Most reliable 16" AR-15 carbine configuration?


August 10, 2006, 04:43 AM
I'm looking to buy an AR-15 as a present for a LEO. The LEO will own it as a personal weapon, so it still has to be "peasant legal", even though the LEO will be carrying it while out on patrol, etc. In doing research on 14" barrels with permanently-affixed 2" muzzle devices, I've found several folks claiming that the higher pressures present in the shorter carbine gas system can potentially adversely impact reliability and service life. So, I'm seeking information on the best way to put together a rifle similar to the following, but with changes as needed to improve reliability as much as possible.

Here are the desired features which I'm told may influence reliability:
14" barrel (1:7 twist) with 2" flash hider permanently attached
collapsable stock
no preference on gas block size or positioning (supposedly mid-length gas systems are best in carbines?)

I'd prefer to have the rifle "complete" from Bushmaster, Armalite, Rock River Arms, or Colt, to avoid any gunsmithing snafus.

That said, are any of those features problematic in terms of reliability? If so, which changes would those of you in the know suggest?

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Gary G23
August 10, 2006, 08:27 AM
I currently own two AR carbines.
One is a 14.5" with a permanent Phanton and carbine length gas system.
One is a 16" with a mid-length gas system.
Both are 100% reliable. I have added Wolff extra power extractor springs to both.
In theory the carbine length guns should get more wear and potentially be less reliable but I haven't seen it. If I had to keep only one I would probably choose the mid-length though because of the longer sight radius and the fact you have more room to put junk on the quad rail.

Nathan Williams
August 10, 2006, 08:43 AM
I am curious as to why you would want a 14.5" barrel with a 2" flash suppresor. Just go with the 16" barrel and keep the accuracy that extra 1.5" gives you. Also I have heard tales of 14, and 16" barrels affecting reliability, however I had a carbine AR with 16" barrel and never had a problem with it.

Gary G23
August 10, 2006, 08:49 AM
There are a couple of reasons for the 14.5" with permanent FS.
1. It looks like a real M4.
2. It is shorter therefore handier.
3. You can use a standard bayonet with this configuration. A standard bayonet won't work with a 16" barrel and carbine length gas system.

August 10, 2006, 09:00 AM

If it has to be 16" in length, you might as well have 16" of barrel. (With the flash hider it'll really be ~17".) If he ever wants to add a freefloat rail system, a welded flashhider will just be one more hassle to deal with.

That said, a 10.5" barrel can be just as accurate as a 20" barrel. It's velocity that suffers.

I highly recommend you get him either a Colt 6920, or a Colt 6250.

The 6520 will be a little lighter, and because it's an A2 upper he can never lose the carry handle. :D The 6920 has an M4 barrel profile (a little heavier), and a detachable carry handle. This will make it easier to attach optics.

Ken Elmore of Specialized Armament offers a reliability upgrade package for Colt rifles. It's $150 and while I haven't had it done myself (I'm not LE/mil and have been fine with my Colt as-is), it's worth investigating.

You might also give him a gift certificate good for a Pat Rogers training class. Everyone can use more practice under an expert instructor, right?

Thin Black Line
August 10, 2006, 09:04 AM
I've seen people argue that you get better velocity with the 16". I've
never chronied the diff. I prefer the 16" heavy barrel over an M4 profile
barrel, though. Heavier just tends to do better.

August 10, 2006, 09:58 AM
supposedly mid-length gas systems are best in carbines?Yes. In my opinion if you are using a 16" barrel, you might as well go with the more reliable midlength gas system. The midlength properly scales down the gas system to the 16" from the 20" barrel.

Copying the gas system from the military M4s is just silly. Even the manufacturers admit reliability issues with the shorter gas system.

CMMG ( sells a very nice 14.5" midlength if you want the ultimate in compactness. You can get it as an SBR or have them attach a permanent FS to it for right at an NFA friendly 16".

