If you skipped my general advice for a first AR15, my recommendation there was to start with a basic lightweight 16″ or 20″ chrome-lined gun with collapsable stock and fixed front sight (from a reputable manufacturer, of course). If this is your first AR, I implore you to go back and read that post. The following advice is geared towards those who have already been shooting their guns and are looking to build a second one, or improve their first.
This post summarizes just about everything I’ve learned about barrels over the years. Note, I am not an engineer, so I’ll be skipping past the engineer speak and focusing on the basic easy-to-understand principles.
First, let’s review my two basic laws of buying AR-15s and accessories:
1. Let the Mission Dictate Configuration
2. Buy Once, Cry Once
I will focus on four main considerations for buying a new barrel: Length, Material, Profile, and Rifling. I will also talk a bit about the gas system at the end. Remember, there is no such thing as a free lunch. Every decision comes with a compromise in some other area. The more specialized you make your rifle for a particular task, the less suited it becomes for others. This is why I always recommend first time buyers start with a “generalized” gun that can do a bit of everything pretty well, but will be outperformed in specific roles compared to specialized configurations in the hands of the same skilled shooters.
Allow me to clarify that last sentence. If you are new to shooting, then you have not developed the skill to take advantage of special configurations. An NRA High Power shooter with a High Master classification shooting a bare bones chrome-lined M16A2 will still outscore any new shooter, even if that newbie has a tricked out match rifle. The tricked out rifle really only makes a difference between two skilled shooters.
Ok, let’s go…
Barrel Length: The Long and Short of it
The barrel is the heart of the gun. The physical principles of gun barrels have not significantly changed since the origins of rifles and cannon. There must always be a combustion chamber, a charge, and a projectile. What has changed over the generations is the method by which ammunition is loaded and the manufacturing technology available to consistently produce high-performing barrels.
Generally, a longer barrel will add more velocity to a projectile at the cost of weight and handling; a shorter barrel will reduce weight and create a more compact, lighter, and better handling gun at the expense of velocity and increased muzzle blast. A projectile is driven down the length of a gun barrel by the pressure of expanding explosive gasses from the powder charge. The cartridge case is mechanically locked in place by the bolt lugs, which means the only direction the gas can expand is toward the muzzle. The more time you give that gas to expand (via longer barrel length), the more energy will be imparted to the projectile. Once the projectile leaves the muzzle, or “uncorks,” then the expanding gasses will vent to the outside atmosphere and the pressure inside the whole system drops back to normal. Whatever energy the bullet has when it uncorks from the muzzle is all that it is ever going to have, and that energy begins decreasing immediately after uncorking.
The shorter the barrel gets, the less time there is for the expanding gasses to impart energy to the bullet. If the barrel is sufficiently short, then the initial powder charge may not have even fully burned yet, resulting in much more violent release of gas upon uncorking (hence the brutal muzzle blast and sparking fragments from short barreled rifles).
This chart (which I grabbed from a great article at the Small Arms Defense Journal) shows the bore pressure from expanding gasses at the moment of uncorking. Higher pressure equates to more blast and concussion, and potentially more pronounced muzzle movement.
Why does velocity matter? The short answer is better trajectory and terminal effect. Contrary to what you may have heard, longer barrels are not inherently more accurate than shorter barrels. Actually, the reverse is true. Shorter barrels are stiffer and less prone to “whip;” but this effect is minuscule and you should not worry about it. The accuracy potential of a barrel has much more to do with its manufacturing method and the quality control (QC) procedures of the manufacturer. Velocity (and, by extension, a longer barrel) provides a “flatter” ballistic trajectory. For a marksman, this translates to less holdover or sight adjustment between different ranges [see this post]. There are many other factors involved here, such as the aerodynamic ballistic coefficient of the bullet. But for now, let’s just simplify by saying longer barrels provide more starting velocity and energy for a given projectile.
