General

The Hammer Forged Barrel Process

An interesting video I came across recently came from The Gun Collective. They took a tour of the Daniel Defense factory, and spent some time showing the hammer forge machine in operation. I briefly discussed how hammer forging works on my guide to AR-15 barrels, but this is a nice visual representation. The key part starts at 1:27 and ends around 6:27 (though the entire video is worth watching).

I’ve been very vocal about the importance of quality control and how it factors into the cost of a component. I found it interesting that the first QA inspector talks about doing random inspections on about every 20th barrel rather than every individual barrel. I can’t say this is the best or worst practice, because it clearly works out for Daniel Defense. Hammer forging is known for very consistent results, and it probably just makes the most cost efficient sense to check every 20th barrel to look and see if the tooling is ready for replacement. The second QA inspector likely does her process on every barrel, since it is a faster process.

The remainder of the video shows the aluminum manufacturing and assembly processes that DD goes through. DD is a known quality company, with prices to match. So, again, you have to ask yourself what steps other companies are skipping in order to give you that cheaper price.

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General

The Skinny on Nitride Barrels

After I wrote about the Faxon barrel I’m using in the latest project, a question arose about the benefit of nitrided barrels. I’m not surprised, since the gold standard for so long has been chrome lining. Nitriding is just the “new thing” that all the cool kids on the internet are talking about.

But, what is it? Does it have any benefits over the tried and true methods? I want to dig into that for a bit.

Here is a quick description of the process from an industrial coating company, IBC Coatings, I’ve bolded some key elements.

Salt Bath Nitriding/Nitrocarburizing was originally created as an alternative to gas nitriding that would produce a more uniform case through surface contact between the substrate and liquid salt. When steel parts are placed into a preheated liquid salt, there is sufficient energy localized near the surface due to differences in chemical potential that then allows nitrogen and carbon species to diffuse from the salt into the steel substrate. The process is carried out at 750-1050°F, making it faster than gas nitriding. Lower temperature cycles produce an S-Phase/Expanded Austenite case in stainless steels. Post-oxidation after nitriding combined with polishing produces coatings with exceptional appearance (black color) and high corrosion resistance (greater than electrolytic chrome plating). To ensure part quality, our salt baths are continuously monitored, with chemistry adjustments made when necessary.

Salt Bath Nitriding/Nitrocarburizing is well known under various trade names, including ARCOR®, TENIFER®, TUFFTRIDE®, MELONITE®, and QPQ®.

The idea here is that the barrel is immersed in a sodium-nitrogen solution and heated to a high temperature. The ensuing chemical reaction causes the nitrogen to diffuse into the surface of the barrel (inside and out) and convert a thin layer of the surface into a very salt-bath-nitriding-dhn.gifhard coating. From what I can find, the surface of a nitrided barrel is in the realm of 60 to 65 Rockwell, while the typically gun barrel steel is 28-32 Rockwell. This surface layer becomes a very corrosion resistant “case” around the barrel steel.

Additionally, the surface layer created by the nitriding process has a much lower coefficient of friction compared to bare metal or chrome. Ostensibly, this would mean that nitriding barrels may present a small boost in velocity. I have read some accounts verifying this on Accurate Shooter, but it was only by about 1% or so. Still, a boost is a boost and nobody will ever turn down velocity.

 

The real benefits of this process is that nitriding performs all the same functions as chrome, such as increasing corrosion resistance and prolonging the life of the barrel, and it does it without the associated negative impacts on accuracy. Since the surface of the bore is being converted into a harder material, rather than adding a new layer of material, the uniformity of the bore is maintained. As I’ve mentioned before, consistency is accuracy.

From my reading, this process is not perfect, though. The surface may be harder than chrome, but it is not as heat resistant. Weapons fired on fully automatic for prolonged periods may burn through the nitrided layer quicker than a comparable chrome layer. This should not really be a factor for semiautomatic weapons, though. Also, per the bolded portion in the paragraph above, the barrel must be heated to a temperature of 750 to 1050 degrees fahrenheit. Coincidentally, this is about the same temperature that barrels are heat treated/stress relieved. There is a very real risk that heating to such temperatures (particularly with stainless barrels) can undo heat treatments already performed by the factory that machined the barrel to begin with. While I haven’t seen anyone mentioning decreases in accuracy after nitriding, I have seen many warnings to not perform the process on a barrel that’s already been fired a significant amount. The micro cracks in the surface of the bore and chamber of such barrels may be aggravated by the high temperatures, making them worse and degrading accuracy.

The Faxon barrel I purchased is my first real experience with a nitrided barrel. I’ve handled at shot some rifles with them before, but I’ve never done accuracy evaluations or had to care for one. For the most part, I’m told that care procedures are the same- except that some products (like those from Bullfrog) will tend to discolor the finish.

I will continue reporting back on my results. If you are waffling back and forth on a standard barrel or chrome lined barrel, I don’t think you can really go wrong either way as long as the barrel is well made. Find one that suits your needs in size, profile, and accuracy and let the manufacturer worry about the rest.

General

The Law of Diminishing Returns: Are We Spending Too Much for Precision?