CMMG 14.5 Mid-Length Upper w/Permanent Phantom

New 14.5" mid-length upper w/permanent phantom hider. Permanent phantom hider brings OAL barrel length to legal 16” This upper is complete with bolt, carrier and charging handle. CMMG semi auto bolt carriers have the shrouded firing pin to protect against hammer wear. Upper receivers are "T" marked and include extended feed ramp cuts. Features include gov't profile mid-length barrel with 11 degree recessed target crown, triangle handguard cap, chrome lined 5.56mm chamber and bore, 1/7 twist, parkerizing under “F” marked forged front sight base and magnetic particle testing on every barrel. When ordering with optional gas block, barrels will not be drilled for taper pins. Barrel is marked CMMG MPC 5.56 NATO 1/7.

August 10, 2006, 10:23 AM
Especially with the .223, I'd personally prefer a longer barrel. However, this one won't be for me, and I've talked with the LEO and that is what is desired.

I figure that having a permanently attached muzzle device isn't such a horrible thing when all that is needed is to buy another upper...

Only thing about the Colts, I hear, is that they do not have a mid-length gas system for the 14" (or 14.5"?) uppers. Hm.

Thank you for the info!

Bartholomew Roberts
August 10, 2006, 11:09 AM
OK, you stated you wanted maximum reliability. So I am going to take that approach; but keep in mind that we are discussing fairly small differences in performance here.

In doing research on 14" barrels with permanently-affixed 2" muzzle devices, I've found several folks claiming that the higher pressures present in the shorter carbine gas system can potentially adversely impact reliability and service life.

There are links in the Reading Library at the top of the forum that explain this in great detail (complete with pressure curves and technical diagrams). Essentially the shorter gas system affects reliability by trying to cycle faster than the system was originally designed. When the rifle is cold, carbine gas system delivers a more gas, more quickly so that the cyclic rate is much faster than the rifle version. As a result, the magazine has less time to push the next round into position before the bolt comes back. As the carbine starts to heat up from prolonged firing, the second thing that happens is the carbine starts to try to extract before the brass has shrunk away from the chamber walls - this can result in torn rims, the extractor slipping off the cartridge, or the carbine using most of its energy pulling the case out and not having enough left to cycle.

The shorter gas system also affects service life due to increased pressure. Basically, it comes down to the pressures on the bolt and internal parts of the carbine being about 1.5x as much as the original rifle design. As a result of that, a recent NDIA Powerpoint noted that M4s subject to a heavy (SOCOM definition of "heavy") firing schedule developed micro-cracks in the bolt as soon as 3,000rds and even with a moderate (again SOCOM definition of "moderate") firing schedule, almost all M4 bolts display these cracks by 6-10k rounds.

With the carbine gas system, you can still adress both of these issues. On reliability, you can add a heavier buffer to slow the cycle time and lower feed ramps so that rounds that are in marginal position feed better. You can also use good quality magazines with stiffer springs and download them by a round or two. Second, you can add a stiffer extractor spring insert, a stronger extractor spring and an O-ring buffer to make sure the extractor stays on the brass.

To address the service life issue, there are several "enhanced" bolt designs that reengineer the traditional AR15 rifle bolt to better withstand the pressures produced in the carbine. Most of these designs also redesign the extractor as well to make the second step mentioned above unnecessary. However, these are all newer designs and may not be fully tested. If you want to stick with what works - buy MP tested bolts that have been proof-fired and replace them every 6-10k.

However, the easiest way to address these issues is just to move the gas port forward about 2" - this causes slower cycling and drops the internal pressure on the bolt. This little change makes most of the upgrades unnecessary (and sometimes undesirable in the case of heavier buffers); but you can add the extractor, mag spring, and feed ramp upgrades and they will perform well in this platform too.

However, now that we have discussed the theoretical limitations of reliability and service life, let's discuss real world practical use. SOCOM uses its weapons pretty hard. During pre-deployment training a Navy SEAL platoon, each man might fire 20,000rds in six months. In addition, there will be a fair amount of full-auto and even suppressed fire. Both of those things are hard on carbines. They can literally shoot a brand new carbine to pieces just during the workup. Very few of us will reach that level of training so it is important to understand that we aren't discussing the difference between reliable and unreliable as much as we are discussing the difference between 99% performance and 95% performance (those aren't actual numbers - just an example).

In my own practical experience, I ran a carbine length gas system with a regular strength extractor spring, regular buffer, no feed ramps, and black follower used GI mags of ancient vintage for about 10,000 rounds. During that time I had no parts breakage of any kind and the number of stoppages that I had that weren't related to ammo or magazines could be counted without taking off my shoes. Using good magazines and good factory ammo designed for the AR (cannelured bullets), will solve most of your problems right away.