In some calibers, particularly 5.56, velocity is also important for terminal effect. This means that the higher the velocity is, the better the bullet will damage the target. This is important for hunters or self defense minded shooters. The 5.56 is a relatively light projectile, and most bullet designs rely on velocity to cause sufficient tissue damage. The lower the velocity dips, the less effective the bullet becomes.
To illustrate, here is another chart from the same article at the Small Arms Defense Journal. In this chart, you see the initial velocity on a tested M855 bullet from different barrel lengths. In order to keep things consistent, they used the same barrel and cut it down one inch at a time from 24″ to 5″. The red line indicates the velocity at which the bullet can no longer be expected to reliably provide terminal effect. Note how the longer barrel lengths impart a higher starting velocity. This increases the possible ranges at which the M855 will reliably perform on target.
Remember, this chart is geared towards M855. There are a lot of other bullet designs out there that will have a different velocity requirement. Some designs are not velocity dependent at all, and you will find these very popular among shooters who rely on very short barreled weapons. However, the underlying principle remains the same: longer barrels increase the possible effective range of a bullet. I’m not saying that the 5.56 isn’t useful once it drops below a certain velocity threshold. Below a certain velocity, though, we’re just poking .22 holes in things rather than causing massive tissue damage. Still, poking .22 sized holes in a living target is still effective as long as that hole is put somewhere vital. If all you’re doing is poking holes in paper on a shooting range, then this doesn’t really affect you.
Let’s discuss the practicalities of specific barrel lengths.
20″ Barrels – Old Reliable
20″ Barrels are the old school workhorse of the AR world. The AR-15, and 5.56 cartridge (particularly M193 and M855, the most common surplus cartridges available), were designed around a 20″ barrel. I would go so far as to argue that the 20″ barrel, and its rifle length gas system, is the optimum length for recoil characteristics, service life, and terminal effect. The chart above demonstrates this quite well. Notice that the peak velocity occurs at 20″, and starts to level off (or drop off) at longer lengths. While some ammunition loadings may be optimized for shorter barrels, the 20″ provides the most consistent performance across the widest variety of loads.
In a discussion I read with a few gentlemen in special units in the military, they almost always say that they will never sacrifice velocity if they don’t have to. While the rest of the AR shooting community is obsessed with going smaller and smaller, professional users only go shorter due to mission considerations (working out of cramped vehicles, weight concerns, confined spaces, etc).
You, as a bona fide member of the civilian world will probably never find yourself kicking down doors, charging out of an armored HMMWV (Humvee), or dropping out the back of a perfectly good airplane with a full combat load. In all likelihood, the most action the average AR owner will see is the time that it takes them to take the rifle out of the safe, drive to the range, shoot it, and drive back home. So why not start with the most reliable and easiest to shoot configuration?
The longer 20″ barrel has a bonus benefit to the marksmanship-oriented shooter. The bit of extra weight out front provides some forward balance. This “hang” off the end of the gun is useful for reducing the wobble zone and decreasing muzzle jump after the shot. I would argue that these are more valuable benefits to the average user than the ability to do a 1-5 drill in a fraction of a second faster by using a shorter and lighter gun.
While the 20″ barrel has partially fallen out of popularity in the last fifteen years, my personal unscientific observation is that it is making a resurgence. The chatter I’m seeing is that people have enough shorter guns (or can’t get SBR for legal reasons), and are looking for something more interesting than another vanilla 16″ AR. A lot of them bought modern versions of the old 20″ as a fun toy, or a clone of a service rifle, but have found that they actually like shooting it more than most any other gun in the collection. That should tell you something.
16″ Barrel- The All-Rounder
The 16″ barrel is kind of an oddity, really. I sincerely believe the 16″ AR is the most common length because it is the shortest allowed length under the NFA of 1934. If people could get an unmolested 14.5″ barrel, the length on the M4 carbine, then that would probably be the most popular. But, since the NFA is in play, the 16″ is it.
