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AR 308 [AR10] Barrels

AR 308 Barrels Frequent Asked Questions

416R Stainless steel is used extensively by manufacturers for their match grade precision barrels. It is the highest grade of stainless steel available, specially formulated to function in Arctic cold conditions down to -40 F.

416R stainless is ideal for machining very precise and polished rifling, chambers, and crowns into the barrel to maximize consistency and accuracy potential. 416R stainless barrels may not match the longevity and toughness of 4150 CMV barrels.

4150 Chrome Moly Vanadium steel is the proven military specification steel type. The barrels in M4 carbines and M240 machine guns are constructed from 4150 CMV steel.

It is difficult to produce a 4150 CMV barrel offering the same accuracy potential as a 416R stainless barrel made using the same manufacturing techniques.

Carbon Fiber may be used in exotic precision rifle barrels as a wrap surrounding the metal barrel. Traditionally used in race cars and aircraft, this material offers incredible strength and lightness and is used to tension and stiffen the barrel for added accuracy without adding significant weight.

It is important to note that a carbon fiber barrel’s rifling is still made of 416R stainless—the bullet never touches the carbon fiber material. Carbon fiber barrels come with a significant price increase and may be more fragile than 416R or 4150 CMV barrels—carbon fiber does not dent or bend, it shatters!

Button rifled barrels are made by pushing an extremely hard tool called a button through the bore (hole) running down the middle of the barrel blank.

The button rotates at the correct twist rate as it travels down the barrel blank under tremendous pressure. Button rifled barrels are sometimes referred to as button cut but this is incorrect.

With the barrel blank at red hot temperature, the button tool actually displaces the metal, hardening and smoothing the rifling as it passes through, leaving a mirror smooth surface behind it.

Many of the most accurate barrels in the world are button rifled, especially if the manufacturer is a master at stress-relieving the finished product. Button rifled barrels that are not correctly stress relieved tend to exhibit shifts in bullet point of impact as they get hot.

Cold Hammer Forged barrels are made by inserting a carbide mandrel into the bore of the barrel blank.

Think of the mandrel as an incredibly hard drill bit in the exact shape of the rifling and chamber. The mandrel does not move at all inside the barrel blank as it slowly passes through an enormous machine that hammers it simultaneously from all sides at up to 50 tons of pressure per strike.

At room temperature, millions of hammer hits squeeze the metal barrel blank against the mandrel with incredible force. When the barrel comes out the other side of the machine it is significantly longer and the diameter has narrowed.

Removing the mandrel reveals the shape of the chamber and rifling inside. The cold hammer forging process results in extremely hard, tough barrels that can withstand intense heat and abuse.

Cold hammer forged barrels also benefit from a long service life as they are very wear resistant.

5R rifling is one of the latest developments in precision rifling technology. Instead of the conventional 6 or 4 symmetrical lands and grooves found in traditional rifling, 5R rifling uses 5 lands and grooves arranged so that each land (raised section of the rifling), is positioned directly opposite a groove (low section of the rifling), in the barrel.

The shape of how each land transitions to the adjacent groove is also subtly changed, using a more gradual slope rather than the traditional 90-degree angle.

5R rifling is intended to deform the bullet less than traditional rifling as it spins down the barrel. Bullets that are less deformed by the barrel rifling will fly more consistently shot after shot, aiding accuracy.

5R rifling is also significantly easier to clean due to the reshaped transitions, which help prevent fouling from building up and sticking in the corners of the rifling.

An alternative form of rifling adopted most famously by Glock and Heckler & Koch, but also found elsewhere.

Polygonal rifling abandons traditional lands and grooves entirely in favor of a twisting hexagon or octagon shape that engages the bullet more uniformly around its entire diameter.

Looking down a polygonal rifled barrel, the absence of traditional lands and grooves appears stark as the polygonal shape forms a very smooth overall swirl pattern. Polygonal rifling allows less propellant gas to escape around the base of the bullet, maximizing velocity.

Like 5R rifling, polygonal rifling deforms the bullet less than traditional rifling and is very easy to clean. Polygonal rifled barrels are also known to enjoy a boost in service life compared to other forms of rifling.

Parkerized barrels were first manufactured on a large scale for US military use during World War Two. This corrosion resistant finish is still the military specification standard today.

The barrel is submerged in a bath of phosphoric acid with high manganese content at a specific temperature range. The process changes the chemical composition of the surface metal and leaves a relatively thick coating of phosphate permanently adhered to the barrel.

Phosphate coated barrels can be easily spotted by their grey colored finish. Phosphate finishes make a great foundation to apply paint, Ceracoat, Duracoat or Norrell’s Moly Resin on top of, because the spray-on coating grabs onto the rough phosphate instead of peeling off.

It is impossible to phosphate coat stainless steel, so stainless barrels are either used bare or with another type of finish applied. In military type barrels, phosphate coating on the exterior is often combined with chrome lining of the interior chamber and rifled bore.

