SOPMOD Block 2

SOPMOD Program Overview

Good information on the SOPMOD Block 1/2 and parts life.

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The contents of various buffer weights.

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Carbine vs. Mid-Length Gas System on a 16″ Barrel

Often times I receive emails and private messages asking why I prefer a mid-length gas system over a carbine gas system on a 16″ barrel or asked which carbine I prefer and why. What is written below is a response to an email, the author was considering a Colt LE6920 and a Bravo Company Manufacturing (BCM) Mid-Length. In my response I explain why I prefer the mid-length gas system over a carbine gas system on a 16″ barrel

Colt 6920 has a 16″ barrel with a carbine length gas system

BCM Mid-Length has a 16″ barrel with a mid-lenght gas system

I own at (3) Colt LE6920’s and (3) BCM Mid-Lengths and have used both at work, in training classes, run and gun rifle matches, etc.

ALL (3) of my Colt LE6920’s have had their barrels changed to Colt 14.5″ SOCOM M4 barrels.

I prefer to use a mid-length gas system on a 16″ barrel.

I do not like a 16″ barrel with a carbine length gas system (same gas system that is on the military M4 carbine)

If I have to use a carbine length gas system I prefer to use a 14.5″ barrel (and I permanently attach a longer flash hider to make the overall length of the barrel 16″).

The 3 most common type of gas systems are:

1) Carbine length gas system – (same length as the US Military M4 carbine) and takes a 7.0″ rail or handguard.

2) Mid-length gas system – the mid-length gas system is 2″ longer than the carbine length gas system (thus the front sight housing and gas hole are 2″ forward of where they are on a carbine length gas system). The mid-length takes a 9.0″ rail or handguard.

3) Rifle lenght gas system – (same gas system on the AR15 or M16A2 / M16A4 with 20″ barrel). The rifle length gas system takes a 12.0″ rail or handguard.

16″ barrel with MID-LENGTH gas system:

16″ barrel with CARBINE gas system:
(pictures property of Bravo Company USA, used with permission)

As you look at both pics, pay close attention to the distance between the front of the receiver and the front sight tower. You can see the mid-length upper has a longer gas system and the front sight housing is 2″ forward as to the location of the carbine length gas system.

When talking about the different gas systems on a 16″ barrel, think about the distance from the gas hole to the end of the barrel. The longer that the bullet is in the barrel after the bullet passes the gas hole, the more gas that is getting pushed back through the gas tube and back into the gas key. The end result is a sharper recoil impulse.

This is why on a 16″ barrel, a mid-length gas system is slightly smoother than a carbine length gas system.

The distance from the gas hole to the end of the A2 flash hider on a 16″ barrel with carbine-length gas system is approximatley 9.5″.

The distance from the gas hole to the end of the A2 flash hider on a 20″ barrel with rifle length gas system is approximatley 7.5″

The distance from the gas hole to the end of the A2 flash hider on a 16″ barrel with mid-length gas system is approximatley 7.5″

The distance from the gas hole to the end of the A2 flash hider on a 14.5″ barrel with carbine-length gas system is approximatley 7.5″

Something to note. The 20″ barrel with rifle length gas system, 16″ barrel with mid-length gas system, and 14.5″ barrel with carbine length gas system all have the same amount of dwell time (distance from the gas hole to the end of the barrel).

The 16″ barrel with the carbine length gas system has a dwell time that is approximtely 1.5″ longer. Thus it’s pumping more gas into the bolt carrier key, forcing it back hard, etc. The 16″ barrel with the carbine length gas system is harder on parts over the long term and you’ll feel slightly more recoil impulse. Not a huge thing, but after shooting all 3 side by side on numerous occasions, I see no need to own a 16″ barrel with a carbine length gas system. I own (18) AR15’s. (2) are short barreled barreled rifles, the rest of my AR15’s are either 14.5″ M4’s with a carbine length gas systems or 16″ Mid-Lengths.

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The Big M4 Myth: “Fouling caused by the direct impingement gas system makes the M4/M4A1 Carbine unreliable.”

By Mike Pannone

March 19, 2010

Here’s my question for those that subscribe to the direct impingement fouling concept:

I fired 2400 rounds of M193 through a 14.5” M4-type upper receiver from Bravo Company Manufacturing (BCM) with no lubrication, and without any rifle-caused malfunctions. So; why can I get my direct impingement rifles to repeatedly do things that conventional wisdom says they can’t do?

