Selecting the “right” AR-15 barrel is probably the most difficult choice to make, especially for beginners. Companies throw a lot of marketing dollars around to convince you that you need the latest and greatest or you’re going to lose the match, be uncool, or even wind up dead in the street!
But they’re wrong.
This article summarizes just about everything I’ve learned about AR-15 barrels. While I’m focusing on the AR-15 platform in particular, the principles apply to any firearms barrel. The physics of ballistics don’t change just because you went from an AR-15 to a bolt action.
Everything I’ve written here comes from years of reading, competing, training, practicing, and writing articles for this blog and elsewhere. In this post, I’m posting a few quality examples of what I’m talking about. I’m linking to these in particular because they are companies that I know, like, and trust- but they aren’t the only game in town. There are many quality barrel makers out there, and I only have direct experience with a subset of them.
Suggestions aside, this article helps you understand the trade offs and quality indicators of a good AR-15 barrel so that you can make that decision for yourself. I’m not here to be yet another internet expert trying to sell you on the “Best AR-15 Barrel” of the moment.
Bottom Line Up Front
If this is your very first AR-15, I suggest a lightweight profile with chrome lining. This will serve you well in 90% of situations, and gives you the opportunity to grow your capabilities in many directions.
Every choice you make that deviates from the classic lightweight profile (AKA the “pencil barrel”) introduces a compromise somewhere. Whether the thing you gain makes up for the thing you lose is worth it or not depends entirely on your intended purpose.
One last point, and I’m going to say it a lot, realize that most people aren’t as good a shooter as they think they are. As Jeff Cooper once put it, a “good shot” is someone who can shoot up to the capabilities of the rifle. A highly-customized weapon, tailored for maximum accuracy, speed, or competition doesn’t do much good unless the shooter is capable of leveraging it.
If you aren’t there yet, I say stick with the classic lightweight barrel and shoot the snot out of it.
Setting the Baselines
This is a long article, and you might be pressed for time. So to get to some quick answers, let’s start with the basics and some hard and fast examples.
The M16A1 adopted in the 1960’s featured a lightweight 20″ barrel. I consider this the starting point of the discussion. The rifle evolved along with a small caliber high velocity (SCHV) projectile dubbed M193. This is a 5.56 mm bullet weighing 55 grains and launched at 3,150 feet per second from a 1/12 twist barrel.
The M16A1 weighed 6.4 lbs unloaded, which made it exceptionally easy to live with compared to full-powered battle rifles of the time. It was a gunfighter’s rifle, meant to be carried much of the time, and then perform well in a firefight.
Since then, technology for manufacturing, ammunition, and optics have come a long way. Because of that, it doesn’t make sense to stick strictly to the M16A1 template. However, we shouldn’t forget the lessons learned leading up to its adoption.
Most folks shopping for their first AR-15 only have some vague notion of what they want it for. Their described need usually falls somewhere between home defense, tactical training, plinking, competition, or the collapse of society. I argue that the classic lightweight barrel template is the best all around starting point, so let’s start with some suggestions there.
General Purpose Lightweight Barrels
This class of barrel performs well at most tasks, and is particularly suited to field rifles that are carried more than shot. If you’re sticking to a budget, then the Faxon Gunner Profile is a great wallet-friendly option. It’s light, accurate enough, and will take care of you for a long time. I used this particular barrel on my Minuteman KISS Rifle pictured here.
If you have a bit more money to spend, then I suggest Daniel Defense’s Hammer Forged Lightweight or the Bravo Company USA (BCM) Enhanced Lightweight Barrel.
Criterion’s new CORE series barrels have also impressed me with their accuracy potential.
Wondering whether you should look at a 16″ vs 18″ vs 20″ barrel? I say either 16″ or 20″, and I cover that in much more detail further down in the article.
General Purpose, Mid-weight AR-15 Barrels
These barrels have slightly more mass, which helps them better tolerate aggressive firing schedules 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 for the materials, manufacturing, and quality control. My favorite barrels in this class is Criterion’s Hybrid Profile as well as Centurion Arms’ hammer-forged options (either lightweight or mid-weight).
