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Early on, selecting a barrel for your AR-15 is probably the most difficult choice you’re going to make. If this is your first AR-15, I suggest that your first AR have a lightweight profile with chrome lining. That configuration will serve 90% of shooters well for a long time.

But what if you’ve taken the next step and want to modify what you already own? Or maybe you are building your second, third, or seventh rifle, and want some advice on making the choice.

Perhaps you want some more information about why I made the recommendation that I did in the buyer’s guide.

This guide is for you.

This article summarizes just about everything I’ve learned about rifle barrels in general, and specifically the AR-15. I’ve learned these lessons from years of study, competition, training, and practice. I avoid as much engineer-speak as I can, but some of it is unavoidable.

This isn’t really a guide on telling you the best AR-15 barrel manufacturer. Though I will throw out some recommendations in a minute. When it comes down to it, a quality barrel is a quality barrel no matter where it came from.

I want to help you understand what makes a good barrel so you can bump the criteria up against other manufacturers and find a barrel that meets your needs.

Bottom Line Up Front (BLUF)

This is a long post, I get it. The objective is to teach you about barrels so you can make a good decision for yourself. If you just want my recommendations for something to buy, you’ll find them here at the start.

Selecting the right barrel for you is a confusing process when you don’t know your needs. Most new shooters have an idea of what they want, but not how it’s going to work for them in the long term.

I’m breaking this down by budget and use.

General Purpose, Lightweight

This barrel class performs well at most tasks, but it particularly suited to being carried more than it is shot. Unless you have some special purpose in mind, like competition shooting, then this is where you should start.

If you are trying to stick to a budget, then the Faxon Gunner Profile is a great option. It’s light, accurate enough, and will take care of you for a long time.

If you have a bit more money to spend, then I suggest Daniel Defense’s Hammer Forged Lightweight option. I discuss hammer forging further down in this post if you want to know more.

General Purpose, Mid-weight

These barrels have slightly more mass. They tolerate aggressive firing schedules a bit better without being too much more cumbersome to carry. 

In this category, I don’t really think you should look for budget options. If you need it to well at everything, then you’re going to pay a little extra. My favorite barrel in this class is Criterion’s Hybrid Profile.

Precision

In this category, weight isn’t a concern anymore. These barrels are designed to deliver tight groups and consistent performance all day. That also means you’re going to pay for it.

At the “budget” end, I suggest Criterion’s fluted stainless offerings. I put that in quotes because, obviously, it’s still more expensive than the low end of the market.

If you have more cash to spend, then check out the Centurion Arms RECCE or Mk12 barrels, which are the same ones used on the military Mk12 rifle.

The Beating Heart

When you look at it, the modern rifle barrel is not far removed from the muskets and cannons used during the Revolutionary War. There is a charge, combustion chamber, projectile, and length of hollow tube known as the bore. As the charge burns, expanding high-pressure gasses propel the projectile down the bore. Upon exit, the bullet begins a ballistic trajectory.

The major advancements in this technology have been in how ammunition gets loaded into the chamber. The aerodynamics of bullets have seen significant advancement as well, but that’s not the topic we’re talking about today.

The barrel, more than any other part of the rifle, will affect accuracy, reliability, and handling of your weapon. It is fair to say it is the most important choice you will make when building your AR-15.

BCM 16" lightweight profile barrel on the Minimum Capable Carbine
BCM 16″ lightweight profile barrel on the Minimum Capable Carbine

When selecting a barrel, you are balancing accuracy, weight, handling, compactness, parts longevity, and recoil characteristics. The choices we have to make to affect these traits include:

  • Length
  • Profile
  • Material & lining
  • Rifling method & twist
  • Chamber dimensions
  • Gas system

There is no getting around it. Every choice you make to the benefit of one area will negatively affect another. The more specialized you make your rifle for one task, the worse it perform at others.

The Compromise

I always recommend that new shooters start with a generalized, “Jack of All Trades” type of rifle. The reason is simple: new shooters don’t have enough skill to take advantage of special configurations. This is not me being judgmental in any way. This applies to myself just as much as it may to you.

The average life of an AR-15 barrel is about 20,000 rounds. Let’s assume, generously, that the average new shooter goes to the range once a month and fires 100 rounds through their rifle per session. At that pace, it would take about 16 years to wear out that barrel.

In contrast, the average professional competition shooter will expend between 20,000 and 30,000 rounds per year

I’m not trying to discourage you. You can still develop a huge amount of skill on 1,200 rounds per year of structured practice. That’s far more than the average gun owner puts in. I’m trying to illustrate how much practice some people really put in at the high levels of shooting.

I want you to understand that the ultra-specialized configurations only make a significant difference when in the hands of highly practiced shooters. A master class level shooter will pick up a bone stock AR-15 off the shelf and still outshoot a novice equipped with a fully custom top-end match rifle.

