This guide covers all the important aspects of an AR-15 barrel from chamber to muzzle. We're covering the various types of manufacturing, the types of gun steel used to make barrels -- including clearing up all the nonsense about "mil-spec steel" -- and we're even going over barrel treatments and coatings, twist rate, barrel length, velocity, and ballistics.
How AR-15 Barrels are Made
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.
Cold Hammer Forged (CHF)
Cold hammer forged barrels are some of the most popular on the market because they're incredibly strong, they're favored by defense personnel and military groups, and they're accurate. CHF barrels are made by taking an over-sized barrel blank (a raw piece of gun steel roughly resembling a barrel) and forcing a shaped tungsten-carbide mandrel through the rough, un-rifled bore. The mandrel has the shape of the rifling imprinted on it. With the mandrel inserted, the barrel blank is literally pounded into its final shape by very large hammers, shown below. This process requires approximately 50 tons of force per hammer.
This is a cold hammer forging machine. Four rounded hammers pound the barrel blank in quadrants. Note the mandrel inside the blank.
It's not really cold
Cold hammer forging isn't done at "frigid" temperatures, but rather at room temperature. The immense pressure of the forging process does heat up the steel. The finished barrel must be stress-relieved by further heating (baking at a few hundred degrees for 1 to 2 hours) after forging. Hammer forging can also be done hot, at temperatures around 80% of the steel's melting point. Hot forging is unnecessary given the advancements in cold forging tech and stress-relieving processes.
- Produces an incredibly dense, strong barrel
- CHF barrels typically have longest rifling lifespan
- 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 barrels are favored by militaries and largely produced by defense contractors
- Production equipment is expensive for manufacturers
- Extremely high-end button- and cut-rifled barrels have potential to be more accurate
Cut-rifled barrels are considered the widespread "gold standard" for the AR-15, even today. While cold hammer forging has made mass barrel production less expensive without sacrificing quality, cut rifling is still favored by many manufacturers and it's still the method of production the U.S. Military employs. In fact, this process has been around for nearly 500 years.
This P & W Cut Rifling Machine was made in 1895. It was used to produce some of the most accurate rifled barrels of the time, until it was decommissioned in WWII.
A cut-rifled barrel is produced as the name implies: A forged barrel blank with a rough bore is inserted in a large rifling machine, which resembles a CNC lathe. The "lathe" pushes a single cutter attached to a long rod through the rough bore, carving out each land and groove of the rifling. Only a few thousands of an inch of steel are removed with each cutting pass. This process is time-consuming but with the right equipment and technique, a cut-rifled barrel is the most accurate barrel you'll find.
- 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
- More expensive to produce than CHF
- More time-consuming manufacturing process
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.
This button rifling machine shows the large rod that pushes the "button" through the bore to form the rifling.
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 black rifle barrels are cold hammer forged or produced with cut rifling.
- Relatively fast and economical
- Well-suited for mass production
- Produces relatively consistent rifling
- Button rifled barrels require stress-relieving treatments
- Rifling buttons are expensive make and maintain
CHF vs. Button vs. Cut Rifling: Which is Better?
You've probably been browsing forums, thread after thread, reading all sorts of opinions about CHF vs. cut and button rifling. The brutally honest answer to your question is simple.
Either a CHF or cut-rifled barrel will serve you just as well as the other.
Both options will provide more accuracy and capability than the average shooter knows what to do with. If you absolutely must pick between one of the three barrel types based on certain advertised advantages, it boils down to this: If you want a barrel that'll afford the longest possible lifespan, stick with a CHF barrel. If you want a barrel that's as accurate as possible, stick with a cut-rifled barrel. Button-rifled barrels provide no distinct advantage. We want to stress, however, that th subtle differences between CHF and cut-rifled barrels will only become apparent if you're purchasing the absolute best of the best in each category.
Even then, these differences will only become apparent to the very rare shooter who's chasing sub-MOA accuracy at long range with match ammo, and throwing 5,000 or more rounds down the muzzle. If that isn't you, either barrel will perform just the same.
Types of AR-15 Barrel Steel
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.
Types of Carbon Barrels Explained
There is a misnomer in the AR barrel market concerning the types of "mil-spec" steels available, but we'll explain as we go along.
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 megapascals, 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.