Precision AR-15 Barrels
In this category, you are no longer concerned with weight. These barrels deliver tight groups and consistent performance all day, so long as you don’t abuse them.
That also means you pay for it.
At the “budget” end, I suggest Criterion’s fluted stainless offerings. I put “budget” in quotes because, obviously, it’s still more expensive than the lower end of the market.
If you have even 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. If this is the path you’re going, then you’re probably building out a Recce Rifle or a Designated Marksman rifle. Check out my build guides for those configurations.
The Marksman’s Guide to AR-15 Barrels
With the quick and dirty recommendations out of the way, the rest of this article explains all of the factors you should know when choosing a barrel for your AR-15 build.
The Rifle’s Beating Heart
The modern AR-15 barrel isn’t far removed from the muskets and cannons used during the Revolutionary War. There is a charge, combustion chamber, projectile, and length of hollow metal tubing. As the charge burns, expanding high-pressure gasses propel the projectile down the bore.
Upon exit, the bullet begins a ballistic trajectory.
Nearly all of the modern advancements in rifle technology focus on the mechanisms for loading ammunition into the chamber as well as better manufacturing technology overall. The aerodynamics of bullets have also come a long way, as well, but that’s not the topic we’re talking about today.
More than any other component, the barrel affects accuracy, reliability, and handling of your weapon. It is fair to say it is the most important component you select when building your AR-15, and you build the rest of the rifle in support of the barrel.
When selecting an AR-15 barrel, you must balance accuracy, weight, handling, compactness, parts longevity, and recoil characteristics. The choices we have to make to affect these traits include:
- Material & lining
- Rifling method & twist
- Chamber dimensions
- Gas system
Every choice you make benefiting one trait negatively affects another. So the best you can do is prioritize which things are more important to you and consider how much “downside” you’re willing to tolerate.
Simply put: the more specialized you make your rifle for a specific task, the worse it perform at others. This is why I suggest your first rifle should be a general purpose “do-all” rifle. New shooters don’t have enough skill to take advantage of the specialized configurations. Here’s some numbers to illustrate what I’m talking about.
The average life of an AR-15 barrel is about 20,000 rounds. Let’s generously assume 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 expends between 20,000 and 30,000 rounds per year.
I’m not trying to discourage you. You can develop a huge amount of skill with small ammo counts and a lot of dry practice. This is only to show you just how much trigger time “pro” shooters actually put in to get where they are.
I want you to understand that the ultra-specialized configurations only make a significant difference in the hands of highly practiced shooters. A master-class level competitor using a bone stock AR-15 will still trounce a novice equipped with a fully custom top-end match rifle.
As an example, 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 another blogger (Shawn at the now defunct Loose Rounds) doing it. That act blew my mind and started me on the path to taking marksmanship more seriously.
A basic rifle is capable of way more than most “experts” give it credit for, and as the saying goes: “It’s the Indian, not the arrow.”
The Golden Rules of Buying Gear
As I get into the technical side of things, there are two rules that I try to abide by for most things:
- Mission drives gear
- Buy nice or buy twice
Make your decisions on what you need it to do. Many of us fall into the trap of buying things for the wrong reason and then try to shoehorn them into roles they’re not suited for.
If you are going to buy it, buy something nice enough to last.
AR Barrel Length: The Long and Short of it
The longer the barrel, the more velocity the projectile starts with. Velocity is important for two reasons:
- It decreases the required adjustment for a given range (otherwise known as shooting “flatter”)
- It increases the effective range of the bullet
Longer barrels generally correspond to lower pressures inside of the rifle. That, in turn, increases the longevity of the bolt, springs, and other components. The price for these benefits is increased weight and more cumbersome handling.
On the other end, short barrels have quicker handling, compactness, and lighter weight, but come at the cost of higher operating pressures, increased noise, and accelerated wear on internal parts. Suppressors practically become a requirement once you get down to 10.5″ barrels. The noise and concussion become unbearable, especially indoors where instantaneous permanent hearing damage will occur without protection.
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 at bullet exit.
