AR15 Gas System Balancing

Table of Contents:

• Abstract

• Reliability is not Enough

• Presuppositions

• Window of Operation in Depth

• Standing on the Shoulders of Giants

• Variables We Control

• Reciprocating Mass

• The A5 Buffer System

• Notes on Buffer Springs

• Barrel Configurations

• Competition Rifles

• Suppressed Rifles

• Conclusion

Abstract:

The goal of this article is to give an engineer-level understanding of the AR15 operating system such that the reader can understand the tradeoffs in the component choices that lead to an optimized system.

With this information, you will be able to build a rifle that will be regarded by a gun autist as optimal, and potential changes will be considered sidegrades rather than upgrades.

An engineering-level understanding is the highest level of understanding, relegated to the likes of Eugene Stoner and Jim Sullivan.

Below engineering-level understanding is armorer-level understanding, enough to assemble and disassemble rifles for maintenance.

Below armorer-level understanding is user-level understanding, enough to effectively use the weapon platform via mastery of the manual of arms as well as knowledge of field stripping and maintenance for the proper care of his fighting implement.

This is about being an engineer and building the best that there can be.

Reliability is not Enough:

You will hear it many times on the gun forums “I use [x] because it is reliable” or “[x] is more reliable than [y]”.

As an end-user, this is perfectly sufficient as all you care about is that your rifle functions properly (does not malfunction).

Reliability, as it is colloquially used by the end-user, is a low-resolution term that generally means “my rifle works properly with both weak and full pressure ammunition, with and without lubrication, and in both cold and warm temperatures, and can go a long time before parts break”.

From our standpoint, this is an inadequate definition because it is low-resolution. One word encompasses many distinct concepts. We need high-resolution terms that afford the proper precision so that ambiguity is eliminated.

One example of this is, for the past 10 years (and likely more), people would often ask “are longer barrels more accurate?”. It’s a fair question, however, if your definition of accuracy is “being able to hit the target”, when the answer of “no, longer barrels are not inherently more accurate” is delivered, it seems unsatisfying by merit of being counter to natural intuition.

Now, let us replace the term “accuracy” with “mechanical dispersion” and “hit probability” and here is the picture it paints:

 An AR15 with a 10.3 inch barrel can have just as good, or better, mechanical dispersion than a 20-inch gun. Mechanical dispersion being the size of the group produced by the rifle when machine-rested (i.e. absent of human error).

However, the 10.3 inch AR15 will have lower hit probability due to the reduced velocity, which reduces allowable error in range estimation (makes the trajectory look more like a rainbow and less like a laser) as well as increasing wind drift. You will also have a longer time of flight, although since most people do not train on moving targets, this effect is less observable.

Just by changing our definitions, now we can see a clear picture, whereas before, it produced an unsatisfying answer.

With that being said, here are our relevant definitions:

Reliability: Describes when a firearm functions properly (no malfunctions) in a given set of conditions (ammunition pressure, lubrication status, temperature).

Window of Operation: Describes the range of conditions in which a firearm operates reliably.

Longevity: Describes the lifespan of parts or the whole system in terms of cycles/round count before part replacement is needed.

Recoil: The magnitude of rearward energy upon firing. Hard to formally define and measure. Can be pseudo-synonymous with “smooth shooting”.

Weight: Weight of the firearm.

Length: Length of the firearm.

Presuppositions

The bulk of this article will be optimizing the component selection of a duty-grade firearm - one that you trust with your life. Practically, this means that we want to maximize our operating window so that our firearm is reliable in any situation that we encounter.

With that being said, here are our axiomatic presuppositions:

1.      This is a duty-grade weapon when life is on the line.

2.      The firearm is unsuppressed.

3.      You are solely using full pressure 5.56x45mm NATO ammunition.

4.      All components are built to spec and pass quality control.

5.      Quality, fresh, undamaged magazines are being used.

With respect to (1), (2), and (3), competition guns, suppressed guns, and underpowered ammo users will have their own section with notes specifically pertaining to those configurations. The point being that once the general concepts are understood, extending it to these cases should be simple. By presupposing these, it creates a good starting point that provides a manageable amount of complexity.