Up until 2014, I would have told you that it wasn’t possible to land hits at 1000 yards with a bone stock M16A2, and then I read about Shawn at Loose Rounds doing it. That act blew my mind and started me on the path to taking marksmanship more seriously.

Remember the Rules

Underpinning everything about the choice you make is the two rules I wrote about in the first time AR-15 buyer’s guide:

  • Mission drives gear
  • Buy nice or buy twice

Make your decision on what you need it to do. Do not make the mistake of buying something because you like it and then trying to shoehorn it into a role it’s not suited for. If you are going to buy it, buy something nice enough to last.

The Long and Short of Barrel Length

The longer the barrel, then the more velocity the projectile will have.

Velocity does two things for us:

  • It decreases the amount of holdover necessary for a given range (otherwise known as shooting “flatter”)
  • It increases the effective range of the bullet

Longer barrels usually mean lower pressures inside the rifle, which increases the longevity of the bolt, springs, and other components like suppressors. This comes at a price: longer barrels mean increased weight and more cumbersome handling.

Shorter barrels are easier to handle, especially in cramped spaces, but come at the cost of higher operating pressures, increased noise, and accelerated wear on internal parts. The use of a suppressor almost becomes a requirement once you get to 10.5″ barrels. The noise and concussion become unbearable, especially indoors where permanent hearing damage will occur without protection. The chart below illustrates this.

Barrel length versus bore pressure curve of an AR-15
Chart of barrel length against bore pressure

This chart comes from a 2012 article at Small Arms Defense Journal about the effects of barrel length. They measured the bore pressure at bullet exit after cutting the barrel down one inch at a time. Note that a 10.5″ barrel has almost twice the pressure of a 20″ barrel. That affects the noise, concussion, flash, bolt carrier velocity, and long-term durability of the weapon.

When I was in the military, a base armorer once told me that he practically never sees broken bolts from the M16A2s they had in the racks. The M4s with the carbine gas systems, on the other hand, were breaking bolts pretty regularly beyond certain shot counts.

Accuracy and Trajectory

Barrel length has little to do with accuracy. Yet, you will see a lot of folks talk as if longer barrels are more accurate and use that as a reason to justify them.

That is wrong, don’t believe it.

If anything, longer barrels reduce accuracy potential. Think of a long piece of 2×4 at the hardware store. The longer it is, the more prone it is to “flex” under stress as you hold it from one end. For a barrel, we call this “whip.” Long skinny barrels tend to whip more compared to shorter and stiffer barrels.

To counteract this, because we want to keep higher velocities and lower pressure, we make barrels thicker and heavier so they remain stiff. However, this is more of a technical curiosity than anything. The actual effect of the whip is very small and not something you should be concerned with until you are a seasoned match shooter. This gets into the harmonics of barrel vibration during the shot, which is beyond the scope of this article.

When people talk about longer barrels being more accurate, they are probably talking about an easier perceived ability to put a hit on target.

So what drives this?

A bullet is moving at its maximum speed when it exits the barrel. From the moment it leaves, its velocity begins to drop and gravity takes over. The increased velocity of a longer barrel means that the bullet drop less at a given distance than a bullet that started off slower.

For you, this means that you could land a hit with less holding over the target or adjustment to the sights. There are other factors involved here, such as the aerodynamics of the fired bullet, but that’s another discussion. I wrote a much more detailed article on the effects of velocity on trajectory, so definitely check that out. As part of that, I put together these two charts.

The first chart shows a 100-yard zero for a 20″, 16″, and 12.5″ barrel and associated impact points at 25, 50, 100, 200, and 300 yards. The point of aim is highlighted in red. I used a nominal 3300 fps, 3100 fps, and 2900 FPS for each of these. Notice how the overall extreme spread of impacts shrinks as velocity increases. It’s particularly noticeable with the 300-yard impact.

The second chart is for a 300-yard zero, and I think it shows the effects much more dramatically. This tighter cluster is what we mean by “flat shooting.” Leveraging this kind of grouping above and below the point of aim is the Point Blank Zero and is an important element in a marksman’s toolbox.

That’s not the whole story though. Landing the hit is one thing, and we can always compensate by choosing a different zero or using holdovers. What will the bullet do when it arrives?

Effective Range

When we talk about the effective range of a bullet, we are talking about the maximum distance the bullet can travel and still have the desired effect on a target. What that desired effect looks like depends on what you’re doing.

The velocity required to punch a hole in a piece of paper or ring a steel plate is one thing. The energy needed to produce a devastating wound channel is very different and much higher.