This difference in pressure 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 on the racks. The M4s with the carbine gas systems, on the other hand, were breaking bolts pretty regularly as they approached round counts around 10,000. That’s not scientific, mind you, but an interesting anecdotal observation.
Accuracy and Trajectory
Many folks claim that long barrels are more accurate. They then use that as a reason to justify them. That is wrong and you shouldn’t believe it. Barrel length has little to do with accuracy. If anything, longer barrels reduce accuracy potential.
Imagine a long 2×4 wood board at the hardware store. The longer it is, the more prone it is to “flex” under stress as you hold it from one end. In barrels, we call this “whip.” Long skinny barrels demonstrate more whip than shorter or fatter barrels.
Don’t let that sway you just yet, though. The effect of the whip is very small and not something you should be concerned with until you are a seasoned match shooter. That gets into advanced topics like barrel harmonics during the shot.
So what do they really mean by more accurate?
When people talk about longer barrels being more “accurate,” what they really mean is that they think it’s easier to make hits at distance. This is a true statement.
A bullet reaches its maximum velocity as it exits the barrel. As soon as it exits, gravity and aerodynamic drag start doing their thing. A higher starting velocity means that the bullet drops less for given distance than a bullet starting at a slower speed.
For you, this means that you could land a hit with less holding over the target or adjustment to the sights. Yes, choosing a different bullet with better aerodynamics might have a similar effect, but this is an article about barrels and not ammo selection.
I wrote a much more detailed article on the effects of velocity on trajectory, as part of that, I put together these two visualizations:
The first chart compares a 100-yard zero for a 20″, 16″, and 12.5″ barrel. The red dot represents the point of aim. The black dots show expected impact points 25, 50, 200, and 300 yards. Notice how the overall extreme spread of impacts shrinks as the barrel length increases. It’s particularly noticeable with the 300-yard impact.
The second chart shows the same comparison for a 300-yard zero, and the effect is much more dramatic.
This is what we mean by “flat shooting.”
The nice thing is that this over/under is predicable using external ballistics calculations, and we can leverage this behavior to our advantage. A battlesight zero, sometimes annotated as “BZO,” means picking a zero distances that keeps all of your shots within +/- some distance of the point of aim.
Beyond the military, we also call this our Point Blank Zero.
Effective Range of the AR-15
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 threat-stopping wounds 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.” I wrote a lot about this in my series on the Small Caliber High Velocity (SCHV) Program that eventually produced the M16 rifle.
Daniel Watters, who I leaned on heavily for the SCHV series, did an outstanding job assembling the history. The short version of the research is that a lighter, faster bullet could be just as effective as a larger bullet up to realistic combat ranges.
What’s a realistic combat range? Historically, it’s about 300 yards, and that was the intended range for the .223 cartridge. Depending on the configuration, the M16/M4 series has listed effective ranges of 460 to 550 meters. Keep in mind that those definitions come from what the military wanted the rifle to do at that distance, namely punching a hole in a steel helmet. It says nothing about fragmentation or other terminal effects you might want as a defensive shooter.
So for civilian purposes as a defensive rifle, consider the AR-15 to be maximally effective out to about 300 meters with a 20″ barrel. That distance shrinks as the barrel gets shorter. Don’t get me wrong, you still poke holes in things and ruin someone’s day at 500-700 meters, it’s just that the rifle wasn’t designed for it.
But I digress.
This chart shows a plot of starting muzzle velocities of an M855 projectile fired from various barrel lengths. The red line represents the minimum velocity for M855 to have good terminal effect. Again, this is only for M855. There is a lot of other new ammunition designs that have good terminal effects down to 2000 fps and below, and you’ll pay for it.
By starting at a higher velocity, you give yourself more time and distance to have maximum effect. 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.
Comparing AR-15 Barrel Lengths
Let’s discuss the most common barrel lengths for the AR-15. Each one has its pros and cons. What you choose ultimately stems from your goals and needs.
20″ Barrels – Old Reliable
A member of the special operations member, who also happened to be a gun nut, once told me something that stuck with me ever since. He insisted 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.