This article is not a direct impingement vs. piston driven operating system debate and does not discuss piston guns at all. It is specifically dealing with a 14.5”AR-15 upper receiver with .062” gas port that’s as close to a Mil-Spec M4/M4A1 upper as I could find on the civilian market.
All I have ever asked and required of myself (and others) as a professional is that everything I say or write must be capable of being substantiated. I am asking some questions and giving my opinions, observations, and conclusions based on my own experience and testing.

For years I have been told, and heard others repeat, incessantly, that the direct-gas-impingement M16/M4 family of weapons is flawed because they deposit gas and powder residue in the upper receiver, and thereby are inherently unreliable with hard use. That sounds good in theory. However, in practice, I have not seen nor experienced it with my guns as a special operations soldier or civilian instructor. Why is that? Why don’t I have said commonly referred-to fouling problem with even excessive use and minimum maintenance?

When I returned from Iraq in 2005, I was a primary instructor on a rifle course with the Asymmetric Warfare Group (AWG). During that time as I have mentioned in previous articles I began a quest to find out what made the M4/M4A1 Carbine run well, and what stopped it from doing so. In that time I spent a year at the 82nd Airborne Division training with infantry units prior to their deployment on the Iraq surge. During this time, I saw every manner of malfunction and never saw a rifle that was not well cared for (the soldiers attending were more senior and specially selected, as well as being members of the highly disciplined 82nd Airborne division). Each time there was a malfunction, if possible, I would run over and observe what had happened, then write it down in my log book. What I eventually realized was that when magazine issues were removed, along with broken parts, about 80% of the malfunctions had been accounted for. The rest were failures to properly extract and eject, and failures to go into battery. That is where I realized my rifles were superior to the ones issued. The only problems I had experienced with my own guns were double feeds which are exclusively magazine caused.

What’s odd is that I was using a civilian version of an M4 that was nearly identical to the ones used by the paratroopers of the 82nd. My rifle utilized a DPMS chrome-lined 16” M4-profile barrel with a Larue free-float forend rail tube. After that barrel was shot out I went to a Noveske 14.5” Afghan barrel, and then finally to a Noveske 14.5” N4 cold hammer forged, double-chrome-lined barrel. Aside from being semi-auto-only instead of select-fire (i.e. burst-fire or full-auto capable), and one having a barrel that was 16.1” vs. 14.5”, they were functionally the same rifle. The difference was that I used a heavier Sprinco buffer spring (correctly called an action spring), a DPMS Extra-heavy buffer (.2oz lighter than a Colt H3 buffer), and a 5 coil extractor spring with a Crane O-ring for added extractor tension. Those drop-in parts made my rifles obscenely reliable, and still do. The spring-and-buffer combo I use works in mil-spec-size gas port rifles (.062” as per NAVSEA Crane a.k.a. Naval Surface Warfare Center, Crane Division) with 14.5” or 16” barrels and a 7.5” carbine gas system. There are some rifles on the market that have smaller gas ports than the Colt M4 in its military configuration, so the spring and weight may not work in them, as they may cause short cycling issues. I had the luxury of shooting my rifle without maintenance in a training environment until it failed. I routinely went well over 2500 rounds with only a few drops of oil and a bore snake run through the barrel every morning. I was convinced there and then that fouling was not nearly the issue it was purported to be, and that the real issue was weak springs and a buffer that was too light.

My Test:

Recently, I received a milspec equivalent (barrel length/gas port size/gas system length) M4-type upper from Bravo Company USA (BCM) to test my theory that a heavier buffer and spring with enhanced extractor tension would give extraordinary reliability with no lubrication or maintenance whatsoever. I have shot over 2500 rounds with the FailZero kit with EXO Technology coating on four separate occasions with no lubricant, as well as a ceramic coated rifle (to include bolt and bolt carrier group) from Next Generation Arms that currently has 4000 rounds on it without cleaning or lubricant, and also no malfunctions. I have also routinely shot a Noveske N4 14.5”-barreled rifle over 2500 rds with only 6-8 drops of oil every 500-700rds fired without any issues. If I used those rifles or parts for my test, many would say “well those are custom coatings/guns and military guns don’t have that.” For that reason, BCM was kind enough to send me a stock 14.5” upper on which to do the test. Prior to the test I did the following:
1. Remove all visible oil and lubricant from the inside of the upper receiver.
2. Disassemble the bolt carrier group (BCG) and remove all lubricant inside and out
3. Put a Crane O-ring on the existing extractor spring
4. Use a lower receiver with a Sprinco standard Blue spring and an H-3 buffer (I used an H3 because it was close to the DPMS Extra-heavy buffer I use in most of my rifles.
(*Writers note: A standard rifle buffer is 5.2oz. For a Carbine receiver extension a standard H buffer is 3.8oz, H2 is 4.7oz and H3 is 5.6oz.)