That’s not a bad thing, though. The extra 1.5″ over 14.5″ does impart a bit of a velocity boost. Quite a few new 5.56 loadings are being optimized for performance in 16″ barrels. The 20″ will still do things better, but the potential gain of going longer might not be worth the extra length and weight penalties. When it comes to home defense, the loss of 4″ off the end of the barrel does make a difference in handling and moving around the house “tactically,” but it really isn’t that significant (especially with a collapsable stock).
While the 20″ may shoot a bit flatter, and push the maximal terminal velocity envelop just a bit further out (about 50 meters), the 16″ will work just fine at the most realistic distances the average shooter will be using it. If your focus is on self defense scenarios, you will most likely never be able to justify taking a shot beyond the practical range of a 16″ barrel.
As a reminder, just because a 5.56 bullet drops below a given velocity, it doesn’t render it ineffective, it’s just not as effective as it could be. The 20″ and 16″ are both equally capable of hitting targets at 500-700 yards. Because of its good balance between velocity, weight, and compactness, the 16″ makes a great all-round performer. This is why I suggested it in my recommendations for the first time buyer.
14.5″ Barrel – The ‘Mil Spec’
14.5″ gained popularity because of the M4 carbine. Like the 16″ barrel, it provides a good all-round balance between weight, compactness, and velocity (being very close to the 16″). The 14.5″ has gained popularity in the last several years despite the NFA restrictions because of the ability to permanently attach a muzzle device to the end of the barrel and bring the overall length back up to legal ground. However, I dislike this path because it severely limits the user from being able to experiment with different muzzle devices, rails, sights, or the other things that we like to swap out on occasion. If you really want to go the 14.5″ route, then you might as well file the NFA paperwork and get your stamp so you can play with even shorter/handier lengths.
18″ Barrel – The Special Purpose Barrel/Competitor
The 18″ barrel has gained prominence for two reasons: 1) It is the length found on the military Mk12 SPR and 2) It provides a nice balance of velocity to weight to recoil characteristics in competition.
The 18″ barrel usually retains the same rifle length gas system of the 20″ barrel (I’ll talk about this below), but loses the two inches of steel from the end of the gun. This usually saves valuable ounces of weight, and takes them from the place that most affects the handling of the rifle. This makes it very popular in the three-gun competition world, especially when everyone is running heavy stainless barrels.
As far as the SPR goes, the 18″ was a compromise in order to keep the length down when using a suppressor. From my reading, most of the guys who had the option ended up ditching the 18″ barrel and going down to 16″ (otherwise known as the RECCE configuration).
I think the 18″ sits in a nice balance of length, weight, and recoil. It will retain the smooth recoil impulse of the rifle length gas system, while having better velocity and “hang” than a 16″ gun. But this is not without tradeoffs (I’ll discuss this in the gas system section).
Short Barrels 10.5″, 11.5″, 12.5″
The Mk18 CQBR issued to “cool dudes” uses a 10.5″ barrel. BCM has a really good explanation as to why they went with a 11.5″ barrel for their signature SBR. And quite a few professionals I’ve talked to have all expressed a preference for 12.5″ when available due to the previously mentioned mantra of “never turn down velocity.” These will all be short, relatively light, and quite handy for moving quickly. But they will also all be very loud due to the aforementioned pressure issues with short barrels.
If you are looking into these barrel lengths, then take your pick. They all have similar terminal performance and are designed to function roughly the same practical ranges. As one military contractor put it, the bad guy isn’t going to know or care if he got shot with an 11.5″ or a 12.5″ gun.
Barrel Material, or, How I Learned to Shut Up and Love Chrome
With barrel material, you basically have two common options: Chrome Moly or Stainless Steel. There is a wide variety of different metallurgical alloys within those categories, as well as surface coatings (chrome, nitride, etc). Rather than dig into engineering, I’ll just simplify as much as I can.