The 5.56 cartridge and original AR-15 design happened in tandem. The barrel supported the new cartridge’s ballistic characteristics. The 20″ barrel was the heart, and it’s the workhorse or the AR world.
I would even argue that the 20″ barrel length is the most optimized for balancing velocity, longevity, and recoil characteristics. Going back to the velocity chart above shows peak velocity occurs at the 20″ length. Everything about it is optimized from velocity to pressure.
I’ve taken many folks accustomed to 16″ and 14.5″ barrels and let them shoot my M16A5, and they are often blown away at just how smooth the 20″ length feels.
20″ AR-15 barrels provide the most consistent performance across the widest variety of loadings. There are plenty of modern specialized loads “optimized” for use in a 16″ barrel, or shorter, but the 20″ length will still do things better for any given loading.
Aside from velocity, the long 20″ barrel gives a balance benefit to marksmen. With the extra length and weight, your rifle has a slight forward balance known as “hang.” This helps reduce the “wobble zone” during unsupported aiming.
The trade-off 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.
There are configurations like the M16A5 which help with the maneuverability aspect, but a shorter rifle is always easier in cramped quarters.
16″ Barrels – The Civilian Standard
The 16″ length is the most common AR-15 barrel length sold today. It doesn’t do anything particularly better than other options, but it has good all-around performance.
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.
That said, the 16″ AR-15 barrels offer a good compromise between the full 20″ length and the short barreled rifle class. It’s still capable of hitting any target the 20″ can hit while only losing about 50 meters of effective range compared to the longer rifle.
When you think about it, the difference between 200-ish meters and 250-ish meters effective range isn’t a big deal if most of your shooting is inside 100 meters.
Combined with a collapsible stock, the 16″ gun is compact enough to make living the rifle plenty more convenient. If you’re trying to be a bit more of a clone dork, then the 16″ barrel is your ticket for building a RECCE pattern rifle.
14.5″ Barrels – Military Carbine Length
14.5″ is the length of the military M4 and M4A1 carbine barrels. The M4 was adopted in 1994 after ten years of design and testing. The original intent was bridging the gap between the 20″ M16 series and the Vietnam-era XM177 “Commando” and its 10.5″ barrel.
It had a great reputation for compactness and ease of carrying, but came with noted shortcomings in low velocity and loud noise signature. In fact, the XM177 came equipped with a 4″ moderator to mitigate some of the noise, effectively bringing the length up to about 14.5″.
For the average person, there isn’t much benefit to dropping from a 16″ to 14.5″ barrel. In fact, it will cause you more pain than it’s 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 does bring the length back up to the legal 16.1″, but it also makes it difficult to do any future modifications.
If this is your first AR-15, 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
Few things trigger a good forum debate like discussing the “best” barrel length for an SBR. The thread will go on for pages and pages while they hash out the tiny differences between 10.5″, 11.5″, and 12.5″ AR-15 barrels.
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 barreled carbines are certainly fun to play with, but I wouldn’t want to seriously use one without a suppressor, especially indoors. Keep in mind that this class of AR-15 is purpose-built for close-quarters fighting. Sure, they can hit a target and poke holes in things at 300 or 400 meters, but I’d rather have something more suited to the job.
As for my personal preference, it’s the 12.5″ barrel. My reasoning goes back to something Monty (Navy SEAL and founder of Centurion Arms) once posted in a discussion on the topic.
18″ AR-15 Barrels – The “Special Purpose” Length
The 18″ barrel length has an interesting story, and it’s well documented by the Small Arms Defense Journal. After the Gulf War, Armalite floated an idea for a light sniper rifle called Special Purpose Rifle, and it went nowhere.
Later, in 1998, the 5th Special Forces Group resurrected the concept and started circulating it around SOCOM. The concept finally rose to prominence when the GWOT started ramping up in the early 2000s. The Navy signed on and dubbed it the Mk12 program.
The SOCOM program started by trying to formalize the Navy SEAL Recce rifle and its 16″ barrel. But the 5th SFG guys wanted a 20″ barrel for velocity reasons. Eventually, the Army won the debate. But as it tends to go, bureaucratic red tape caused more headaches.