With respect to (4), there are no free brownie points for being in-spec and passing QC. These things are assumed because this article’s target audience is not buying a “budget” AR15 and calling it “just as good”.

(5) Approximately 80% of stoppages are magazine-related. This is important, however, autism will not be directed toward this subject matter. AR magazines are the cheapest magazines on the market and acquiring the best (Magpul Gen 3 Window PMAGs) should be trivial.

Window of Operation in Depth

The most important element we are trying to optimize is the window of operation. The window of operation is the range of conditions in which the rifle will operate reliably.

This de facto implies that reliability is binary, meaning that (assuming no parts breakages):

• if the rifle is operating reliably, it is inside the window of operation.

• if the rifle is not operating reliably, it is outside the window of operation.

A practical way to think of this is to ask “how strongly is my gun cycling?”, or colloquially, “am I overgassed?” or “am I undergassed?”.

When an AR-15 rifle is operating well within its window of operation, it will have a cyclic rate of 700-800 rounds per minute.

Slower tends toward leaving the left bound of the window of operation and experiencing weak-cycling malfunctions such as a short-stroke (the bolt carrier does not have enough energy to bottom out, enough so that the next round cannot be loaded, or more severely, the current round is not even extracted).

Faster tends toward excessive recoil and lowered parts longevity, and can lead to your bolt carrier outrunning your magazine (bolt over base malfunction), this is especially prevalent on magazines with weak springs, where the bolt carrier can miss the round completely, or it can push the round before it has seated against the feed lips, potentially causing you to miss the barrel extension and hit the upper receiver (this was the reason why M4 feed ramps were introduced).

Standing on the Shoulders of Giants

Over the 60~ years of development the AR15 has gone through, I believe that the best place to start is with the M16.  For our purposes, the distinction between A1/A2/A3/A4 is not relevant as all basic operating components are the same.

M16A2

• 20 inch barrel with rifle length gas system

• 11.6 oz bolt carrier group

• 5.0 oz rifle buffer

• Rifle action spring

The full 20-inch barreled rifle is renowned for being a soft-shooting rifle that generally does not break parts. This was the original configuration, so it went through arctic testing, jungle testing, etc., so window of operation is not in question here. (Yes, there were historical issues resulting in the addition of chrome plated bore and chamber, as well as the ill-fated switch from IMR4475 to WC846 powder, but these would be resolved in time).

For much of the subsequent development of the M16 family of weapons, it was a saga of making it shorter and then fixing the problems that shortening the barrel, gas system, and buffer system caused.

One example of this is that the most common place that AR15 bolts break is at the cam pin hole. On short carbine AR15s, past a certain round count, this is such a frequent occurrence that you would think that the original engineers Stoner and Sullivan would have fixed it. However, this was not an issue on the original 20-inch barreled rifle.

The full-length rifle setup will be considered ideal. Shorter rifles will inevitably have more recoil and lower parts longevity, however, they can still be optimized within the end-user’s length constraints.

Variables We Control

Here is a list of the things we can change:

• Barrel length

• Gas port diameter

• Gas port location (affects pressures and dwell time)

• Carrier mass

• Buffer system

There exist “enhanced” bolts and carriers on the market, however, they are out of the scope of this article. While many of these options verifiably improve bolt life, the point here is that a properly balanced rifle will improve parts longevity on “standard” mil-spec bolt carrier groups (BCGs) and enhanced bolts alike (although the metallurgy on some enhanced bolts is so good that you would struggle to measure such things).

Reciprocating Mass

We will start with reciprocating mass because it is the simplest. Reciprocating mass is anything that moves back and forth, which includes the bolt carrier group and buffer. While the buffer spring may technically be part of reciprocating mass, the physical mass of the spring is not something that can be meaningfully changed.

Here is most peoples’ experience with reciprocating mass:

In the United States, there are two tiers of AR15 ammo, full pressure 5.56 NATO and lower pressure .223 Remington. It’s not necessary to get into the specifics of 5.56 NATO or .223 Remington, just know that .223 Remington is significantly “weaker” and produces less energy to cycle your firearm.

The weak stuff, especially steel-cased .223 Remington, sells at an approximately 30% discount relative to full pressure ammunition, so it is very common (it may be predominant, but I have not looked at the exact numbers, suffice to say, it’s common enough for manufacturers to tune their guns for it).