Here are the desired features which I'm told may influence reliability:
14" barrel (1:7 twist) with 2" flash hider permanently attached
collapsable stock

I don't like permanently attached muzzle devices. They limit the amount of work you can do on the AR (can't remove the front sight base). They reduce velocity a small amount and the .223 relies heavily on velocity to be effective. Finally you only get about 1" reduction in length over a 16" barrel, which doesn't make much difference in handiness for me.

no preference on gas block size or positioning (supposedly mid-length gas systems are best in carbines?)

I'd prefer to have the rifle "complete" from Bushmaster, Armalite, Rock River Arms, or Colt, to avoid any gunsmithing snafus.

If you are looking for maximum reliability, I would drop Bushmaster from that list (and I own and have been very happy with several Bushmasters) because their carbine doesn't come equipped with all the things I mentioned earlier (heavier buffer, upgraded extractor, feed ramps, MP proof tested bolt) and they do not make a mid-length gas system.

Both Rock River and Armalite offer midlength gas systems; however like Bushmaster they don't include many of the upgrades I mentioned in their carbine gas system offerings.

Colt doesn't offer a mid-length gas system; but the LE-series carbines they do sell have all the appropriate mods we discussed earlier.

That said, are any of those features problematic in terms of reliability? If so, which changes would those of you in the know suggest?

All of the manufacturers you mention are good manufacturers and all of them put out the occasional lemon. I believe that the dealer is more important than the manufacturer when buying an AR.

I don't own any carbine gas-systems anymore. They are all 16" midlengths. I find the midlengths to be smoother in recoil and I am convinced (though I don't have complete personal experience equal to the carbine yet) that in the long term they will have better service life and be more reliable as well. Having said that, I got excellent reliability out of my carbines and had no issues with parts breakage or service life in the 10k rounds on the most used carbine.

Much more important than the gas system or the manufacturer are the ammo and magazines. Those are really the key to making this particular design run. Being in LE, your friend may chamber and clear ammo quite a bit before firing it. In that case, ammo with hard military primers and a cannelured bullet is important. The military primers prevent the repeated light indentation of the free-floating firing pin from setting off the round and the cannelured bullet keeps the bullet from being set back into the case and creating unsafe pressure levels. If his department doesn't already specify a particular load, I would look at the law enforcement offerings from Hornady, Federal and Winchester.

The second thing is good mags - if you are buying new magazines, you want USGI magazines, green USGI follower or Magpul self-levelling follower, and new chrome-silicon springs. Even then, you will need to try them out to make sure you didn't end up with any lemons; but once they all test good all your will need to do is keep them clean of major chunks of debris. Remember that mags are designed to be disposable items. When one starts giving you trouble on the range, ditch it and replace it. Currently, a lot of people offer "high reliability magazines." I am skeptical that some of these offer an improvement. Some of these, like the H&K HRM, have offered improved feeding, easier seating of the mag, and stronger material but have suffered from new issues caused by the change in design (corrosion, spines cracking). The basic USGI mag is cheap, available and a proven item that has worked well for me, so I would probably be reluctant to try many of the "super reliability" magazines in a scenario where I might really need the rifle until I had a lot of range time with them.

August 10, 2006, 11:13 AM

A chrome lined barrel is a $40 option; other options include a Smith Vortex flash suppressor instead of the A2, and a winter trigger guard. The price is suggested retail; you can probably do better than that.

August 10, 2006, 11:19 AM
The only ARs I've ever seen with barrel-length related reliability problems have been SBR-type weapons with very short (10" or 12") barrels (and, depending on manufacturer, these may work perfectly fine, as well). My issue weapon with 14.5" barrel functions just fine, as do my privately owned ARs/upper receivers, including both 16" versions and 11.5" versions with the long flash suppressor to make it civilian legal.

August 10, 2006, 05:12 PM
There are a couple of reasons for the 14.5" with permanent FS.
1. It looks like a real M4.
2. It is shorter therefore handier.
3. You can use a standard bayonet with this configuration. A standard bayonet won't work with a 16" barrel and carbine length gas system.