The Navy had trouble requisitioning 20″ match barrels because “the system” kept telling them that 20″ barrels already existed in the supply chain. The fact that these were standard M16A2 and A4 barrels not up to the accuracy requirement for a light sniper rifle didn’t matter.
So the program office changed the specification to 18″ and said that there wasn’t anything in the system. They got their barrel.
Regardless of its origins, the 18″ AR-15 barrel is very popular in the competition world. The rifle-length gas system offers a very smooth recoil impulse, while the 2″ shorter length shaves a few ounces off of the end of the rifle. The length also maintains good velocity for reaching out with a flatter trajectory.
The 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 when I get to gas systems.
As a side note, the Army still wanted their 20″ accurate barrel, and the idea lived on in the SDMR (Squad Designated Marksman Rifle) and Marine SAM-R (Squad Advanced Marksman Rifle).
AR-15 Barrel Length Takeaways
Those who carry rifles professionally do a lot more living with their weapons than shooting them. 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 unless things have gone very very wrong in the world.
How are you actually going use your rifle? I argue that the benefits of shootability and effective range of longer barrels outweigh perceived inconveniences about “living with the weapon” for rifles that only ever move from the safe to the range and back.
Also, as a bonus, we civilians can take advantage of configurations the military never fully adopted. Such as the M16A5. This configuration combines 20″ barreled upper with collapsible stock. That reduces some of the pain associated with full sized rifles and still provides all the benefits. This is actually my favorite configuration, and the Canadians liked it well enough to adopt is at the C7A2.
Now that we’ve covered AR-15 barrel length, let’s talk about profiles.
The Skinny (and Fat) on AR-15 Barrel Profiles
The profile of a barrel refers the thickness of metal at different points along its length.
The way mass distributes along the length of the barrel has dramatic effects on how the barrel performs for different tasks. For example, a thin profile all the way down, the “pencil” profile, makes a very lightweight and easy to carry weapon that heats up quickly. In contrast, a thick profile from end to end, the “HBAR” profile, makes a heavy rifle that sustains accuracy over a long string of shots.
AR-15 Barrel Heat Management
The balance here is one of heat management versus handling.
From a cold bore, there’s little practical difference in accuracy between a light barrel and heavy barrel for the first few shots. The differences appear as the barrel heats up.
You might remember from chemistry classes way back in the day that metal expands as it heats. For rifle barrels, this expansion negatively affects the accuracy of a barrel. So the faster it heats up, the quicker your accuracy potential starts dropping.
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 are also slower to cool down.
I should note that a few manufacturers, like Faxon Firearms, claim that the heat treatment they’ve done on their barrels greatly reduces the effects of heat on accuracy. That could be true, or false, I’ve not tested it myself.
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 dissipates heat well. Criterion uses this in their “Hybrid Profile” as well as their CORE serries AR-15 barrels. Ballistic Advantage calls their version the “Hanson Profile.” Faxon’s “Gunner Profile” fits this category as well.
There is also the “Government” profile found on the M16A2, M4 carbine, 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 handles heat.
You can read more about that in my article on weight and balance.
Choosing Your AR-15 Barrel Profile
Rifles that are carried a lot and see easy to moderate shooting schedules are the best candidates for light barrels. This is most people, including military members who live with their rifles.
Heavy barrels are suited to rifles that need to maintain higher levels of accuracy over long strings of shots. This describes match shooters and precision rifles shot from relatively fixed positions.
There’s also an argument for heavy barrels when it comes to sustained high rates of fire, as with an “automatic rifleman” type of project (though with something like a binary trigger rather than actually being fully automatic). The heavier barrel helps soak up the heat from those long strings, but just remember that it’s also going to take time to cool back down. I’ve personally never had interest in trying this AR-15 project.
As the late Hognose of Weapons Man once put it, “You can carry it all day, or you can shoot it all day, but not both.”
Most people are far better served by lighter profiles.
AR-15 Barrel Composition
When it comes you barrel steel, you’ll choose between Chrome Moly or Stainless. Those descriptions are very broad, with many sub-varieties falling under those labels. Some alloys are more useful than others.