After I had done that, I fired 2400 rounds of M193 through it in six sessions, often shooting it so hot that I could not hold the forend without gloves. The first of such sessions was in the presence of two Border Patrol BORTAC snipers, and it consisted of 330 rounds in 25 minutes. This included zeroing the optic so the bulk of the rounds were fired in a 20 minute period by all three of us. (Note: At the conclusion of this, I pulled the bolt carrier group out and held it by the lugs with my bare fingers. That’s another myth (to debunk) for another article. I did this a second time later during the test where I had shot the rifle so hot I needed gloves to hold the forend, then shot 120 rds in 2:35 and again held the bolt by the lugs with bare fingers.) The rifle had no issues other than some test magazines that did not feed the last round properly. Once those test magazines were removed, the rifle always locked to the rear on the last round fired and did not feel sluggish.

With good magazines–I used USGI aluminum of various makes so as to replicate military use as closely as possible–there were no issues until I reached 2450 rounds fired. At 2450 rounds the rifle would not complete the recoil cycle due to the additional friction caused by the fouling and no lubrication, and exacerbated by the extra buffer weight. Once the rifle began short cycling, it did so every shot. In diving medicine, that’s called “dramatic onset of a symptom”. It was as though a switch had been flipped and the rifle just stopped working.

Rounds fired per session were: 330, 510, 540, 450, 450, 120* (Note: Failure point was end of 6th magazine/2440rds. Problem: chronic short cycling due to excessive fouling caused friction.)

At the failure point I replaced the H3 buffer with an H buffer, and the rifle ran reliably again. I finished the remaining rounds in the 6th magazine of the session, and continued shooting. At 2500 rounds, the rifle ran, although quite sluggish in counter recoil. Then, nearly on cue, the rifle stopped again, this time at the 2540 round mark, and the last ten rounds were accomplished by tap-rack (performing a tap-rack-bang drill) each time. Just to isolate the issue I put the BCG in another dirty but oiled upper of same design and it ran easily (with H3 buffer reinstalled). I returned the BCG to the original upper, oiled it, and the rifle immediately came back to life firing another 90 rounds smoothly and without issue (2630 total rounds fired for test + 30 in replacement receiver cited above).

Here are the findings of my testing:
· When the rifles become fouled, they have more drag (friction) inside the upper receiver, which slows down the bolt carrier group. This along with the pressure on the bottom of the bolt carrier from a loaded magazine will slow the BCG down enough to keep it from reliably going into battery during the counter-recoil cycle. The heavier buffer and spring completely remedy this, but there is a crossover point. That crossover point on a bone-dry stock M4/M4A1-type AR carbine upper is about 2400rounds fired. At that point, if there is enough buffer spring tension to drive the BCG into battery, then it cannot fully cycle. And, if the spring is light enough to allow the weapon to fully cycle to the rear, it does not have enough force to go fully into battery. The changing from an H to an H3 buffer only gave an additional 80 rounds of reliability. And, given the parameters of the test (no lube) and the dramatic increase in shootability using a heavier buffer, I am still a proponent of a buffer heavier than an H.
· With the Sprinco enhanced Blue action spring (or comparable extra-power spring) and an H2 orH3 buffer, unless there is a rigid obstruction present in the barrel extension, the rifle will reliably go into battery. Note: I routinely take “damaged” or discarded rounds (see first article on M4 reliability) that have been lying around or have deformed cases from the malfunctions block I conduct and load them into my magazines. I will shoot them all without issue, unless they are catastrophically disfigured or the projectile is pushed back into the case (creating a safety issue due to increased chamber pressure). The heavier buffer and added spring tension effectively resizes the case and fires it.
· A benefit of the additional spring/buffer weight is that it slows down the unlocking and extracting tempo, increasing the locked chamber dwell time and allowing for much more reliable extraction and ejection. This is because the longer dwell time allows the chamber pressure to recede more, as well as transferring heat from the case to the chamber walls. It also offers a softer-shooting rifle because the recoil impulse is transmitted over a longer period of time, hence lower ft-lbs/second received at the shoulder.
· With an enhanced extractor spring (BCM 4 coil, Sprinco 5 coil or comparable) and a Crane O-ring, I have not experienced any failures to extract except for faulty ammunition (specifically Radway Green training ammunition used by the 82nd in 2006) The SOPMOD bolt upgrade kit (new extractor and pin, 5 coil extractor spring, Crane O-ring and new gas rings), first fielded by SOCOM, should be standard on all M4’s used by the military or law enforcement.