These barrels are submerged in a bath of alkali cyanate at relatively high temperatures. The hot salt bath thermally reacts with the barrel’s metal, creating a thin but extremely hard, corrosion and scratch resistant nitrogen-based finish that is uniform and shiny compared to phosphate.

Variations of the salt bath nitride process are used in a huge variety of firearms and parts and are referred to as Melonite, Tufftride, Tenifer, Ferritic Nitrocarburization, and more.

This process works well on many types of firearms grade steel and is often used to add an extra layer of corrosion and wear protection to stainless steel rifle barrels.

These barrels are submerged into a liquid chromium solution, with an anode run through the barrel. When the anode is electrically charged, chrome bonds to the inside of the chamber and the surface of the rifling.

The resulting chrome lining is typically a few thousandths of an inch thick.

Chrome lined, phosphate coated barrels are still US military issue. Because the rifling must be made oversized, with the addition of chrome plating then adding the last bit of surface material, the resulting rifling will not be as polished or consistent as bare stainless steel or salt bath nitride barrels.

Therefore, compared to the match grade barrel options, chrome lined barrels suffer a very small accuracy penalty in exchange for their excellent service life and corrosion resistance.

.308 Winchester is the caliber Eugene Stoner designed the original AR-10 to use way back in the 1950s. This versatile caliber has a long and rich history of proven performance worldwide in a variety of situations and applications.

Although there are minor technical differences, rifles chambered in .308 Winchester can also safely shoot 7.62 NATO specification ammunition, and vice versa.

Many shooters will casually plink using less expensive 7.62 NATO ammo, then switch to more expensive, dedicated match grade or hunting ammunition as needed. Loadings for various .308 Winchester applications use an enormous variety of bullet types ranging from 125 to 200 grains in weight.

Every major ammunition manufacturer in the world produces at least one .308 Winchester product.

6.5 Creedmoor was introduced in 2007 as a target shooting caliber suitable for precision target engagement past 1,000 yards. 6.5 CM features an excellent ballistic coefficient, giving it much less bullet drop and less wind drift relative to .308 Winchester at medium to long range.

With common barrel lengths and velocities, 6.5 CM will stay supersonic and stable past 1400 yards. 6.5 CM is also an excellent hunting round, as it retains energy extremely well even at long range.

There is no cheap imported or military surplus 6.5 Creedmoor available, so building a 6.5 Creedmoor rifle comes with a commitment to always shooting premium ammunition.

Other calibers compatible with the AR-308 platform include: .260 Remington, a worthy competitor to 6.5 Creedmoor and .243 Winchester, a popular bolt-action hunting round that is based on the .308 casing.

This is measured by how many inches of barrel the bullet will travel through before it completes one 360-degree revolution.

A twist rate of 1:8 means that the rifling (and the bullet passing through it) twists 360 degrees in eight inches of barrel length. If the 1:8 barrel is 16 inches long, the bullet will spiral like a football two full times before leaving the muzzle.

Barrel twist rate determines the range of bullet weights that the barrel will stabilize best for optimal accuracy. Although barrel twist is matched with bullet weight for easy reference, the ideal twist rate depends on a variety of factors including bullet weight, how much length of the bullet engages the rifling, bullet velocity, and more.

In general, faster or tighter twist rates are intended to stabilize heavier bullets at lower velocities, with less twist rate needed for lighter bullets traveling at higher velocities.

Most barrels chambered for .308 Winchester range from 1:12 twist (best for 168 to 170 grain bullets) to 1:10 twist (best for 170 to 220 grain bullets).

In 6.5 Creedmoor, 1:9 twist barrels are intended for bullet weights up to 130 grains, and 1:8 twist barrels are intended for bullets weighing 130 grains or more.


Twist RateIdeal Bullet Weight Range
1:14150-168 grains
1:12168-170 grains
1:10170-220 grains


Twist RateIdeal Bullet Weight Range
1:9Up to 130 grains
1:8130 grains or More

Carbine length gas systems replicate the gas system length on the military standard M4 carbine. Carbine length gas tubes measure approximately 9.75 inches long and are normally found on 16 inch or shorter barrels.

Mid length gas systems bridge the gap between carbine and rifle length gas systems and are reputed to give 16 inch to 18 inch barrels a smoother felt recoil impulse than the carbine length gas system. Mid length gas tubes measure approximately 11.75 inches long.

Rifle length gas systems are paired with longer barrels, producing a very smooth recoil impulse. Rifle length gas tubes measure approximately 15 inches long and are normally found on 18 inch or longer barrels.

Extended rifle length gas systems are rare and found only on long barrels firing high pressure cartridges. Because extended rifle length gas systems are unusual and not standardized, barrels featuring these gas systems are normally sold with the correct hardware included as part of the package.

If you have any questions, don't hesitate to send us a message at info@primaryarms.com or call us at 713-344-9600.

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