Some bullet designs, such as the light 55gr FMJ or 62gr M855, require higher velocities to produce “terminal effect.” If you look at the excellent history of the 5.56 cartridge documented by Daniel Watters, the original theory focused on using high velocity smaller caliber bullets to produce equal wounding characteristics of a .30 caliber round. In short, a lighter, faster bullet could be just as effective as a larger bullet.

That also had other benefit. A lighter weapon and ammunition meant hits were easier to achieve and more ammunition was carried. If you have ever studied small unit tactics, more ammunition means more time in contact.

This chart illustrates the initial velocity for an M855 projectile fired from different barrel lengths

Mission drives the gear, so how much range do you need? This is all assuming that you are trying to kill something or stop a threat. If you are interested in shooting paper for competition, then it’s a moot point.

Barrel Length Comparisons

Let’s discuss the most common barrel lengths for the AR-15. Each one has its pros and cons. What you choose is ultimately driven by your goals and needs.

20″ Barrels – Old Reliable

I once had a conversation with a member of the special operations world who also happened to be a gun nut. Something that stuck out to me was his insistence that we should never sacrifice velocity if we don’t have to. He later started up a very successful company that produced well known AR-15s, barrels, and accessories.

That mantra always stuck with me. 

A 20″ rifle with collapsable stock

The 5.56 cartridge and original AR-15 were designed as a pair. The 20″ barrel was the heart. It’s the workhorse or the AR world.

I would go so far as to argue that this barrel length is the most optimized for balancing velocity, longevity, and recoil characteristics.

Look at the velocity chart above and how peak velocity occurs at the 20″ length. Everything was optimized for it. People accustomed to 16″ and 14.5″ barrels are often blown away at just how smooth the 20″ length is to shoot.

The 20″ barrel provides the most consistent performance across the widest variety of loadings. There are plenty of modern specialized loads “optimized” for use in a 16″ barrel, but the 20″ length will still do things better.

Aside from the benefits of velocity, the long 20″ barrel gives a balance benefit to marksmen. With the extra length and weight, your rifle will have a slight forward balance known as “hang.” This helps reduce the “wobble zone” during aiming. In other words, the rifle and sights stay more still in your hands.

The trade-off, of course, is that a 20″ barrel is longer and heavier than other options. Clearing a house or riding in a vehicle is more difficult. One Security Forces Airman put it to me this way: the 20″ barrel is easier to shoot, but harder to live with.

16″ Barrels – The Civilian Standard

The 16″ length is the most common barrel length sold today.

It doesn’t do anything particularly better than other options, but it makes a good all-around length. The National Firearms Act (NFA) of 1934 limited a rifle’s barrel to a minimum of 16.1″ before more taxes and restrictions come into play. I suspect that we would see a lot more 14.5″ barrels on civilian rifles, the length of the M4 Carbine, if the NFA was not a factor.

16″ barrels offer a good compromise between the full 20″ length and the short barreled rifles. It’s still capable of hitting any target the 20″ could hit, albeit with a little more holdover. It only loses about 50 meters of effective range compared to the longer rifle. The difference between 150-ish meters and 200-ish meters effective range isn’t a big deal if most of your shooting is inside 100 meters anyway.

Combined with a collapsible stock, the 16″ gun is compact enough to make living the rifle plenty more convenient.

14.5″ Barrels – Military Carbine Length

This is the length of the military M4 and M4A1 carbines. The M4 was adopted in 1994 after ten years of design and testing.

The original intent was to bridge the gap between the 20″ M16 series and the experimental XM177.

The XM177 “Commando” had a 10.5″ barrel and saw service in Vietnam. It had a great reputation for compactness and ease of carrying, but the shortcomings of its low velocity and very loud noise signature were a problem. In fact, it came equipped with a 4″ moderator designed to mitigate some of the noise, effectively bringing the length up to about 14″.

There isn’t much benefit to dropping from a 16″ to 14.5″ barrel. In fact, it will cause you more pain than it is probably worth due to dealing with NFA paperwork, taxes, and regulation.

Some people circumvent this by using 14.5″ barrels and permanently welding extended flash hiders to the muzzle. This brings the length back up to the legal 16.1″, but makes it difficult to do any future modifications. If this is your first AR, don’t do this to yourself. Give yourself room to experiment without tripping over federal law.

10.5″, 11.5″, 12.5″ – The Short Barreled Rifles

A member of the US Navy using a MK 18. Photo credit to the US Navy

If you want to see a bunch of forum fanboys go at it with one another, ask them the best length for an SBR. The thread will go on for pages and pages while they hash out the tiny differences.

The truth is that these lengths all do the same thing, and they do it well. They all make for very compact and easy handling carbines. They also come with increased pressures, loud noise, and reduced velocity.