*I have heard of some rifles that will not function properly with both an enhanced extractor spring and a crane O-ring installed. The symptom is the extractor does not release the brass from the bolt face causing a failure to eject. I have never experienced this with my personal rifles, but am currently working with Lou Patrick of on finding the reason for this. Lou is one of the most overall knowledgeable gunsmiths I have ever met, and is also a former gunsmith for the Army Marksmanship Unit (AMU).
**Test-fire any enhancements before fielding.


Fouling in the M4 is not the problem. The problem is weak springs (buffer and extractor), as well as light buffer weights (H vs. H2 or H3). With the abovementioned drop-in parts, the M4 is as reliable as any weapon I have ever fired, and I have fired probably every military-issue assault rifle fielded worldwide in the last 60 years as a Special Forces Weapons Sergeant (18B). An additional benefit of the heavier spring/weight combo is that it transmits the energy impulse of the firing cycle to the shoulder over a longer duration, lowering the amount of foot pounds per second and dramatically reducing the perceived recoil. Follow-on shots are easier to make effectively, and much faster, especially at 50 meters and beyond.

I reliably fired 2400 rounds (80 magazines) on a bone dry gun, and I would bet that is a lot more than any soldier or other armed professional will ever come close to firing without any lubrication whatsoever. So, disregard the fouling myth and install a better buffer spring, H2 buffer, enhanced extractor spring and a Crane O-ring (all end user drop-in parts). With normal (read “not excessive”) lubrication and maintenance, properly-built AR-15/M4 type rifles with carbine gas systems will astound you with their reliability and shootability.

About the Author: Michael Pannone a.k.a Mike Pannone is currently the owner/operator of, and senior instructor for, CTT Solutions, which is a tactical training (including tactical shooting) and consulting firm. He’s also a certified Colt Armorer. Mr. Pannone is a former operational member of U.S. Marine Force Reconnaissance, U.S. Army Special Forces, and specially selected elements of the Joint Special Operations Command. He has participated in stabilization, combat, and high risk protection operations in support of U.S. policies throughout the word as both an active duty military member, and a civilian contractor. During his military career, Mr. Pannone was the Distinguished Honor Graduate of a Level 1 SOTIC held at Ft Bragg. He currently instructs U.S. military, law enforcement (LE), and private citizens around the country as an adjunct instructor with several different organizations. He can be contacted via e-mail at

© Copyright 2010 and Mike Pannone. All rights reserved. This material may not be published, broadcast, rewritten or redistributed without receiving permission and providing proper credit and appropriate links.

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M4/M4A1 Carbine Reliability Issues Part II: Diagnosing the root cause.

March 15, 2009

If your Explorer gets a flat does that mean “Fords are unreliable”?
If you put bad fuel in your Tundra does that mean “Toyotas run crappy”?

Stay with me…

When a weapon malfunctions, we often hear “it jammed” or some such non-descript statement that gives us no more information than what we already witnessed–your rifle didn’t fire. To be truly capable of correcting malfunctions and, more importantly, preventing them, the operator must be able to identify which of the five areas are the root cause, and then correct the deficiency. It is very simple and should be taught to every individual issued an AR-15/M16/M4/M4A1-type weapon.

The 5 root causes of malfunctions:…

– Shooter Induced

– Magazine Failure

– Weapon Related

– Operating Environment

– Ammunition

Let’s take a look at these one at a time understanding that some fall under multiple categories. I am not going to cover every possible problem but I want you to understand the methodology so it can be applied to your instruction and training.