Now you end up with two common situations:

• The manufacturer optimizes the gun for the underpowered .223 Remington, the end-user fires full-pressure 5.56 NATO in it and now the gun is “overgassed”.

• The manufacturer optimizes the gun for full pressure 5.56 NATO, the end user fires underpowered .223 Remington in it, perhaps also while neglecting to lubricate, and perhaps in a cold environment, and then they complain that the manufacturer makes defective products because their gun doesn’t cycle.

In either of these situations, some percentage of these people go to online forums and ask “how do I fix my overgassing or weak-cycling problem?”, and the answer given is to change their buffer weight.

For reference, the standard AR15 carbine buffer (CAR) contains 3 steel weights inside. Replacing one of those weights with a tungsten weight yields the H1 buffer. Continuing to do this yields H2 and H3. Here are the 4 standard weights:

• CAR: 3.0 oz

• H1: 3.8 oz

• H2: 4.6 oz

• H3: 5.4 oz

Since steel is cheaper than tungsten, budget AR15s typically ship with a CAR buffer and come with oversized gas ports to allow proper function with underpowered steel .223 Remington ammunition. When used with full pressure 5.56 NATO, the gun will cycle too quickly/violently and be considered “overgassed”, prompting the user to replace the CAR buffer with, say, an H2.

The recoil “feels” softer, he may have solved a malfunction related to having a too-high cyclic rate. The user is now happy.

In the converse situation, the user may have purchased an AR15 from a high-end manufacturer, who gasses their guns for professional end users (like a police department), and those people do not use cheap, underpowered, steel cased .223 Remington. The AR15 comes with an H2 buffer. The regular user now puts his cheap ammo through the gun and gets sluggish cycling, eventually leading to short strokes as friction increases due to fouling, and perhaps the temperature got colder so you lose even more pressure. He asks the forum, gets his answer, replaces his H2 with a CAR buffer, and now his gun works again.

In both of these cases, the end-user is acting rationally since replacing a buffer is a trivial parts swap that yields the intended result of getting reliable function. This is much easier than disassembling the rifle to drill out a gas port (increasing gas flow) or installing a gas block/gas tube/adjustable carrier to restrict the flow of gas as both of these operations require armorer-level disassembly with the requisite tools and fixtures.

When designing our optimized AR15 from the ground up, this is opposite of the optimal approach. The above end-user is working around his gas system by adjusting reciprocating mass.

We do not want to do that.

We want to set our reciprocating mass first, and then size the gas port appropriately.

People often cite wanting more reciprocating mass for reliability, yet there are the competition shooters who want less reciprocating mass for reduced recoil. Many people subconsciously know, for example, it’s important not to use low-mass carriers for duty-grade rifles, yet cannot explicitly describe why.

Let us go back to one indicator of a well-balanced system: cyclic rate. If your rifle is cycling at 700-800 rpm, you are in a good spot. You can get there with standard reciprocating mass and the appropriate gas port size, or you can cycle at the same rate with lightweight components and reduced gas. If both are cycling at our target speed, why is more mass better?

Reciprocating mass is akin to the heat capacity of water. Much like how water can experience widely varying temperatures with a relatively small change in its own temperature, a firearm with high reciprocating mass (I use the word “high” relative to low mass. In a military/duty context, it would be considered “appropriate” mass) can function better in a variety of operating conditions.

An example: if a high reciprocating mass and low reciprocating mass are both being driven at the same velocity, the high reciprocating mass will have more inertia to power through and continue cycling in a dirty, unlubricated firearm. If you increase the gas drive on the low reciprocating mass firearm to perform in adverse environments, now it is cycling too fast in normal environments (increased parts wear).

Which brings us to the optimal setup.

For bolt carrier groups, this is simple. Use a mil-spec M16-profile bolt carrier group, which should weigh 11.6~ oz. Avoid ones advertised as lightweight, or AR15-profile carriers (thankfully these are going away now).

For buffers, there are 3 main formats on the market:

• Rifle Buffer

• Carbine Buffer

• A5 Buffer

If you are using a fixed-stock rifle that accepts a full-length rifle buffer, you are done. Use the standard 5.0 oz rifle buffer. Not much else to say here.