The two alloys have different wear characteristics. It looks something like this: 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 its accuracy declines much more quickly.
This is a generalized example and ignores the quality of the barrel, rate of fire, heat, or load pressure of the ammunition. All of these things dramatically affect the life of a rifle barrel.
Any barrel might last through many thousands more rounds shot at a slow cadence than it will doing regular mag dumps. This heat management issue is why you see such a wide discrepancy between the mechanical cyclic rate of an AR-15 (700-800 rounds per minute) and the sustained rate of fire (12-15 rounds per minute, or about one shot every 4 to 5 seconds). The sustained rate of fire is the heat management tipping point where the weapon could sustain that rate of fire indefinitely without overheating.
Chrome Moly Barrels
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% carbon– hence the 4140 and 4150. In real terms, 4150 is a harder alloy and better stands up to temperature extremes, such as fully automatic fire or very cold weather.
The US Government 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.
The third blend in this spec includes Vanadium. This blend is called Chrome Moly Vanadium, or 4150 CMV. You may also see it called 41V45 steel. The vanadium in this alloy helps promote a finer grain structure, wear resistance, and strength.
A manufacturer may claim their barrel is 4150 steel, but that doesn’t mean it’s actually certified under MIL-B-11595E. Being both 4150 and 11595E certified is more difficult, and thus more expensive.
A manufacturer that’s gone to the trouble of utilizing certified barrel steel will advertise the fact. If they don’t, then you may be getting a barrel made with plain AISI 4150 steel, which has looser tolerances on its chemical composition.
If you shop around enough, you’ll come across retailers selling AR-15 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’s a variety of 41V45.
Stainless Steel AR-15 Barrels
When we start talking about stainless steel barrels, everyone assumes we’re talking about precision rifles and match grade barrels. To dispel that, understand that 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 is able to machine, rifle, polish, and lap a barrel in one place according to their own quality control standards. Due to a shortened supply chain and skilled craftsmen all working in one place, the finished product is cleaner and more consistent from barrel to barrel. It’s this improved consistency of the barrel and its finish on the inside of the bore that improves accuracy.
Speaking of finishes inside the bore, another factor is protective linings. Since stainless barrels have a much higher amount of chromium in their chemical composition, upwards of 12% compared to about 1% in chrome moly barrels, they do not need any additional coatings or treatments to resist corrosion. This is important because the application of chrome lining to rifle barrels historically meant that the bore surface became uneven and inconsistent compared to a bare metal finish. These inconsistencies disturbed the bullet and reduced accuracy.
Due to the easier process, and all things being equal, good stainless barrels are 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. The necessary inclusion of sulfur in the metal raises a risk of developing sulfide stringers. That’s a fancy way of saying that the barrel could suffer catastrophic failure under fatigue.
What is fatigue? For barrels, it comes from extreme temperatures. That could mean very cold environments, very high rates of fire (and therefore heat), or some combination of the two. The best way to prevent this fatigue to increase the barrel thickness and avoid wild temperatures.
Stainless Alloys for Rifle Barrels
The most commonly used alloys for stainless AR-15 barrels are 410 and 416r.
410 is harder and more durable, owing to its lower sulfur content. The trade-off is that without the extra sulfur, the barrel becomes more 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 rated to temperatures down to -40 degrees Fahrenheit. This is the most common alloy you’ll come across. It has more sulfur than 410, so it comes with increased risk of failure from sulfide stringers.
IMPORTANT: 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 all together. I know they are out there, and that’s because people ask for it even if it’s not a good idea. Such barrels might work fine for a bolt action hunting rifle that only get shot once every few hours, but it certainly isn’t meant for the higher rates of fire you see in defensive carbine classes or tactical matches.
Protective Linings for AR-15 Barrels
This is where the real differences between chrome moly (CM) and stainless barrels become apparent. CM and stainless barrels, machined bare, have about the same level of accuracy. But you’re rarely going to find a bare CM barrel on the market. CM barrels require extra lining, typically composed of chrome, to resist corrosion.
To improve corrosion resistance and wear characteristics of combat rifles, it’s important to line the bore and chamber of a CM barrel with chrome.