There is a story about the late Colonel John Boyd in which he admonishes a young crowd of Air Force officers about progressing through their careers.

And you’re going to have to make a decision about which direction you want to go.” He raised his hand and pointed. “If you go that way you can be somebody. You will have to make compromises and you will have to turn your back on your friends. But you will be a member of the club and you will get promoted and you will get good assignments.” Then Boyd raised his other hand and pointed another direction. “Or you can go that way and you can do something – something for your country and for your Air Force and for yourself. If you decide you want to do something, you may not get promoted and you may not get the good assignments and you certainly will not be a favorite of your superiors. But you won’t have to compromise yourself. You will be true to your friends and to yourself. And your work might make a difference. To be somebody or to do something. In life there is often a roll call. That’s when you will have to make a decision. To be or to do? Which way will you go?

The message here is essentially one warning young officers about careerism; a phenomenon I liken to Pournelle’s Iron law of Bureaucracy: an observation on how those who work hard to perform the mission and carry out the vision of an organization will usually advance slower than those that work to support the organization itself. But this is not a post about the perceived organizational woes of the Air Force. This is a blog about marksmanship. How does Boyd’s advice apply to shooting?

There is a strong tendency among shooting enthusiasts to always be looking for the bigger better deal; to get to the next level with a minimum of effort. This often manifests itself as spending exorbitant amounts of money on a rifle in pursuit of producing tighter and tighter groupings. There is certainly a time and place for that in bench rest competitions. But what about practical applications of marksmanship?

Day after day, we see newer shooters on the boards asking advice about their first rifle for hunting, target shooting, or even self-defense. We often see them planning around very accurate rifles, often capable of ½ MOA or better. Others in the discussion will agree and point the person in the direction of the top gunsmiths and custom rifle builders in the country, who charge thousands of dollars per rifle, and who already have year-long backlogs of work.

I simply have to ask: At what point are we just spending money in order to say we have something?

The Precision Rifle Blog put up a post last month concerning the relationship between a rifle’s ability to shoot tight groups and actual hit probabilities at long range. Examples were modeled, using Brian Litz’s software, for a 10″ plate at 700 yards and a 20″ plate at 1000 yards. The simulation assumed perfect marksmanship fundamentals regarding trigger control, rifle cant, follow through, and stability. The only variables were the rifle’s precision (1 MOA to .1 MOA), and variance in wind call of 2.5 mph (which is also an expert-level wind call).

The analysis showed that the probably of it for a 10″ plate at 700 yards goes from 70% with a 1 MOA rifle to 78% with a .5 MOA, and to 80% with a .1 MOA rifle. 

The end result shows that for a 20″ plate at 1000 yards, there is effectively a 3.8% improvement in hit probability by going from 1 MOA to .5 MOA, and only a 1.2% improvement by going from .5 MOA to .1 MOA. Think about that for a second.

How much more money does it cost in equipment to go from a 1 MOA to a .5 MOA capable rifle? Many very reasonably priced factory bolt guns are already capable of shooting about 1 MOA out the door. Many will gladly spend a thousand dollars to get that extra .5 MOA– for an extra 3.8% probability. Furthermore, those statistical results were assuming perfect marksmanship fundamentals, world-class wind calls, and nearly perfect consistency of ammunition velocity. How many people looking to purchase or upgrade to rifles capable of that kind of precision can claim they have already mastered these other elements? At what point is a shooter better off buying ammunition to practice with than buying a new barrel/upper/trigger? At what point are we just wasting money on perceived capability that we cannot actually take advantage of?

My argument here is that John Boyd’s advice relates to shooting as much as military careers. There comes a point in your shooting career where you have to decide if you are going to “be someone” and just continually buy top-end equipment for the sake of saying that you own it; or you are going to “do something” and practice enough to use the equipment you have to its fullest capability. Ideally we could do both, but unless you are independently wealthy or have a list of sponsors buying your equipment, then you are going to have to make a choice for the time being.

A marksman that practices the fundamentals endlessly so that they have the ability to shoot any rifle in nearly any condition to the maximum capability of their equipment will always far outperform the one who shoots a couple boxes of ammunition per year, but owns top of the line equipment in every respect.

As the old saying goes, “Beware the man with one gun…”

General

Origin of the Government Profile Barrel

In light of The New Rifleman’s post about 20″ barrels, I felt this subject was fitting. One of the great mysteries of the firearms world is how the US military decided on the M16A2 barrel profile, or what has come to be known as the Government Profile. To most observers, this profile seems backwards. You would typically want more mass around the chamber and first several inches of the barrel to help with heat an stiffness. The government profile is thin in this area, but widens up at the muzzle.

When the M16 was first adopted, and all the way through the years of the M16A1, it had what was called a standard profile. The standard profile of the day was what we now call a lightweight, or pencil, profile.

Early-AR-barrel
M16A1 barrel, photo from Weaponsman

This barrel profile proved to be quite good for a general issue combat weapon. It was lightweight, so it would be less cumbersome when walking on long dismounted patrols. Combat weapons, generally, are carried a lot and fired relatively little. When fired, the barrel was still accurate enough for combat purposes, especially when compared to the competing AK-47 design of the time.