If you intend to use a collapsible stock, the best option is the A5 Buffer System with an A5H2 buffer, which weighs 5.3 oz. This is closest in weight to the rifle buffer. There will be a subsequent section with additional details on this system.

If you intend to use a collapsible stock and are averse to the A5 Buffer system for some reason, I would recommend balancing around the H2 buffer, which weighs 4.6 oz.

Since everything is a tradeoff, the downsides are increased weapon weight and the opportunity cost of optimizing for recoil.

It is a truism that no soldier has ever asked for a heavier rifle, however, sacrificing window of operation to save a handful of ounces (less than the weight of a single loaded 30-round magazine, which is approximately 17 oz) is a fool’s errand. In the same light, a competition rifle sacrifices window of operation for optimal recoil, which is also a tradeoff that a duty/military rifle should be unwilling to make.

The A5 Buffer System

Unlike the rifle and carbine buffer systems, there is much misunderstanding about what the A5 buffer system is and is not.

The background being that since the AR15’s inception in the late 1950s, the standard buffer system was the rifle receiver extension, with rifle buffer, and rifle spring.

In the 1960s and 1970s, Colt engineers set upon shortening their AR15 and they collectively called these shorter guns “CAR-15” (carbine AR15s), in addition to shortening the barrel and gas system, the buffer system was also shortened. CAR-15 variants have a carbine receiver extension, with carbine buffer, and carbine spring. All 3 of these components are shorter than, and not interchangeable with the longer rifle systems.

Circa 2004~, the United States Marine Corps was having issues with the ergonomics of their M16A4 rifles. The fixed stock was too long, causing issues for small-statured marines as well as those wearing body armor. The short eye relief of the issued Trijicon TA31-RCO-A4CP 4x32 prism optic also did not help with this.

It was well known at the time that M16 rifles had less issues than shorter guns, so the USMC wanted a collapsible stock while retaining the reliability/window of operation that the rifle buffer system provided.

Nick Wantland, of Vltor Weapon Systems, developed this new buffer system for the Marine Corps. It was named the A5 Buffer System because it would be one of the primary upgrades for the M16A5 PIP (Product Improvement Program). The M16A5 ultimately was never adopted, with the USMC deciding to acquire M4A1 and M27 systems instead, but the buffer system remained and gained traction on the civilian market.

The A5 Buffer System has 3 components:

• A5 Receiver Extension

• A5 Buffer

• Rifle spring

The A5 receiver extension is 0.750 inches longer than a carbine receiver extension.

The A5 buffer is 0.750 inches longer than a carbine buffer. Additionally, there is an internal biasing spring that keeps the internal weights pushed up against the face of the buffer. The internal weights are not free floating, unlike a carbine buffer or a rifle buffer.

The A5 system uses a rifle spring, which is interchangeable with the buffer spring used in fixed-stock rifles. The A5 buffer system DOES NOT use carbine springs.

There are some common misconceptions of the A5 buffer system, first and foremost, that it increases the reciprocal length of the bolt carrier group. This is incorrect. The length of travel is exactly the same between rifle, carbine, and A5 systems. There are products that change this (such as the Surefire OBC system), but they are out of the scope of this article.

With respect to the internal biasing spring, on paper, it appears to provide additional consistency because the buffer weights are always stacked in the same place at the beginning of the firing cycle, however, I have absolutely no idea if this is a benefit in actuality. At worst it does not seem to adversely affect performance.

The most obvious improvement is the use of a rifle buffer spring. These are known to have more longevity than carbine springs, and this is because if the reciprocal length is fixed, having an additional 0.750 inches for the spring reduces the compression ratio of the spring, thus leading to longer spring life.

Additionally, it has been observed that when you have an AR15 upper that short-strokes with an H3 buffer (5.4 oz) yet functions flawlessly with an A5H2 buffer (5.33 oz), this indicates that the A5 system has an increased window of operation because the rifle could cycle a heavier reciprocal mass than before, despite having the same level of gas.

This is the primary advantage of the A5 system – it increases the window of operation of your firearm.

It has also been subjectively noted that it changes the recoil impulse to be softer and more similar to that of a rifle buffer system, but I will leave it at that because recoil is hard to objectively quantify.