In the 21st century, manufacturers like Criterion have figured out how to get a more even and consistent layer of chrome in the bore. I have another barrel from Centurion Arms made of FN’s machine gun steel and double-thick chrome lining that shoots about 1 MOA with good ammunition.
The bottom line is that chrome lining does not deserve the negative accuracy reputation it has today- so long as it comes from a quality manufacturer.
Nitrided AR-15 Barrels
Another popular option is nitriding. I’ve written a much longer article about this nitrided barrels, so this is just a snapshot. Nitriding has several trade names such as Melonite, Tenifer, QPQ, Salt Bath Nitride, and others.
Nitriding is a surface conversion process whereby manufacturers submerge the barrel in a nitrogen-sodium solution and heat to a high temperature, usually between 750 and 1200 degrees. Remember this temperature for later.
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’s very corrosion resistant and has a much lower coefficient of friction compared to bare metal or chrome.
The “killer feature” of nitriding is that the accuracy of the barrel is maintained while the corrosion and wear characteristics improve. Nothing is free, however.
There are two downsides that I know of. The first is the temperature required to complete the process, which is very close to those used for stress relieving barrels after rifling- specifically stainless barrels. 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. More on that in a second.
Secondly, the nitride case is much less heat resistant than chrome. It is not an ideal solution for barrels that 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.
So Which AR-15 Barrel Material Should You Choose?
99% of shooters are best served by a quality CM/CMV barrel. Either chrome lined or nitrided. Well-made barrels are more than capable of turning in 1.5 to 1 MOA groupings or better. 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.
It doesn’t make sense to spend $500 on a custom stainless match barrel capable of 1/2 MOA, and then feed it cheap bulk ammo that can’t group better than 3 MOA.
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.
AR-15 Barrel Twist Rate and Rifling
Rifling describes the grooves cut down the length of a barrel’s bore. These grooves “grab” the bullet and impart rotation. Like a spiraling football, this spinning action increases the accuracy and range of the projectile. We measure how stable a bullet is by applying a gyroscopic stability factor.
The stability factor is a combination of the spin rate and the shape of the bullet. Before I get into determining twist rates, let’s quickly look at how we create rifling.
AR-15 Barrel Rifling Methods
There are three methods that manufacturers use to form the rifling of a barrel: cutting, button rifling, and hammer forging. Each of these has pros and cons and 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 is the original “old school” way. A machine cuts each groove one at a time over many 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 quality controls required.
This video shows a machine performing cut rifling
Button rifling is the military specification and involves forcing an extremely hard “plug” 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.
Many qualities barrels are button rifled, so don’t let anyone fool you into thinking that it’s inferior to other methods.
First developed by the Germans during WWII, this method uses a tungsten “negative” mandrel inserted into the smooth bore of a “blank.” A large hydraulic 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.
This video demonstrates what the process looks like. It comes from GFM Machine, which produces most of the hammer forging machines in the world. One of the original engineers who designed the process founded the company.
Stress Relieving a Rifle Barrel
Applying high amounts of force on steel results in stress. To combat this, we need to stress relieve the barrel. Without this step, the rifle’s barrel would lose accuracy very quickly after starting to heat up.
To summarize the process, 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. 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? It’s about 1000 degrees. That is right around the upper range of the nitriding process. Unless the company doing the nitriding is very careful to control temperatures, there is a risk of undoing the important stress relief work. At that point, the barrel will become very inaccurate as it heats.
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.
AR-15 Barrel 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 calculation accounting for a bullet’s diameter, length, and weight distribution. More length, and more mass, for a given diameter typically requires a faster twist to stabilize it.
The original AR-15 barrels had a 1/14 twist rate. Later, the specification changed to 1/12 for use with M193 55gr bullets. When the M16A2 arrived, the twist rate increased to 1/7 to better work with the long tracer rounds.
For a long time, most civilian AR-15 barrels had a twist rate of 1/9, which is actually a really good compromise for shooting most common bulk ammo between 45gr and 62gr. But since the military specification is 1/7, it has been deemed “most desirable” by serious users.