Shooter Induced

Failure to properly load is the most common cause of a failure to fire. Some say only load 28 rounds in a 30 round magazine so it is easier to load with the bolt forward. Here’s a tip, load thirty and use proper technique. If you’re doing a magazine exchange to top off your gun then you have enough time to do it properly. Don’t short yourself 2 rounds for a training deficiency.

An improperly loaded magazine is also a common problem. The most common is 31 rounds in a 30 round magazine. This makes it almost impossible to seat in the well. If you do hammer it in, it will do damage to the feed lips (which if repeated will result in feed lip separation and double feeds) and cause the rifle to malfunction on the first shot due to the extreme pressure on the bottom of the bolt carrier by that 31st round. The next problem is one I have witnessed many times in HK steel magazines (and magazines of similar dimension) that have a wider spine than the standard aluminum GI magazines [a.k.a. USGI magazines]. When using a stripper clip and standard GI stripper clip guide a case can get stuck in the spine and bind the magazine. *This is of critical importance to military personnel for the following reason. If you get a resupply and those steel magazines are present, who loaded them? They were probably loaded by someone who was tasked to “load 100 magazines as fast as you can?” With that you won’t know that your magazine is not serviceable until your rifle doesn’t fire when you need it to do so.

Shooter induced failure to feed or double feed happens in several ways. When an operator loads with the bolt forward on an empty chamber and then manually cycles the bolt but doesn’t pull it all the way to the rear or allow it to go forward under its own spring tension often a malfunction is induced. A bolt over base malfunction is caused when the bolt is not pulled back far enough to clear the top round in the magazine (short cycling) and the bottom lug catches the case about 1” from the base and pins it in the feedway. A shooter induced double feed is caused when the bolt is ridden forward so it rests on the base of the case in the chamber but does not have enough force to snap the extractor around the rim (failure to lock). When the hammer falls and the weapon does not fire SPORTS (Slap-Pull-Observe-Release-Shoot) or tap-rack will cause a double feed. Another version of that is when clearing a stove pipe improperly (using SPORTS instead of pulling the case out manually).

The last is far too common and stems from a lack of understanding. This is when the operator puts a magazine in the well with the bolt locked to the rear and then slaps it hard enough to dislodge a round from the feed lips. When the bolt is released it causes a double feed. The only time and the only reason you slap the bottom of an M4 magazine is when loading a rifle in the bolt forward configuration or conducting SPORTS. The operator is slapping the base of the magazine to overcome the tension of the compressed spring as transmitted through the top round against the bottom of the bolt carrier. When conducting SPORTS it can break loose a follower that may be jammed. Slapping the magazine with the bolt rearward is prone to cause a malfunction and at best unnecessary.

Magazine Failure

Failure to feed is most commonly caused by a weak spring. If a relatively clean magazine fails to feed then the spring is fatigued and the magazine is not serviceable.

Double feeds are caused by feed lip separation. When the feed lips are wider than the factory spec (use a new magazine to compare to your old ones) then it is unserviceable and can cause the worst type of malfunction you will encounter: the bolt-override double feed. If a magazine double feeds once, it will do it again. They don’t get better so get rid of them. *(If a magazine is repeatedly loaded with 31 rounds and forced into the well, it will not only cause a malfunction of the rifle but will also force the feed lips wider because the rounds cannot go down, and they will damage the magazine. I have witnessed an operator split the spine on two polymer magazines when loaded with 31rds.) This is a magazine failure caused by the shooter using improper technique, and thereby causing damage.

Failure to drop free from the magazine well is a sign that the feed lips are separating and the magazine is borderline unserviceable, or the magazine is manufactured out of specifications. Regardless of reason, all magazines should drop free when empty. If they will not, replace them.

Weapon Related

Broken parts happen to any machine, and if you read the previous article on M4 reliability, you’ll see that weapon-related malfunctions are relatively uncommon if you conduct proper maintenance.

A serviceable failure is when an otherwise fully functioning, properly maintained weapon, magazine and ammunition fail to function. This can happen but it is extraordinarily uncommon. I can’t remember when I had or witnessed a malfunction for no apparent reason…but I guess it could happen since no machine can be expected to work perfectly, 100% of the time.

Operating Environment

Sand, salt, cold and humid environments can wreak havoc on any machine, so the weapons-maintenance schedule must be modified, and PM (Preventative Maintenance) performed more frequently. Sandy environments do require lubrication, but only on the friction-related areas, so as not to attract excessive dust. Cold environments require a low-viscosity, cold-weather lubricant, and humid and salt environments necessitate a lot of lubricant to help resist corrosion. You can minimize the effects of your operating environment by modifying and adapting your maintenance protocols.