There have been some studies that indicate that the A5 buffer system is no more reliable than a standard carbine buffer system, and to that, there are questions to be raised about the methodology of such tests. The entire point is to increase the operating window, yet if both systems are tested in optimal conditions (well within the operating window of both systems), there would be little difference between them.

From an engineering standpoint, this is a return to the original design of the AR15 by Stoner and Sullivan, and the usage of the longer rifle spring lends credence to the A5 to being a superior buffer system to the carbine buffer system. It has led respected industry members to comment that “The A5 buffer system should have been the original carbine buffer system; when Colt shortened the rifle, they went too far”.

Notes on Buffer Springs

The last component of the buffer system is the action spring (colloquially known as the buffer spring).

Some people think that using enhanced-power (more power than mil-spec springs) buffer springs make their rifles function better. In fact, Mike Pannone of CTT solutions has written on this topic: https://defensereview.com/the-big-m4-myth-fouling-caused-by-the-direct-impingement-gas-system-makes-the-m4-unreliable/

The summary being that, as guns get fouled, the increased return-to-battery force of a stronger spring helps push through the additional friction, thus letting the firearm function better in adverse conditions.

There is not enough evidence for me to definitively say this is correct, and I believe it is a plausible theory, however, at this time, I still recommend the use of mil-spec action springs.

There seems to be plenty of “try [x]” or “try [y]” spring with words like “chrome-silicon” or “flat-wire” or “braided-wire” thrown around. Longevity aside, it is difficult to find evidence of the benefits of these types, unlike that of the A5 buffer system. This is not a recommendation against them, but it appears to be a deep rabbit hole for little return.

Barrel Configurations

This is the most significant and most variable part of your AR build. Typically you can have the same A5H2 buffer and M16 BCG and rifle action spring and use that on any barrel configuration you want since there is little reason to change it.

Here are the things you can change:

• Barrel length

• Gas port location

• Gas port diameter

• Dwell distance (dwell time)

The first three are straightforward, and the fourth is a derivative of barrel length and gas port location. Dwell time is the amount of time the gas system remains pressurized after the bullet has moved past the gas port and before the bullet has left the muzzle. This is important because there needs to be some duration of time for the gas pressure to impart force onto the BCG and cycle the action.

Since it is easier to think about dwell time as “the amount of barrel past the gas port” rather than in microseconds, heretofore, we will refer to it as “dwell distance”.

When making selections, it is a sequential process:

1. FIRST, decide on barrel length

2. THEN, decide on gas port location

3. THEN, decide on gas port diameter

When selecting barrel length, the relevant lengths are between 10.3 and 20.0 inches, inclusive. People desire short guns because they are handier in confined spaces or if a suppressor is being used. There are legitimate reasons to have a short barrel, however, one should choose a barrel “no shorter than necessary”, which is also why no serious user employs a 7.5 inch barrel.

Once you have selected a barrel length, then you have to decide on gas port location, which is referred to as “gas system length”, example being rifle-length, mid-length, or carbine-length. You want your gas system length to be as long as possible while retaining adequate dwell distance. Having inadequate dwell distance, such as with the original M16 dissipators (Colt Model 605), is evidenced by having temperature sensitivity issues affecting cycling.

Colt Model 605

Once gas system length is set, then size the gas port for appropriate function, and you are done.

            For reference on gas system lengths (length measured is the length of handguard/rail that can fit with a front sight base):

• A rifle length gas system is 12 inches long.

• A mid-length gas system is 9 inches long.

• A carbine-length gas system is 7 inches long.

There are more lengths than that, but these are the most common.

Before we discuss gas port diameters, it is important to note that due to the different gas pressures and timing when you place the gas port at different locations, gas port sizes are only directly comparable when they are at the same port location. A 20-inch M16 has a 0.093 inch gas port, yet a 14.5-inch barreled M4A1 has a 0.062” gas port. Because the gas system length is different, you cannot deduce that one is more heavily gassed than the other.

With that being said, here are some reference military configurations:

• M16 (all variants): 20” barrel, rifle length gas, 0.093” port.

• M4/M4A1: 14.5” barrel, carbine length gas, 0.062” port.