Short barrels are certainly fun to play with, but I wouldn’t want to use one seriously without a suppressor, especially indoors. Keep in mind that this group was purpose-designed for close quarters fighting at very short ranges. Sure, they can hit a target at 300 or 400 meters, but I’d rather have something more suited to the job.

18″ – Special Purpose Rifle

The 18″ barrel length has an interesting story, one well documented by the Small Arms Defense Journal. After the Gulf War, an idea for a light sniper rifle called Special Purpose Rifle was floated by Armalite.  It promptly faded away. Later, in 1998, the 5th Special Forces Group brought back the idea and started circulating it around SOCOM. The concept finally rose to prominence when the GWOT started ramping up in the early 2000s.

I used to think that the 18″ length was a compromise to keep weight down with a suppressor. That idea might be wrong. I recently learned a different story. In this one, the 18″ requirement came about because of bureaucratic red tape. The Navy was having trouble requisitioning 20″ match barrels because “the system” kept telling them that 20″ barrels were already in the supply chain. Of course, those were standard M16A2 and A4 barrels and did not meet the accuracy requirement.

So the Navy changed the specification to 18″ and said that there wasn’t anything in the system. They got their barrel. Given my experience with military acquisition programs, this sounds like a reasonable story.

Whatever its origins, the 18″ barrel is very popular in the competition world. It offers a very smooth recoil impulse due to its full-length rifle gas system. The length also maintains good velocity for reaching out with a flatter trajectory. Its primary trade-off is a loss in dwell time between the gas port and the muzzle. This can make it a little touchy about ammo selection in cold weather. I’ll talk more about this in the gas system.

Takeaways

Those who carry rifles professionally do an awful lot more living with their weapons than shooting it. You probably aren’t riding around in vehicles all day with your rifle, doing yard work with it slung across your back, or taking it with you to the bathroom. So worrying about living with your weapon probably isn’t a concern.

Mission drives the gear. How are you going use your rifle? I don’t mean fantasy land expectations of the apocalypse, either. I would argue the benefits of shootability and effective range outweigh perceived inconveniences about “living with the weapon” for rifles that only move from the safe to the range and back.

Also, as a bonus, we civilians can take advantage of configurations the military never adopted. A great one is combining a collapsible stock with a 20″ barreled upper. That reduces some of the pain associated with full sized rifles and still provides all the benefits. This is actually my favorite configuration.

Barrel Profile – The Skinny (and Fat)

The profile of a barrel refers to its overall shape. Where mass is distributed along the length of the barrel has dramatic effects on how the barrel performs for different tasks. A thin profile all the way down, the “pencil” profile, makes a very lightweight and easy to carry weapon. A thick profile from end to end, the “HBAR” profile, makes a heavy rifle that sustains accuracy over a long string of shots.

Heat Management

The balance here is how the barrel handles heat versus how easy it is to maneuver. There’s little difference in accuracy between a light barrel and heavy barrel for the first few shots. They both place the round where you aim them. The differences show up at the barrel heats up.

As metal heats, it expands. This deformation affects the accuracy of a barrel. The faster it heats up, the quicker this decrease in accuracy happens.

If you heat any barrel up enough, you’ll eventually destroy the protective linings, rifling, and harm the steel itself. This was the case at the infamous Battle of Wanat. Most of us will never shoot that much.

Skinny barrels heat up faster, so you expect their accuracy to drop off faster. But they also cool down quicker.

Heavy barrels are slower to heat, so they retain their accuracy longer, but also slower to cool down.

Handling Characteristics

There are far more profiles out there than “skinny” and “heavy.” It’s common to have a heavier mass of steel at the rear of the barrel, near the receiver, and then the barrel gradually tapers to a skinny profile near the muzzle. This produces a well-balanced rifle that handles heat well. Criterion uses this in their “Hybrid Profile” barrel I suggested at the start of the article. Ballistic Advantage calls their version the “Hanson Profile.” Faxon, which I also recommended, calls their version the “Gunner Profile.”

An 18" Faxon "Gunner" profile, which continuously tapers from chamber to muzzle
An 18″ Faxon “Gunner” profile, which continuously tapers from chamber to muzzle

There is also the “Government” profile found on the M16A2, M4 carbine (not the M4A1), and many civilian rifles. This has a skinny profile near the receiver and a thicker profile towards the muzzle. There is kind of a funny story behind how that profile came about. The only real benefit of it is that it moves some of the balance forward for shooters who want a bit of “hang” without increasing the overall weight too much.

The compromise between weight and balance comes down to how easy you want the rifle to carry, how quickly you want to bring it to a target, how well it settles in your hands for precision shooting, and how well it sustains accuracy for long strings of fire. You can read more about that in my article on weight and balance.

Make Your Choice

Light barrels are suited to rifles that are carried a lot and see easy to moderate shooting schedule. This is most people, including military members who live with their rifles.