Stainless barrels, despite common perception, are not inherently more accurate. They are more common in precision rifles because their wear characteristics are more consistent and they are easier to machine. A quality CMV barrel will start to see accuracy drop off at X number of rounds and then consistently lose accuracy after that point until it becomes unserviceable, a stainless barrel will last a bit longer than ‘X’ before starting to show accuracy degradation, which will then occur at a much faster rate. Even then, a well made stainless barrel will drop off from outstanding accuracy to “really good” accuracy and still outperform most chrome lined CMV barrels on the market. ADCO posted a pretty good torture test on a 11.5″ stainless barrel that demonstrates this characteristic.
Stainless barrels also show a marked improvement to corrosion resistance over the CMV metals, meaning that they will rarely be chrome lined. Because they do not need lining, the supply chain for producing stainless barrels is shorter. This economic benefit is one of the driving reasons why small business that produce custom barrels tend to focus on stainless barrels. This lets them used the saved money towards better machining and QC practices, providing a more accurate barrel for the price.
However, nothing is free. Stainless barrels, due to their molecular structure, are not as “tough” as CMV barrels and are more prone to problems in harsh temperatures.
There are three common metal alloys used for stainless barrels: 410, 416, and 416R. 410 is the “hardest,” and probably the most desirable. Rather than rehash it, go check out this thread on M4C, it is very informative and quotes quite a few experts. The bottom line recommendations on buying a stainless barrel are:
- Avoid lightweight or thin profile stainless steel barrels. This recommendation is echoed from multiple barrel manufactures due to the potential issues with temper embrittlement in martensitic stainless steels. The thicker walls of a medium, heavy, or bull profile barrel will strengthen the barrel and make up for the shortcomings of 400-series stainless steels as a barrel steel.
- Avoid standard 416 stainless steel barrels. 410 and 416R stainless steels both have a lower sulfur content, making them less prone to developing sulphide stringers which may result in catastrophic barrel failure.
- If the rifle will never see freezing temperatures, 410 stainless steel will likely be your best option. 410 stainless steel has the lowest sulfur content of the three grades we discussed, and will be the least likely to develop sulphide stringers. Avoiding sub-zero temperatures and using a barrel of adequate thickness should also minimize the temper embrittlement issue.
- If the rifle may see freezing temperatures, 416R stainless steel will likely be your best option. 416R stainless steel is rated for use at temperatures as low as -40 degrees Fahrenheit and has a lower sulfur content than standard 416 stainless steel. However, you should still ensure the barrel is of adequate thickness.
CMV refers to a general blend of chrome-moly vanadium. The mil-spec metal, and the and the expected standard, is 11595-E. Better quality barrels will be made from a 4150 blend, while more inexpensive barrels will be made from a 4140 blend. 4150 is stronger and more heat tolerant than 4140; but I would say that the average AR buyer is not going to push their gun hard enough to notice a difference. That’s for you to decide, however.
Remember, this is just a basic rundown. At least one of my barrels is made from a different alloy designed for light machine gun use, and therefore has better heat tolerance than even 4150.
Machined bare, without coating, CMV barrels are every bit as accurate as stainless. They can be machined and profiled thinner and suffer fewer problems relating to temperature. On the other hand, they are also more prone to corrosion and have a lower lifespan. Though, being honest, the average AR buyer will probably never wear out their first barrel.
There’s not much else to add, because the real difference between CMV barrels and stainless comes down to the linings.
History makes a big deal about the lack of chrome lining on the original M16s during Vietnam. Chrome provides a protective layer in the chamber and bore that increases the life of the barrel and reduces risk of corrosion.
Common wisdom is that consistency is accuracy, and that chrome lining reduces the accuracy of a barrel because the thickness of the coating is not consistent all the way down the bore. That remains mostly true. These days, manufacturers have figured out how to still make very accurate barrels with chrome lining (Lothaen, of The New Rifleman, is experimenting with one of these barrels right now from Criterion). I have a barrel from Centurion Arms that is chrome lined, and it has proven to be a solid 1 MOA performer. For comparison, the standard spec for a military barrel is 3-4 MOA, and the spec on bolt action sniper rifles used to be 2 MOA.