• MK18/CQBR: 10.3” barrel, carbine length gas, 0.070” port.

• MK12 Mod 0/1 SPR: 18” barrel, rifle length gas, 0.100” port.

• M4A1 “Block 3” URGI: 14.5” barrel, mid length gas, 0.076” port.

We shall go down the line on the best configuration for your chosen barrel length:

20.0 inches: This should have a rifle length gas system with a 0.093” gas port. This is the original setup by Stone and Sullivan and as such, should remain that way.

18.0 inches: This should have a rifle length gas system with a larger than 0.093” gas port. The MK12 program did cold weather testing and due to the reduced dwell distance compared to a 20” barrel, they compensated for it with additional port diameter.

16.0 inches: This should have a Knight’s Armament Company (KAC) E3 gas system. This is also the same as the Black River Tactical (BRT) EXT gas system.

13.7 – 14.7 inches: This should have a mid length gas system. Port should be 0.076-0.082 inches.

12.5 inches: This should have a BRT EXC gas system (Carbine + 1 inch).

10.3 – 11.5 inches: This should have a carbine length gas system. Port should be 0.070” on the 10.3 inch and be between 0.070 and 0.062 on the 11.5 length.

The 18 and 20 inch barrels are fairly self-explanatory. While Noveske Rifleworks has developed the “intermediate” length gas system, which is exactly in between rifle and mid length, however, this has never achieved widespread proliferation. There also exist mid length 18 inch barrels (do not buy these).

The 16 inch barrel is the most popular civilian barrel length because barrels below 16 inches take more effort to get or have to be configured as a “pistol”. The M4 Carbine was adopted in 1994, the Global War on Terror started in 2001, the Federal Assault Weapons Ban expired in 2004, Call of Duty 4: Modern Warfare came out in 2007, and Magpul Dynamics (the training company; Magpul Industries makes the polymer products) released The Art of the Tactical Carbine in 2008.

Already, the classic image of the fuddy gun owner who carried a 1911 and purchased wood and steel guns and said “you don’t need an AR15” started to fade.

At this time, everybody wanted an M4, the so-called “M4Gery”, and the most renowned example was the Colt model LE6920. Other manufacturers soon followed suit. Because of the National Firearms Act of 1934, barrels could only be a minimum of 16 inches, but the M4 and M4A1 had 14.5 barrels. Manufacturers simply added the additional length and sold them that way.

It wasn’t long before somebody realized this was suboptimal and decided to create the midlength gas system. (Bravo Company Manufacturing sold many midlengths early on, but I am unsure of who started the trend).

The midlength gas system became the industry standard for 16 inch guns. Since, 2013~, If your 16 inch gun did not have a midlength gas system, it was either a budget-tier gun, or a Colt purchased for historical reasons. This is still the case today (there is no technical advantage to buying a 16 inch carbine length gun).

However, in 2008, Knight’s Armament Company released their version of the AR15, known as the SR15 E3 IWS, which featured a 16 inch barrel with a longer-than-midlength gas system (yet shorter than intermediate gas). It was not until Black River Tactical released their Optimum line of barrels in 2019 that people caught on to this trend (Now we have the EXC gas system and carbine+1 systems from other manufacturers which create a longer-than-carbine but shorter-than-midlength gas systems to optimize for things like a 12.5 inch barrel).

In this regard, KAC was an innovator with their E3-length gas system and E3 bolt, which dramatically improved the longevity of the AR15 bolt. Outside of these two things, the mindless touting of KAC as the best of the best on internet forums is likely a status-driven phenomenon.

With respect to 14.5 inch barrels, the military generally has done proper configuration on most of the other barrel setups with the exception of the M4/M4A1. The 14.5 carbine length gun was a carryover from earlier 11.5 inch carbines, which they extended to 14.5, ostensibly for bayonet use, and they kept the same gas system, likely to simplify logistics. However, performance would have been better had the M4 used a midlength gas system.

There has been plenty of Internet buzz back in 2009 when Paul Buffoni of BCM explained why they did not sell 10.3/10.5 inch barrels. They noted that the 11.5 inch barrel has a 40% increase in dwell time (technically dwell distance) over a 10.5 inch barrel.