Heavy barrels are suited to rifles that are not carried as often and need to maintain higher levels of accuracy over long strings of shots. This describes match shooters and precision rifles shot from relatively fixed positions.

Most people are far better served by lighter profiles.

Barrel Composition

When selecting your barrel, you have to choose between Chrome Moly or Stainless Steel. Those descriptions are very broad. There are a lot of sub-varieties within those types of steel, and some are more useful than others.

The two different steels have different wear characteristics. Let’s say chrome moly barrels last to ‘X’ number of rounds before accuracy starts gradually declining until the barrel is spent. A stainless barrel might last slightly longer than ‘X,’ but then declines much more quickly.

This does not take into account the quality of the barrel, rate of fire, heat, or pressure of the loads being fired.

Chrome Moly

This is the most common type of steel encountered in AR-15 barrels. It’s a broad category that encompasses several different types of metal. If you read my guide to buying your first AR-15, I mentioned two of them: 4140 and 4150. So let’s start there.

4140 and 4150 are two common steel alloys that include Chromium and Molybdenum. These are called “ordnance steels.” 4140 includes about .40%  carbon, and 4150 includes about .50% (hence the 4140 and 4150). In real terms, 4150 is more tolerant of high heat situations, such as fully automatic fire, will wear slower, and can be machined thinner.

The US Government actually has a specification for small arms barrel steel, known as ORD 4150. This specification is MIL-B-11595E, and lists out the chemical composition required to be compliant.

Of note, the third blend in this spec includes Vanadium. This blend is called Chrome Moly Vanadium, or CMV. You will sometimes see this called 41V45 steel.

A company can claim their barrel is 4150 steel, but that doesn’t mean it’s actually certified under MIL-B-11595E. Being both 4150 and certified under 11595E is more difficult, and thus more expensive. A manufacturer that has gone to the trouble of certifying their barrel steel will advertise the fact since it’s a sign of quality.

If you shop around enough, you will come across retailers selling barrels made from “machine gun steel.” FN USA makes these barrels using their blend of steel intended for the M240 and M249 machine guns. It is some variety of 41V45. FN usually adds extra thick chrome lining. They can withstand higher levels of heat than the standard 11595E alloy, but I’m not sure that’s all that valuable to the average shooter.

Stainless Steel

When you bring up stainless steel barrels, the assumption is you want precision.

USMC Staff Sgt. Benjamin J. Lacasse firing at 200m in competition with a match M16A4. Photo by Sgt. Alicia R. Leaders

However, the barrel’s material has little effect on the accuracy of the weapon.

Stainless barrels are popular for match rifles for one primary reason: they are easier to machine and finish. The same small shop can machine, rifle, polish, and lap all in one place. With a shorter supply chain and skilled craftsmen, the finished product is cleaner and more consistent from barrel to barrel. It’s primarily the higher quality finish on the inside of the bore that improves accuracy.

Speaking of finishes inside the bore, another factor is protective linings. Stainless barrels are more resistant to corrosion, so they do not need chrome lining or other treatments. In the past, the application of chrome linings meant that the bore was uneven and inconsistent. These inconsistencies disturbed the bullet and reduced accuracy.

In general, due to the manufacturing differences, good stainless barrels will be about 30% more accurate than the Chrome Moly equivalents. Of course, this varies from manufacturer to manufacturer.

The trade-off is that stainless barrels do not handle adverse conditions as well. Due to the necessary inclusion of sulfur in the metal, there is a risk of developing sulfide stringers. That’s a fancy way of saying that the barrel could suffer catastrophic failure under fatigue. The best way to prevent that is to increase the barrel thickness and avoid very cold or very hot temperatures.

Stainless Alloys

410 and 416r are the top two stainless alloys, both make great barrels. 410 is harder and more durable, owing to its lower sulfur content. The trade-off is that without the extra sulfur, the barrel becomes brittle in temperatures below freezing. To compensate for temper embrittlement, the walls of the barrel must be thicker.

416r is an alloy formulated by Crucible Industries specifically for gun barrels. It’s easier to machine and is rated to temperatures down to -40 degrees Fahrenheit. This is the most common alloy you will come across. It has more sulfur than 410, so there’s a slightly increased probability of sulfide stringers under fatigue.

Do not confuse 416r with regular 416, they are not the same. If you see a barrel advertised as 416 stainless, ask if it is actually 416r before you buy it.

Because of the temper embrittlement and sulfide stringer fatigue concerns, it’s a best practice to avoid lightweight profiled stainless barrels.

Protective Linings

This is where the real differences between CM and stainless barrels become apparent. CM and stainless barrels, machined bare, have about the same level of accuracy. But because CM barrels have lower chromium content in the alloy, they are more prone to corrosion. The M16 had a terrible reputation for reliability early in its life because of this.