Another, newer, treatment is nitro carburizing. You will see this called a lot of trademarked names: Melonite, Tenifer, Salt Bath Nitride, and others. It is a chemical surface conversion of steel to make it harder and more corrosion resistant. Perhaps even more so than chrome. The “wow” factor is that it does this while not affecting accuracy. On the surface, it seems like a good process. Being honest, though, I don’t know enough to say if it is the best way to produce a barrel just yet. There is some concern about the high temperatures the barrel must be heated to in order to complete the process. These temperatures, especially for stainless barrels, are high enough to undo some of the important stress relief work that helps maintain accuracy (this will be covered later).
What should you get?
My honest opinion is that 99% of users are best served by a CMV barrel from a reputable manufacturer. Either chrome lined or nitrided will do. Stainless has its place for expert users who really want to squeeze out that extra 1/2 MOA from a barrel made at a boutique shop (Krieger, Bartlein, Shilen, etc), but most people really don’t need it, nor are they capable of taking advantage of it. The issues with cold environmental temperatures and necessity of thicker profiles might be too much of a trade off, especially for hunters located in northern latitudes stalking through mountains in the colder months of the year.
That said, if you absolutely need better than 1 MOA, and don’t mind the temperature/weight trade offs, then go for stainless. Otherwise, get a good CMV in a profile that works for you.
The Skinny (and Fat) on Barrel Profiles
These barrels represent a selection of Lothar Walther barrels in various profiles (heaviest to lightest) available from Spikes Tactical.
There is only a minor difference in mechanical accuracy between a lightweight and a heavy barrel. They will both put your first shots where you aim them. Though, as with barrel length, there are trade offs to be made between profiles.
When metal heats, it expands. The hotter the metal gets, the more it deforms. This deformation is what causes accuracy loss in hot barrels. If you get a barrel sufficiently hot, it also destroys the rifling and severely shortens the life of a barrel. The underlying question of barrel profile is how long can the barrel sustain fire without becoming overheated to the point of unacceptable accuracy loss or destruction. Part of that depends on your definition of “acceptable accuracy loss.” For a combat shooter, going from 2 MOA to 5 MOA is still roughly “minute of bad guy.” But for a precision shooter, a 3 MOA drop in accuracy means the difference between hitting or missing a small target a 300 meters.
Let the needs dictate the configuration.
Lighter barrels will save valuable ounces (or pounds) off the end of the weapon, making it ‘point’ faster and be less burdensome to carry. However, light barrels will heat up faster and begin losing accuracy sooner. The average AR owner will probably not shoot enough ammunition in a year, much less in a few minutes, to cause a lightweight barrel to overheat to the point of failure. It is important to note that while they may heat up faster, light barrels also cool down faster.
In short, lighter barrels are better for for rifles that will be carried a lot and see light to medium duty shooting schedules.
Heavier barrels heat slower, and therefore show less deformation over the same firing schedule as a light barrel. They also take longer to cool down due to the extra material serving as a heat sink. This equates to a heavier and more cumbersome weapon, but one that will retain its accuracy over a longer string of shots.
Put these two together, and you have the basics to understand the trade off. If you have a gun that will be carried a lot, and shot on occasion (such as in hunting, patrolling, or surviving the zombie apocalypse), it is probably more beneficial to have a lighter profile. The original M16 followed this pattern with its pencil profile barrel.
If the gun will spend most of its life sitting on a bench and/or need to keep tight groups over long strings of shots, such as in competition, then a heavier profile may be in order.