To be clear, both 10.3/10.5 and 11.5 inch barreled rifles can be run reliably, however, the 10.3/10.5 will have to be gassed significantly harder, not only to directly compensate for the loss of dwell distance, but also to compensate for the temperature sensitivity that a very short dwell distance brings.

In case that was ambiguous, this is different than going from a 14.5 carbine length to a 12.5 carbine length (0.062” to 0.064”~, versus going to 10.3, which requires 0.070”) since in both cases, there is more than enough dwell distance, so the port adjustment would be rather minor. Contrast this to going from a 20 inch rifle gas to a 16 inch rifle gas (modern dissipator) requires opening the gas port from 0.093” to 0.120” (this is basically the maximum allowable size on the gas block) since dwell distance was so short, and this still has extreme temperature sensitivity issues, which is why true dissipators are no longer a thing.

Because of this reason, I do not personally recommend the use of 10.3/10.5 inch barrels in unsuppressed applications.

You actually see many people online coming to this conclusion as well. Criterion Barrels is a manufacturer of high quality, match-grade, chrome-lined barrels. On February 27, 2020, they introduced the Criterion CORE line of barrels, which is a line of meta taper-profile barrels for general purpose use (the barrel profile is still rather light for full match accuracy, as evidenced by its 0.625” gas block journal). For all intents and purposes, it is a top tier barrel, and Criterion, who has been a relatively unknown company outside of certain circles just a few years ago, is now generally regarded as a top tier manufacturer.

As of 2022-10-16, the only CORE barrel in stock is the 10.5 inch carbine gas. All other options (11.5, 12.5, 13.9, 14.5, 16, 18, 20) are backordered or out of stock. That is a clear indicator that the trend is away from 10.3/10.5 inch barrels for general use (i.e. not for cloning military MK18 configurations).

Competition Rifles

In our context, competition rifles are ones used for 3-gun and other rapid shooting sports where minimizing recoil is important. The solution is simple, minimize the reciprocating mass, minimize the gas, and now your rifle is cycling with minimum recoil. The downside is that you have significantly reduced your window of operation, so use of a competition rifle in a duty capacity is not advised.

Suppressed Rifles

Suppressors affect the way that the gas system on AR15 rifles are balanced.

In today’s day and age, there are two categories of suppressors: conventional baffle suppressors, and flowthrough suppressors.

Flowthrough suppressors are likely the future of suppressor technology since they do not significantly affect the operation of your firearm. You may only observe a less than 5% change in cyclic rate (700 rpm becomes 735 rpm, well within normal operating bounds). The US Military has adopted the M110A1 (a 16” barreled version of the HK417) in 2017and it comes with a flowthrough suppressor made by OSS/HUXWRX.

The recently adopted M5 rifle (Sig SPEAR) (2022) also features a flowthrough suppressor.

Flowthrough suppressors are great because they are simple. No tuning or special consideration is required. When building a rifle, treat the gas system as if it were unsuppressed. This is good for the military since it eliminates the need to operate unsuppressed only to be overgassed when suppressed (A baffle suppressor can lead to a 20% increase in cyclic rate, turning 700 rpm into 840 rpm – quite significant).

For conventional baffle suppressors, the choice is to optimize for unsuppressed usage, or suppressed usage. We will be discussing optimizing for dedicated suppressed usage (which assumes the rifle will not be able to function reliably with the suppressor detached).

Since baffle suppressors add backpressure, the optimal barrel configuration is actually to use the short dwell distance barrels such as 10.3/10.5 carbine length, 11.5 midlength, and 12.5 midlength.

Gas intake can be restricted with an adjustable gas block/gas tube/carrier, or one can take a longer barrel with the requisite gas system length and have it cut down to the desired length while the gas port remains unchanged.

Since the barrel will be dedicated suppressed, a muzzle brake can be used as a sacrificial baffle, extending the life of your suppressor’s baffle stack.

Conclusion

Fundamentally, this is about thinking and less about specific configurations. You may disagree with some of my axiomatic presuppositions, or how heavily I base my conclusions off the original rifle specifications. It is my hope that this has provided insights such that even if you disagree with specific configurations, it enables you with the knowledge to make better choices in your rifle builds.