Chrome

To improve corrosion resistance and wear characteristics, it was important to line the bore and chamber of a CM barrel with chrome. This lining had a negative effect on accuracy, though.

To line a barrel, steel from the bore is removed and replaced with chrome. Little imperfections in the process affected the bullet as it traveled the length of the barrel. Consistency, as the saying goes, is accuracy.

In the 21st century, manufacturers have figured out how to get a more even and consistent layer of chrome in the bore. I have a barrel from Centurion Arms made of FN’s machine gun steel and double thick chrome lining that shoots about 1 MOA with good ammunition. Criterion is known for producing match quality accuracy with chrome lined bores.

Today, chrome lining does not deserve the negative accuracy reputation it has- so long as it comes from a quality manufacturer.

Nitride

Another interesting option gaining popularity is nitriding. I’ve written a much longer article about this nitrided barrels, so this is just a snapshot.

This process, also called Nitriding, has several trade names: Melonite, Tenifer, QPQ, Salt Bath Nitride, and others. Nitriding is a surface conversion where the barrel is submerged in a nitrogen-sodium solution and heated to a high temperature. This is usually done between 750 and 1200 degrees.

The end result of the nitriding process is a very hard “case” surrounding the barrel steel. This case is about 60-65 Rockwell, as opposed to 28-32 found on normal barrel steel. It is very corrosion resistant and has a much lower coefficient of friction compared to bare metal or chrome.

The primary bonus of this process is that the accuracy of the barrel is maintained while the corrosion and wear characteristics improve.

Nothing is free, though. There are two downsides that I know of.

The first is the temperature required to complete the process. The temperature used in the for nitriding is very close to those used for stress relieving barrels after rifling.

Specifically stainless barrels.

I’ll get to this later, but stress relief is a huge factor in a barrel’s accuracy. It is possible to undo that important work and hurt a barrel’s performance.

Second, the nitride case is much less heat resistant than chrome. It is not an ideal solution for barrels that will see very high volumes of sustained rapid fire. The lining will fail and result in faster erosion of the barrel’s steel.

Again, if you would like more detail about the nitriding process, its positives, negatives, and what exactly I mean by high volumes of sustained rapid fire (hint, it’s a lot), then head over to my article specifically about nitriding.

Which Should You Choose?

99% of shooters are well served by a quality CM/CMV barrel. Either chrome lined or nitrided. Well-made copies of these barrels are more than capable of turning in 1 to 1.5 MOA groupings. That is more accurate than the average shooter is capable of, especially when most users are blasting cheap ammo that isn’t capable of better than 3-5 MOA to begin with.

Spending $500 on a custom stainless match barrel capable of 1/2 MOA, and then shooting mostly cheap bulk ammo that can’t group better than 3 MOA is a waste.

If you absolutely need better than 1 MOA accuracy, and you only plan to feed it quality match ammunition, then go ahead and get a stainless barrel. If you live where it gets below freezing, stick to 416r stainless.

Rifling Method and Twist

Rifling of a 90mm M75 cannon
Rifling of a 90mm M75 cannon dated from 1895. Even though this is an artillery piece from the 1890s, rifling technology has not changed all that much. Photo credit to Petar Milošević.

Rifling describes the grooves cut down the length of a barrel’s bore. These grooves “grab” the bullet and impart rotation. This rotation stabilizes the bullet in flight and increases accuracy.

Rifling Methods

There are three ways that manufacturers form the rifling of a barrel: cutting, button rifling, and hammer forging. Each of these has pros and cons. The balance is between how long it takes, the consistency of the end result, and how much stress the steel undergoes during the process.

Cut Rifling

Cut rifling is the original way. A machine cuts each groove one at a time over several passes. This produces the least amount of stress on a barrel but requires the most care to get right. Generally, these are the most expensive barrels due to the extra time and care required.

This video shows a machine performing cut rifling.

Button Rifling

Button rifling is the military specification. It involves taking an extremely hard “plug” and forcing it through a smooth “blank.” This cutting plug creates the grooves as it passes through the bore. This is a much faster method but produces much more stress on the barrel steel.

Hammer Forging

Germany developed this method during WWII as a way to accelerate machine gun barrel production. Manufacturers insert a tungsten “negative” mandrel into a smooth “blank.” A large machine hammers the blank around the mandrel until the desired rifling pattern forms. There is a great article from a 2005 edition of Precision Shooting that covers the history and process well.

This is a fast way to mass produce very consistent rifle barrels. It has a side benefit of creating a metal grain compression pattern and a slightly stronger barrel. The downside, of course, is that it puts a very high amount of stress on the barrel steel.