There is always a happy medium. There are a lot of interesting profiles out there, more than I really want to talk about here. I will say that I think the government profile (which is lightweight at the rear by the receiver, and heavier at the muzzle) is counterintuitive. But it’s readily available, and it works [Note: I have since written about the origin of this profile]. My preference for the average user would be a pencil profile all the way through (like the old M16A1) or a medium taper profile that starts heavier by the chamber and then thins out as it reaches the muzzle (such as Criterion’s Hybrid profile, or Faxon’s Gunner profile).
In any case, based upon your usage, choose the profile that makes the most sense for you. Most people are far better served by a lighter barrel over a heavy one.
The Basics of Rifling
Twist rate and rifling is a hot topic. The standard wisdom is that you want a 1/7 twist, which means the rifling makes one full turn every seven inches of barrel length. You will also see 1/8 twist (usually in boutique stainless barrels), and 1/9 (more common among inexpensive models). The original AR-15 had a 1/14 twist, which later become 1/12 in the M16. The M16A2 increased the twist rate to 1/7 in order to accommodate the longer tracer rounds.
Bullet twist is not necessarily matched to bullet weight; it is matched to the length of the bullet. Longer bullets for the caliber, such as a 77gr SMK, will perform better with faster twists. If the twist rate is too slow, the bullet will not be stabilized and will be very inaccurate. There is a train of thought that talks about “overstabilization” of lighter bullets in fast twist barrels. My research tends to show that this usually applies to thin-jacketed or unjacketed cast lead bullets that will tend to “explode” if there is too much rotational velocity. For most people shooting standard match or surplus ammunition, overstabilization is not a thing to worry about and you will be fine shooting lighter/smaller bullets in faster twist barrels.
In any case, 99% of AR shooters will be well served with a 1/8 or 1/7 twist.
There are two main types of rifling patterns: land & groove, and polygonal. There is a lot of marketing hype around the latter, with a lot of companies putting their own spin on it (Remington’s 5r and Shilen’s Ratchet Rifling, for instance). This refers to the shape of the section of the barrel that ‘grabs’ the bullet and guides it down the barrel. Polygonal styles claim some advantages in velocity, barrel life, and ease of cleaning. But they are about the same in accuracy, which depending more on the manufacturer than anything else. Either works fine.
On the left is an example of traditional rifling, the right is Shilen’s ratchet rifling.
When it comes to producing rifling, there are three methods: cut, button, and hammer forging. Cutting is the oldest method, and works by cutting each groove of the bore one at a time over many passes. Krieger is one of the most well known makers of “cut rifle” barrels, and they have a great reputation for accuracy. Cut rifling produces the least amount of stress on the barrel during formation.
Button rifling is essentially pulling a cutting “plug” through the bore to form the grooves. This is a newer technique compared to cut rifling, and is also the mil-spec for the M16 family. Very accurate barrels can also be made using this method, Criterion is one of the bigger names in the business, but there are many others out there. Button rifling is the most popular method.
The third method, hammer forging, is the newest, being developed by the Germans in 1939 as a way to mass produce barrels. The manufacturer creates a mirror-image ‘negative’ of the bore they desire and inserts it into an oversized ‘blank.’ The machine then pounds the blank down to size around the ‘negative’ mandrel and produces the desired rifling pattern. There is a lot of marketing hype surrounding hammer forged barrels, so be wary. They can still be very good barrels (I have one), and the way the metal grain is compressed can produce a slightly stronger barrel, but the real world gains are questionable. Be wary of marketing claims that hammer forged barrels are more accurate or possess some other magic voodoo that makes them worth more than other barrels. There are only a few hammer forging machines in the US. FNUSA has one, as well as Daniel Defense, Freedom Group (Remington/Bushmaster/etc), and IWI. There might be one or two more floating around. Just about any hammer forged barrel you see on the market was probably made at one of these facilities.
An important factor to consider is stress on the barrel during manufacture. Each of these methods produces stress in the metal, with hammer forging and button rifling created far more than cutting. Part of the manufacturing process is to “stress relieve” the barrels. This is an important step, as poor stress relieving means the barrel will be far more reactive to temperature, and the heat deformation could be terrible for your accuracy.