During the product improvement efforts of the M16A2, the barrel profile was changed. The new barrel kept the same lightweight profile between the receiver and the front sight base, but increased the diameter of the barrel from the gas block forward. The general internet buzz is that this was done in response to GIs using their rifles as pry bars to open ammunition crates and rations, and bending the thinly profiled barrel in the process. Another line of thinking is that bayonet drills using the thin profile lead to a lot of bent barrels. To an outside observer, it would seem counterintuitive to only beef up the diameter of the barrel forward of the bayonet lug, since force would be applied to the entire length of the barrel and not just the forward end.

BA20GVT-7
The M16A2 “Government” Profile

I came across a post concerning the M16A2 development effort:

Since The M16A2 Product Improvement Program (1980-1983) was my program, this is the down & dirty on the barrel thickness issue.

We (Marines) were replacing a lot of “bent” barrels that were determined to be “bent” because the Armorer’s Bore Drop Gauge would not freely pass through some barrels during Ordnance Inspections (LTI’s). So the Logisitcs people had “Barrels Bending” on their list of “M16A1” things to “Improve” right after listing “Handguards Breaking.”

We “experts” thought this bending was from rough handling like during bayonet drills, etc., as an absence of any mid-barrel handguard damage in these rifles made one assume the fulcrum of such bending was the bayonet lug. So we made that part of the barrel thicker because we did not want the excess weight of a full length heavy barrel.

In testing using the bayonet lug as a fulcrum, and applying calibrated mechanical pressure to the muzzle, the new barrel was about 9 times more resistant to bend and take a set than an M16A1 profile. So we went with this “improvement.”

However, soon after I started using a bore scope with a video recorder and monitor to inspect “bent” barrels. What I found was a mound of bullet jacket material at their gas ports. This build up was caused by a burr left from drilling/reaming the gas port. This was where the Armorer’s Drop Gauge was geting stuck. When we removed this “mound”, the barrels would all pass the Drop Gauge.

We let Colt know what we had deduced, and that is one reason they kept models of “A2’s” in their line-up with A1 profile barrels. However, the A2 profile was already down the road for the US Military. So about the only advantage of the A2 profile was to give the rifle a little more muzzle hang. This was noted by most all the Operational Test participants, especially when they fired the standing/off-hand leg of our rifle qualification course.

So my advice to military armorers is to never replace a bent barrel until you visually check the gas port, or at least scrub the hell out of the gas port area with a new bore brush and an electric drill. And thank God for chrome bores!

Edit: Reader Daniel Watters informed me that the author of this passage, who goes by the name “coldblue” on AR15.com is, in fact LTC Dave Lutz (USMC-Ret), the former VP for Military Operations at Knight’s Armament Company. Given what I have come to know of the military acquisition and project management process, especially during the late 70’s and early 80’s (ever seen the movie The Pentagon Wars? Watch it!), it seems entirely plausible. The product improvement contract was already written and things were already being manufactured, it’s hard to stop that kind of institutional inertia and go back to the previous profile.

Now, there isn’t that much of a weight difference between the two profiles. The A2 weighs about 4.8 ounces more. That weight is also all out front, which does provide a bit more hang when shooting from certain positions (as noted in the above quote). My estimation is that some groups of shooters will tout the benefits of this hang over the expense of 4.8 ounces. But, if I had my way, we would go back to the standard pencil profile for general issue. These rifles were never meant for extraordinarily high volumes of fire, that’s what light machine guns are for. Malfunctions and accuracy loss due to increased rates of fire beyond the designed specification are another issue entirely.

General

AR-15 Buying Guide: Barrels

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 made that commitment, and are looking for ways to improve what they already have. It’s still good information, though, in case you were curious.

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 barrel function 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).

barrel5

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 (either through tumbling or other effects, dependent on bullet construction). 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.

Screen-shot-2012-06-05-at-1.59.51-AM

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; but below a certain velocity, 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. Weight is simply not a concern, even if people really really want to imagine that it is. 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 had their fill of 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 it’s good balance between velocity, weight, and compactness, the 16″ makes a great all-round performer for the shooter who isn’t quite sure what they really want to do yet. 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 a lot of popularity 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 Steel

Stainless barrels, despite common perception, are not inherently more accurate. They are desired for accurate shooting, though, 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, small barrel manufacturers like to use stainless because they can machine the barrel and send it out, minimizing their supply chain. 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 Barrels

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 (but that’s for you to decide). 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.

Protecting Linings

History makes a big deal about the lack of chrome 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, though, 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 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 can follow the tips above about stainless, then go for stainless. Otherwise, get a good CMV in a profile that works for you.


The Skinny (and Fat) on Barrel Profiles

photo-34

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

barrel_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 (77gr SMK, for instance) 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.

ratchetRifle

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 they aren’t the end all be all. 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, Daniel Defense has one, the Freedom Group (Remington/Bushmaster/etc) has one, and 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

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.

387bf-ar15-gasimpingement

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.


The End

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.

Last revised 26 Jun 2016