As the popularity of 77gr SMK for match shooting and defensive use took root, 1/7 twists now reign supreme.
I’ll note that a 1/8 will shoot just about anything 77gr and below well, and is the one I recommend most if you don’t mind deviating from the pack.
I put together this chart to help show you the ideal twist rate for various bullets.
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 ones, you could see some issues with high RPMs causing the bullet to come apart during flight.
Ballistician Bryan Litz does point out that there is an optimum twist rate for maximum accuracy, though. 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.
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.”
If you do enough shopping around you’ll eventually come across a manufacturer touting their various special polygonal rifling patterns. Remington calls theirs 5r, Shilen calls theirs “ratchet rifling,” and there are other names. The bottom line is that they all effectively do the same thing by using gentle “swells” and smoother angles to do the same job as traditional grooves.
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.
AR-15 Chamber Dimensions
The .223 and 5.56 are very closely related. In fact, most people think they are identical. Dimensionally, they pretty much are.
The difference comes down to the pressures that each generates. 223 is a SAAMI specification, so everyone knows how to follow it for the civilian market or reloaders. On the other hand, 5.56 is a military specification and one that’s 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.
Rifles with 5.56 chambers have slightly more room on the inside of the chamber 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.
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. The gas required to do this bleeds out of a small hole in the barrel, known as the “gas port.” This is true of just about every modern semi-automatic rifle design whether it’s piston-driven or direct impingement.
On an AR-15, the gas port is on the top of the barrel under the front sight tower. Gas flows down a tube, where it meets the gas key of the bolt carrier group. It then flows inside the bolt carrier, where the pressure pushes against the back of the bolt and the rear of the carrier. This forces the two components apart and unlocks the bolt. The rearward momentum then continues to cycle the rifle.
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.
What makes this calculation particularly tricky is that different lots of ammunition have different amounts of pressure, so what works for one lot of ammunition on a hot day may not work on another on a cold day. To combat this, most manufacturers will “overgas” a rifle slightly to ensure it works across a variety of ammunition in a variety of environments.
Match shooters who always shoot the same load of ammunition through the same rifle are the only ones who should consider trying to run on the raggedy edge of cycling reliability.
AR-15 Barrel 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. As the bullet “uncorks,” the pressure in the system equalizes back to “neutral” atmospheric.
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 like 18″ and many of the SBR lengths. 18″ barrels with rifle-length gas systems have two inches less “dwell time” when compared to their 20″ cousins. That means the gas port must be enlarged to let more gas in the system before the bullet uncorks.
The inverse applies as well with long dwell times. 16″ barrels with carbine gas ports need smaller holes.
This dwell time issue becomes very important on the very short 10.5″, 11.5″, and 12.5″ barrels, as they all use the same 7″ carbine gas system. Suppressors add yet another layer of complexity, as they also increase pressures and manufacturers have to find a way to work well both suppressed and unsuppressed.
To date, I’ve seen companies deal with this in two ways. The first is some kind of adjustable gas block design, so that the user can adjust how much gas enters the system according to their needs. The second way is more interesting, as it involves using smaller gas ports set closer to the chamber, intentionally increasing the dwell time and siphoning off that pressure for longer.
Something else manufacturers have to content with is “leaky” gas block fitment, where some portion of the gas vents out at the base of the gas block rather than traveling through the gas tube and into the system. I keep repeating myself on this point, but this is why buying from quality manufacturers matters.
Selecting Your Gas System
The hard-and-fast rule here is the longer the gas system, the lower the operating pressure. That means a more gentle recoil impulse and longer parts life. That’s not always true, however, it’s just a general rule. Again, buy something quality and let the manufacturer figure this out for you.
I want to leave you with one more piece of advice, because I see it all of the time in message boards and social media. 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 delivered 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 negotiates 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, on the same machines, but one has a much higher spec product held to tighter tolerances.
I’m not saying that Athena’s barrels are junk. They might even perform quite well, though not as well as Wildcard’s spec. I would also expect that the factory, FN in this example, also wants to maintain a positive reputation and wouldn’t push junk. 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.