4150 Carbon Steel
4150 steel is also 4000-series steel that contains chromium and molybdenum like 4140 steel. However, the "50" denotes that it contains 0.50% carbon, which is 0.10% more than 4140 steel. The higher carbon content requires additional stress-relieving treatments. This makes 4150 more expensive and harder than 4140. It also produces a higher tensile strength of 730 megapascals, 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
4150 CMV is the most popular type of steel you'll see advertised when shopping for an AR-15 barrel or upper assembly. The problem is, this label is a misnomer. The phrase "4150 CMV" is a term barrel makers came up with decades ago that sticks around to this day.
Before the AR-15 became popular, most civilian barrels were made from 4140 steel because it was more affordable. But military-issued Colt rifles had (and still have) barrels made from CMV. When the civilian market grew, shooters didn't want 4140 barrels anymore. They wanted the barrels the military used. Unfortunately, someone in the civilian market started producing barrels made from 4150 and called them "mil-spec" for the AR-15. Although many AR-15 barrels are made from 4150 to this day, this steel was only ever mil-spec for the M14 and M1 Garand.
This misnomer caught on, leaving barrel makers with a problem: Should they produce 4150 barrels because that's what civilian AR-15 buyers thought they wanted? Or should they produce CMV barrels because that's what civilian buyers actually wanted? The solution was to buy CMV steel and make barrels to M16/M4 military specifications, and simply label them "4150 CMV". If manufacturers couldn't get CMV steel, they would buy 4150 steel with the same elements and properties as CMV and add Vanadium to it.
Either way, per MIL B-11595, "4150 CMV" = 4150 Steel + Vanadium = Mil-spec CMV.
What is MIL B-11595?
Advertising "MIL B-11595" as a barrel steel creates yet another misconception because it isn't a type of steel at all. MIL B-11595 is the military's specification document that lists the metals and elements contained in all mil-spec barrels. Looking at the spec sheet below you can compare 4150 to CMV: Both contain generally the same percentages of carbon, manganese, phosphorus, sulfur, silicon, and chromium. CMV might contain 0.05% more molybdenum, though this is negligible.
Types of Stainless Barrels Explained
Stainless barrels don't suffer the same confusion as carbon steel AR barrels. There are four "tiers" of stainless steel barrels: 410, 416, 416R, and 17-4 PH.
Advantages of Stainless vs. Carbon Steel
There are two major advantages to picking a stainless barrel over a carbon steel barrel: First, it'll provide better corrosion resistance with better accuracy. A carbon-steel barrel could last longer if it has a chrome-lined finish, but chrome-lining affects rifling and shot placement. Second, a stainless barrel will provide better "toughness". Stainless barrels are more resistant to heat and abrasion than carbon steel. All other factors being perfectly equal, a stainless barrel will last longer than a carbon steel barrel.
Barrels made from 410 stainless steel include the element molybdenum. The inclusion of molybdenum is meant to reduce the risk of developing sulphide stringers. This stainless steel is actually more durable than 416 and 416R stainless, offering the longest barrel life of the three. The only drawback to 410 stainless is its ability to withstand cold temperatures. Firing rounds through a 410 stainless barrel while below freezing temperatures (32 degrees F) increases the risk of cracking the barrel or causing a catastrophic failure due to sulphide stringers in the metal.
416 stainless boasts the highest machinability of any stainless steel. This stainless can be easily cut like a carbon steel barrel, allowing for better rifling and more consistent performance. Unfortunately, 416 stainless is so "workable" because it contains more sulphur than 410 stainless. This higher inclusion of sulphur greatly reduces the corrosion resistance, weldability, and formability when compared to 410 stainless.
We recommend avoiding purchasing a typical 416 stainless barrel.
416R stainless is a newer, proprietary stainless alloy produced by Crucible Industries, an industrial metal supply company. 416R boasts high machinability (and thus accuracy) like regular 416 stainless, except it also includes molybdenum (like 410 stainless). It also contains less sulphur than 416, reducing the risk of catastrophic failure from the inclusion of sulphide stringers. Crucible confirms an AR-15 barrel made from 416R stainless can be safely fired at temperatures as low as -40 degrees (F).