1. The only plus of "looking like an M4" with that notchy barrel profile is that he can mount a grenade launcher to it (if it is actually government profile and if pigs should ever fly so he can get a grenade launcher). Otherwise the M4 profile has no advantage whatsoever. So you are buying for looks.

2. The m4orgery is shorter by an inch or so. On the other hand it is less lethal by over an inch or so.

3. True, but the bayonet will work just fine with a 16" barrel and a midlength gas system. Which is what everyone is telling you to buy.

August 10, 2006, 06:57 PM
Man, that was a really great post Bartholomew Roberts. That whole post should be in a FAQ somewhere here.

August 10, 2006, 07:44 PM
or shorter, I would personally look for something with a 1:8 or 1:9 twist. I know that you cannot over stabilize a light bullet, but the 1:7's a re really made to shoot the heavy stuff (well as far as .223's go), and as many people have mentioned, the .223 relies on velocity to fragment it's bullets for maximum performance, and a anything above a 68gr, maybe a 72gr, bullet has a tough time hitting that mark with a short BBL. If you don't believe me pick up a copy of "the book of the AR", which is not a book, but a magazine. They did EXTENTSIVE research on how barrel length, twist, and bullet weight affect penetration and fragmentation.

Just my two cents..

August 10, 2006, 08:08 PM
i've been in a mental fog all day, so maybe it's just me, but that last post made no sense whatsoever.

if you cannot overstabilize a light bullet, then why not go with the fastest twist? i.e. 1:7 instead of 8 or 9
(you can in fact spin them too fast)

why would anything above 68g have trouble hitting the mark with a short barrel? and if they DO have trouble, why wouldn't you solve it by going to a faster twist?

August 10, 2006, 08:33 PM
I've only fired a few diff't ar's. In my experience, the nices ones are from Colt, Bushmaster, RRA and lately I've had access to a Stag that seemed very nice too.
In terms of configuration, I'd go w/ a middy flat top

August 10, 2006, 10:12 PM
how about , krebs, robinson, keltec, or maybe robinson arms, for a gas piston driven ar?

August 11, 2006, 01:03 AM

A. He does not want to bother with NFA stuff or his PD will not allow it.

B. His PD allows or specifies ammo other than ball.

Both of these are pretty solid assumptions. If they're true...

1. Don't worry about any velocity lost due to shorter barrel length. Lessened velocity is only really a terminal-performance problem for ball ammo. He won't be using ball ammo, I am willing to bet.

2. Midlength carbines (16" bbls, midlength guards) are a very good blend of short length, good sight radius, balance and durability/reliability. All that for the length of a flash-hider (16"+FH vs 14.5" with pinned FH). There is nothing not to love.

I'd go with a middy flat top with a flip up rear or detachable carry handle. Go with a major maker and you'll be fine. Stick with a 1:9 twist for flexibility, don't worry about a bayo lug (if it's there, great. If not, no biggie). Standard front sight tower is fine, they don't interfere with optics and anything more complex is one more thing to go wrong. He can use this setup as-is until he puts the optic of his choice on it.


August 11, 2006, 01:31 PM
Get a Colt M4 Upper and be done with it, it works for our servicemen why not you? Hav one and not one complaint, and a Bayonet on a 16" carine? :eek: If they get that close you need more range time!!!:what:

August 12, 2006, 01:22 AM
If you can find one, get him a Colt LE6920 and have it sent directly to Specialized Armement Warehouse ( for their reliability package.


Our company receives constant inquiries on the precise modifications (mod’s) which create this unique upgrade. The “Reliability Package:” (RP) is a PROPRIETARY package developed at Specialized Armament (S/A) and tested around the world from 1995 to present.

The Improvements listed:

4-6 Barrel mods
3 Gas system mods
2 Bolt group mods
Upgraded buffer assy

These mods give all late model COLT (.223/5.56mm) carbines greater reliability and better overall function in a combat proven weapons system which is already world-class. Each modification is small on its own, but when grouped together, these upgrades positively affect the weapon’s performance under all conditions. No further S/A Technical Data will be released concerning this COLT Reliability Package.