There are only a handful of companies with hammer forging machines in the US. Daniel Defense, Ruger, FN USA, Remington, and IWI all have them. If you are buying a hammer forged barrel, it probably came from one of these plants.

This video demonstrates what the process looks like. It is from GFM Machine, who produces most of the hammer forging machines in the world. One of the original engineers who designed the process founded the company.

Stress Relief

Applying high amounts force on steel results in stress. If fired immediately after rifling, accuracy would quickly degrade as the steel heats up. The stress relief process minimizes this deformation.

In short, stress relief means heating the barrel up to about 1000 degrees. This helps removes stresses that built up during the rifling process. This also results in a slight softening of the steel, which is expected. Of note, cut rifled barres do not require this process, which means they are slightly harder than other finished barrels. 

Notice the temperature used during the stress relief process: 1000 degrees. That is right around the upper range of the nitriding process. Unless the company doing the nitriding is very careful, there is a risk of undoing the important stress relief work. At that point, the barrel will become very inaccurate as it heats.

Takeaway

The rifling process is interesting but is honestly not that important. Worry less about marketing hype. Buy from a quality manufacturer and let them worry about how the barrel gets made.

Twist Rate

A barrel’s twist rate describes how quickly the grooves spiral. Twist rate is denoted as 1/7, 1/8, 1/12, etc. Pronounced “one in seven,” for example, this means that the groove completes one 360-degree rotation around the bore every seven inches. The smaller the number on the right, the faster the twist rate.

You may have heard that you should tie the twist rate to the weight of the bullet you want to shoot. Technically, that is wrong. If you want the long and detailed version, check out my write up on twist rates. The short version is that the ideal twist rate is a combination of a bullet’s diameter, length, and how weight is distributed. More length, and more mass, for a given diameter needs a faster twist to stabilize it.

The original AR-15 had a 1/14 twist rate. Later, the specification changed to 1/12 for use with M193 55gr bullets. When the M16A2 came out, the twist rate increased to 1/7 to better work with the long tracer rounds. For a long time, most civilian AR-15s had a twist rate of 1/9.

The 1/9 twist is actually a really good compromise for shooting the most common bulk ammo of the time, between 45gr and 62gr. With the popularity of 77gr SMK for match shooting, 1/7 twists now reign supreme, but a 1/8 will shoot just about anything 77gr and below well..

A reference chart for stabilizing .224 caliber bullets. I selected a worthy representative from each “weight class” of .224 bullet to give you a good picture. More information and charts like these are found in my article about twist rates.

Some people speculate that you can over-stabilize a bullet if you fire it through a barrel with too fast a twist rate. Generally, that is wrong. But, if you fire thin jacketed lightweight bullets or smaller cast lead, you could see some issues.

Ballistician Bryan Litz does point out that there is an optimum twist rate for maximum accuracy. If you reference the chart I put together above for twist rates, bullets do best when the stabilization factor is between 1.5 and 2.0.

Rifling Shape

Traditional land and groove rifling on the left, polygonal rifling on the right.

You will eventually come across a manufacturer touting their various special rifling patterns. Remington calls theirs 5r, Shilen calls theirs “ratchet rifling,” and there are other names. Traditional rifling consists of grooves cut down the length of the bore with edges that “grab” the bullet. This pattern is called “land and groove.” Polygonal rifling uses swells and smoother angles to do the same.

There isn’t much difference in performance between them. Don’t let the marketing hype fool you. If anything, polygonal rifling is slightly easier to clean up due to the smoother surfaces, but you aren’t likely to see any real difference in accuracy.

Chamber Dimensions

The .223 and 5.56 are very closely related. In fact, most people think they are identical. Dimensionally, they pretty much are.

.223 cartridges in front of AR-15

The difference comes down to the pressures that each generates. .223 is a SAAMI specification, so everyone knows how to follow it. 5.56 is a military specification and one that has changed a lot over the years as velocity requirements have changed. SAAMI and the US Military measure pressure differently, so there really isn’t a way to directly compare them.

The short answer to this is that 5.56 is generally higher pressure than .223. How much higher? Well, there really isn’t a good way to tell. Since there isn’t a real specification out there.

Rifles with 5.56 chambers have slightly more room on the inside to deal with those higher pressures. Common wisdom is that firing a 5.56 cartridge in a .223 could result in catastrophic failure due to overpressure. I think those fears are a little overblown, but I understand where they’re coming from.

There are other chambers that split the difference between the two, like .223 Wylde or Noveske’s Match Mod 0.

Due to tighter dimensions, you should expect .223 chambers to be slightly more accurate. But 5.56 chambers work safely with a wider variety of ammunition.

I think most new shooters are just fine starting with a 5.56 chamber.

Gas Systems

Now we have reached the last consideration: the gas system.