Despite this, don’t get wrapped up in rifling method. Buy from a quality manufacturer. When you buy a quality barrel, you’re not really paying for the rifling method. You are paying for the skill, polish, and QC methods used to ensure the barrel is free of deformations and defects that might negatively affect performance. BCM, Daniel Defense, Centurion Arms, Lothar Walther, Criterion, Krieger, Bartlein, and many others all make or sell great barrels. Pick a length, profile, and twist that suits your needs- let the manufacturer worry about how it’s made.
Realize that not all barrels are the same even if they came from the same factory. Facilities that mass produce barrels perform up to a contracted specification. For instance, if Company X contracts FNUSA to produce 5,000 barrels to a very tight tolerance and high accuracy demand, that barrel will be more expensive due to the extra care required and to make up lost costs due to out of spec rejects. If Company Y has a looser tolerance to spec, and a lower accuracy demand, then they will get a barrel that meets that looser specification (maybe even barrels that were rejects from Company X) and sell them at a lower price point. Keep this in mind whenever someone says, “But this one was made in the same factory as that one, and it’s $150 less!”
And Finally, Gas Systems
There are really two components that affect the gas system performance in an AR. First is the length of the gas tube itself. Longer gas lengths mean lower operating pressures, which translate to smoother recoil impulse and reduced parts wear (remember what I said about 20″ barrels having the smoothest operation? This is why). But the second part of it is the actual gas port hole that is drilled into the barrel.
In order for the weapon to cycle the bolt and load a fresh cartridge, some of the expanding gas is bled off from bore. It does this through the gas port. If the hole is too large, then the operating pressure is increased and the action of the rifle is more violent. This may lead to malfunctions and premature parts wear. If the port is too small, then not enough gas enters the system and induces malfunctions like short stroking. There is a delicate balance between the size of the gas port and the length of the gas system. Not only that, but barrel makers must account for the gas port slowly enlarging over the life of the barrel due to the erosion forces of hot gasses and debris.
Eugene stoner designed for about an optimum 7″ between the gas port and the muzzle. This distance allows sufficient dwell time for the bullet to pass the port (when some gas will enter the system) and allow pressure to build before the bullet uncorks and depressurizes the whole system. If the distance is shorter than 7″, then the gas port must be enlarged to allow more gas. If the distance is longer than 7″, then the gas port will be smaller in order to avoid overpressure.
On a 20″ barrel, the gas port is about 13″ up the barrel. On an M4 carbine (14.5″ barrel), it’s at about 7.” On a 16″ midlength, it’s at 9.” All of these are relatively optimized. But when we start getting into oddball barrel lengths like 10.5, 11.5, 12.5, or 18, things get more complicated. The shorter barrels (10.5, 11.5, 12.5) all use a 7″ carbine length gas system found on a 14.5″ barrel, but must use oversized gas ports to make up for the lost dwell time.
With an 18″ barrel using a rifle gas, the same is true. The gas port must be enlarged in order to make up for the two inches of lost dwell time. This has a side effect of making 18″ somewhat finicky about ammunition. Overall, an 18″ rifle gas barrel will still shoot very smooth, but probably not as smooth as a 20″ barrel since it needs a larger gas port. One company (Noveske) did come out with an “intermediate” length gas system for their 18″ barrels that would be optimum for the 18″ length, but it is fairly propriety and not widely available.
The bottom line is this: gas length is related to barrel length and gas port size. A quality manufacturer will have already engineered this out for you. If you buy quality (BUY ONCE, CRY ONCE!), then you don’t need to be overly concerned about this minutiae.
If you read all of that, then I salute you. Hopefully I’ve given you at least a decent understanding of the considerations involved in buying a new barrel. Remember, based on what you read here, be honest about your needs and intended use and choose accordingly. Never sacrifice velocity if you don’t have to. Never add weight if you don’t need it. Buy from a quality manufacturer.