighted in green. Velocities highlighted in yellow show that adding any more barrel length decreases belocity. Across all loads, a 9" barrel is most effective for supersonic stability.

Want to go subsonic with your AR9?

You'll need a 5" barrel to keep 147-grain loads subsonic, or a 4" barrel to get the lightest (115-grain) loads subsonic.

Best Twist Rate

All 9mm loads require one twist rate, and that's 1:10. This twist rate provides good stability from 90 grains up to 147 grains.

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Summary

You could write a book on AR-15 barrels, and we nearly did here. Here's a summary of the important stuff:

• AR-15 barrels are made by cold hammer forging and cut rifling.
• Most carbon barrels are made from 4150 or CMV. Most stainless barrels are made from 416R.
• If you want a mil-spec barrel, look for one labelled "4150 CMV", "4150 V", "41V50" or "MIL B-11595".
• Nitride coatings provide the best performance, while chrome-lining and phosphate work well for rapid fire.

Here's a recap of the best barrel lengths and twist rates for each caliber:

• 5.56/.223: 16" barrel, 1:8 twist.
• 7.62/.308: 20" barrel, 1:12 twist.
• 6.5 Creedmoor: 24" barrel, 1:8 twist.
• 6.5 Grendel: 16" or 20" barrel, 1:8 twist.
• 300 BLK: 9" to 11" barrel, 1:8 or 1:7 twist.
• 9mm Para: 5" to 9" barrel, 1:10 twist.