The AR-15 is a gas powered rifle, meaning that it uses the expanding gasses from a fired cartridge to eject the spent case and load the next round. It uses some of the expanding gasses in the bore by bleeding it from a hole. This hole is on the top of the barrel, under the front sight tower. The gas flows down a tube, where it meets the gas key of the bolt carrier group. It then flows into the bolt carrier, where the pressure pushes against the back of the bolt and the rear of the carrier. This triggers a reward momentum that unlocks the bolt and cycles the rifle.

This is a good illustration of the firing cycle. Gas flows from the bore back down the gas tube, where it drives the bolt carrier group like a piston. 

Manufacturers must manage the amount of gas entering the system. Too much gas and the rifle action becomes too violent. Too little gas and the action will not fully cycle and cause malfunctions. Manufacturers also have to account for the gas port slowly widening over time as the hot gasses erode the steel.

Dwell Time

Dwell time describes the amount of time between when the bullet passes the gas port, allowing gas into the system, and when the bullet exits the bore. When the bullet “uncorks,” the pressure in the system equalizes again.

On average, for government spec configurations, there is about 7″ of barrel between the gas port and the muzzle. This is pretty well optimized for a 20″ rifle, 16″ midlength, or 14.5″ carbine barrel. Where you see some trouble is the oddball lengths.

For example, 18″ barrels with rifle length gas systems have two inches less “dwell time” compared to their 20″ cousins. That means the gas port must be enlarged in order to let more gas in the system before the bullet uncorks. It’s sorted out now, but 18″ barrels used to have a bad reputation for being undergassed in cold weather because of this. The same applies the other direction, with 16″ barrels and carbine gas ports. The extra 1.5″ meant the gas port was slightly smaller.

This dwell time issue becomes very important on the very short 10.5, 11.5, and 12.5-inch rifles, as they all use the same 7″ carbine gas system. This gets even more challenging when suppressors are thrown into the mix.

Selecting Your Gas System

Generally, the longer the gas system, the lower the operating pressure. That means a more gentle recoil impulse and longer parts life. For most people, this really shouldn’t be something you are concerned with.

My advice is to simply buy a barrel from a quality manufacturer, and let them sort out the gas system engineering.

Manufacturer Spec

There really isn’t a need to back and review all of the points above. I want to leave you with one more piece of advice:

Let’s take a fictional gun company getting their start. I’ll call them Wildcard Arms.

Wildcard Arms contracts with FN USA to produce a run of very high-quality hammer forged barrels. The specs on the bore are very tight, and the accuracy expectation is high. Not every barrel FN makes will meet this specification, so there is a fairly high “reject rate.” This is expected and built into the higher cost that Wildcard Arms has to pay per barrel. But because each barrel is made to a high spec and consistency, Wildcard gets to charge a premium price for the product.

Another company comes along who specializes in bargains. We’ll call them Athena Defense. Athena approaches FN with a much looser specification and is looking for a lower price point. FN looks at the inventory and sees they have a lot of Wildcard’s rejects on hand, and offers a good deal on them. Athena accepts the shipment, and away they go.

Both companies are now selling barrels from the same factory, but one has a much higher spec product.

I’m not saying that Athena’s barrels are junk, as FN still a reputation to maintain as well. But I am saying that there is a reason for the price difference. It is foolish to discount that just because they came from the same factory.

Buy nice, or buy twice.

Wrapping it Up

I hope you found this guide useful. Buying a barrel is an intimidating process because of the huge number of choices that you need to make. The length, profile, material, lining, and gas system all have an effect on the performance of the gun. But, despite all the text above, the difference from one configuration to the next is not as important as the quality of the manufacturer.

Don’t spend an excessive amount of money chasing accuracy. A quality basic barrel is capable of holding 1 to 1.5 MOA. That is more than accurate enough for even most match shooters, much less a new shooter just getting into things. The mechanical accuracy of the barrel is far less important than the marksmanship skills of the shooter.

Let me know if this article was helpful, and feel free to ask any other questions you might have. Whatever it is, you’re probably not the only one thinking it.

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Bill
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Bill

Does adding a suppressor effectively change the barrel length increasing the projectile velocity.

Matt
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Matt

great post. former jarhead here with a classic a4 w/ carry handle (nostalgic inspection arms practice) from palmetto. looking to build a more tactical for the wife (and myself lol). very helpful info!! cheers!!

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Guest

That was one of the best articles I’ve read in a long time. Very informative. Thanks!

Kyle
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Kyle

I’m looking into building an AR. I’m a marine and my issued weapon is the h and k m27. Which has a gas piston operated bolt. I’ve found it to be much easier to clean and I like the operation better than the m4 I was originally issued. Do you have any articles on piston operated systems that have info to help me decide which to buy?

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