17-4 PH Stainless
Many precision shooters laud the advantages of 17-4 PH stainless over the more typical 416R and 410 stainless options. The "PH" in 17-4 stainless stands for precipitatino hardening. This process involves using heat to make the alloy stronger by hardening it and introducing precipitates (fine, solid impurities like magnesium, aluminum, titanium, and nickel) to the steel. This process also reduces deformations and warping, producing an incredibly honed barrel.
17-4 PH also contains 17% chromium, lending a natural hardness, resistance to heat, and smoothness to the rifling and bore. 17-4 PH barrels do not suffer the same cold-weather concerns as 400-series barrels. On the opposite extreme, the inclusion of precipitates also allows the barrel to withstand extremely high temperatures without deformation, meaning shooters can stay on the trigger and put more shots down-range without the barrel expanding and loosing accuracy. 17-4 PH barrels are usually hardened to around 36 to 44 RC. 416R barrels only harden to 28 RC.
The only downside to 17-4 PH barrels are their ability to withstand sustained, rapid fire. Once the metal is heated nearly to its tempering point through sustained fire, it becomes permanently soft and the barrel is no longer capable of performing accurately (or safely). But getting 17-4 PH to such temperatures is difficult and would take hundreds (if not thousands) of rounds. A 17-4 PH barrel is superior in every way, offering sub-MOA accuracy out to hundreds of meters. They typically cost 3 to 5 times more than a 416R barrel.
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)
Ah, the classic and the most widely-marketed type of barrel treatment: Chrome lining. This age-old process involves coating the inside of the barrel and chamber with chromium. Chrome lining adds to the physical dimensions of the barrel itself, so the rifling and barrel must be over-sized if this process is to be performed. Once applied, chrome lining is visible as 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 it creates.
A properly 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.
Manganese Phosphate/Parkerized (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 Finish (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.
AR-15 Twist Rates & Barrel Length
Twist rate defines the number of times your barrel’s rifling twists in the barrel, measured in inches. For example, a 1:7 twist rate means that every seven inches, the rifling makes one full rotation. For a 16" barrel, simple math says the fired round will spin just over two times (2.3 times, to be exact) before exiting the barrel.
What Twist Rate is Best?
The answer to this question depends on the length of your barrel, and the resulting velocity of the bullet. But above all else, the grain count, or weight of the bullet, will determine what twist rate is best suited for stability and accuracy. So to answer this question, you need to ask another question:
What Ammo Will You Shoot?
Most AR-15 owners keep it simple and shoot the two most common 5.56 NATO rounds: M855 (62-grain steel core penetrator), or M193 (55-grain full metal jacket). If you're shooting .223 Remington, then you're also most likely shooting 55-grain bullets. Lucky for us, endless ballistic testing and data are available that easily tell us what twist rate is best for the "average" AR-15. It's important to remember that this data applies to standard 16" barrels, which is the minimum barrel length for any rifle. You can only invest in a shorter barrel if you're building an SBR and an NFA tax stamp, or a pistol.
Twist Rate and Bullet Weight Chart
Although the U.S. military uses a 1:7 twist in their rifles' barrels, their barrels are also short -- 14.5" -- than the legal minimum limit for civilian rifles. The chart above illustrates what twist rate is best for a 16" barrel. We can see that a 1:7 twist can still safely manage M855. Although it'll "heavily stabilize" the lighter 55-grain M193 rounds, most shooters report favorable accuracy of approximately 1 MOA (1-inch groups) at 100 yards. If you want to shoot most 5.56 NATO or .223 Remington ammo, a 1:8 or 1:9 twist rate will likely work best. If you want to get incredibly accurate (sub-MOA) by shooting Mk262 or Sierra Match-Kings (80-grains), a 1:7 twist is still the best option.
Like we said, you could write a book on the AR-15 barrel. Here's a quick summary of the important stuff:
- Ar-15 barrels are mostly made by cold hammer forging and cut rifling.
- Either barrel will perform with better performance than most can manage.
- If you must pick one or the other, go with CHF for lifespan. Pick cut rifling for accuracy.
- Barrels are made from seven types of steel: 4140, 4150, CMV, and 410, 416, 416R, and 17-4 PH stainless.
- 4150 CMV technically doesn't exist. It's just mils-spec 4150 with Vanadium added, or mil-spec CMV.
- 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.
- For the typical 16" barrel, a 1:7 to 1:9 twist rate is best. A 1:7 twist rate is best for match-grade, heavy loads.