Ammunition Related

Duds, i.e. improperly manufactured or environmentally compromised ammunition, do occur, but they’re rare if simple steps are taken. Proper storage at ASP/AHA (Ammunition Supply Point/Ammunition Holding Area), along with rotation of operational ammunition, can dramatically reduce the likelihood of ammunition-related failures. *Be aware that .223 Remington and 5.56x45mm NATO are dimensionally different. 5.56 often use longer, heavier bullets and this necessitates a lengthening of the throat, lead and free bore in the 5.56 mm chamber. While .223 Remington can be fired in a 5.56 mm chambered guns without issue, firing 5.56 mm ammunition in a .223 Remington chamber can produce excessive pressure even above the 5.56 mm specifications due to the shorter throat, lead and free bore present in military chambers. Know what your rifle is chambered for!

In order to solve a problem, you must know its source. Breaking malfunctions down into the above five root cause categories, or combination of these categories, will simplify this task. With this template, one can remedy a problem simply, rapidly and effectively, and help prevent it from happening again. Although there are many more problems under each heading, for the purpose of brevity, I did not try to include them all. The goal here was to present a logical framework on which to base your solutions, and guide your training and pre-operational weapons checks. Know your tools and master them. Remember, a gunfight is a gunfight, no matter who signs your checks–or what you’re wearing!

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About the Author: Michael Pannone a.k.a Mike Pannone is currently a Senior instructor for both Team VTAC (Viking Tactics) and Mid-Atlantic Training Resources (MATR), and a certified Colt Armorer. He is also a former operational member of U.S. Marine Force Reconnaissance, U.S. Army Special Forces, and specially selected elements of the Joint Special Operations Command. He has participated in stabilization, combat, and high risk protection operations in support of U.S. policies throughout the word as both an active duty military member, and a civilian contractor. During his military career, Mr. Pannone was the Distinguished Honor Graduate of a Level 1 SOTIC held at Ft Bragg. He currently instructs U.S. military, law enforcement (LE), and private citizens around the country as an adjunct instructor with several different organizations. He can be contacted via e-mail at ctts at live dot com.


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Specs on the MK18

One of two proposed special mission receivers that were planned for inclusion into the SOPMOD Block II kit, the CQBR has taken off on its own. Like the proposed Special Purpose Receiver, the Close Quarters Battle Receiver has been more or less taken on by the Naval Surface Warfare Center, Crane Division (often referred to as NSWC-Crane or just “Crane”) as its own project following the CQBR’s removal from the SOPMOD program. Just as the Special Purpose Receiver morphed into the Special Purpose Rifle, and was type-classified as Mk 12 Mod 0/1, the complete CQBR-equipped carbine has been type-classified as the Mk 18 Mod 0.

The short 10.3 in (262 mm) barrel length requires special modifications to reliably function. The gas port is opened from 0.062 to 0.070 in (0.16 to 0.18 mm). A one-piece McFarland gas ring replaces the three-piece gas ring set. The standard four-coil extractor spring is replaced with a commercial off-the-shelf (COTS) five-coil spring. An O-ring surrounds the extractor spring. The standard M4 flash hider has been replaced with the M4QD flash hider for suppressor compatibility.

  • MK18 type 1: NSWC assembled carbine with contracted and /or overhauled parts complete with all accessories to current standards at time of build will work with H or H2 and could have shipped with either. Units will balance buffer to ammo requirements:
    • H okay with M193/M855. (all ship board MK 18’s should have H buffers as this was the standard at the time of assembly).
    • H2 needed for MK 262/ R2LP & will run M193/M855.
    • H3 may be needed for an extreme cases or when the gas port is eroded but the barrel still groups.

Barrel: The CQBR uses a 1:7 in (178 mm) twist M4 barrel that has been modified in length to 10.3 in (262 mm). The diameter of the barrel under the handguards is 1.17 in (29.7 mm). The KAC M4-QD flash suppressor is fitted, allowing use of the KAC QDSS-NT4 suppressor (National Stock Number 1005-01-437-0324).

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I started this blog as a place to store and share information that I come across about Eugene Stoner and Gaston Glock type firearms. Links to the pages will be posted at the bottom of the posting. Thanks for reading.

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