1. The RP will work on all Colt Carbines (1992-present) regardless of fire contol system (Auto, Burst, 4-Way, Semi).
2. Older Colt Carbines (1960’s-1991) can also be upgraded, but require further modification before the RP work.
3. No, the RP will not work on NON-COLT “AR’s.” They are substantially different. Yes, we’ve tried it—results were #$%^#@.
4. This is not an RP for Colt Rifles. The full-size weapons require a slightly different set of mods.
5. This RP will not work on Colt 9mm SMG’s and Carbines. These blow-back operated weapons are a completely different design with their own special requirements.
6. No, The S/A Reliability Package for Colts is not available from any other source—no matter what is claimed!
7. We have installed the RP on countless Colt Carbines over the past 10 yrs and these weapons virtually never malfunction (unless the ammo is absolute garbage or the magazine fails). It’s pretty impressive. Ask around.

Lock-N-Load (Ken)

August 12, 2006, 01:36 AM
5.56 mm duty loads (;f=78;t=001244), Tactical Forums, DocGKR, posted 08-01-2006 07:13

Only after proper foundational and ongoing repetitive refresher training, cultivating warrior mind-set, and ensuring weapon system reliability do you need to worry about ammunition selection. Most folks would be far better off practicing with what they have, rather than worrying about what is "best". As long as you know your what your weapon and ammo can realistically accomplish, it is all just a matter of training and shot placement. If you need to delve into the arcane subject of agency ammunition selection, below are the state of the art choices in .223/5.56 mm:


For general purpose combat use with 1/7 twist barrels from 0 to 600 yards, I would choose one of the combat proven 5.56 mm (ie. 5.56 mm NATO pressure loads, not .223 SAAMI pressure loads which run about 200 f/s slower) heavy match OTM loadings: either the Hornady 75 gr TAP (#8126N) using the OTM bullet w/cannelure or the equally good 77 gr Nosler OTM w/cannelure loaded by Black Hills, followed by the 77 gr Sierra Match King OTM-which, while exceedingly accurate, offers slightly reduced terminal effects. If your expected engagement scenario is at more typical LE distances, say out to 200 yards, then the .223 SAAMI pressure loads are fine. The experimental BH loaded 100 gr OTM exhibits impressive fragmentation, even at relatively low velocities, however, their trajectory is like a rainbow-definitely for use under 150 - 200 yards.

NOTE: For general purpose LE use, if stuck with 1/9 twist barrels, the heavy 70+ gr match OTM loads are not universally accurate in all rifles and the 69 gr SMK OTM, the 68 gr Hornady OTM, the Winchester 64 gr JSP (RA223R2), or one of the new Federal 64 gr TRU (223L) JSP, Hornady 60 gr JSP, or Nosler 60 gr Partition JSP's are the best choices to most likely run accurately in the majority of 1/9 twist rifles. You are screwed with 1/12 twists, I would probably choose the 55 gr Federal bonded JSP load (Tactical--LE223T1 or identical Premium Rifle--P223T2) in order to ensure adequate penetration.

If routinely engaging vehicles, the only LE .223 loads which effectively penetrate automobiles are the 62 gr Federal bonded JSP Tactical (LE223T3) and the similarly performing 55 gr Federal bonded JSP load. While not bad choices, neither the new Hornady 60 gr “barrier penetration” JSP nor the 60 gr Nosler Partition JSP bullets were quite as effective as the proven Trophy Bonded Bear Claws against glass in our testing. None of the OTM bullets, even the heavy 75 - 100 gr loads, offer good performance through automobile glass. FWIW, contrary to what many believe, 62 gr M855 FMJ also is not very good against glass. For military use, the M995 AP is the best choice for vehicles and glass.

If a short barreled 5.56 mm weapon, such as the Colt Commando, LMT/Crane Mk18 CQBR, HK416, HK 53, HK G36C, etc… is used with a 1/7 twist barrel, the 75 gr Hornady OTM, 77 gr Nosler OTM, 77 gr SMK OTM, and 100 gr BH OTM loadings offer acceptable performance, as do the LE Fed 55 or 62 gr Tac bonded JSP's and the 60 gr Nosler Partition JSP bullet; for LE with a 1/9 twist, stick with the Fed 55 or 62 gr Tac bonded JSP's or the 60 gr Nosler Partition JSP bullet. Remember, with barrels under 14.5”, the effective engagement distance is significantly reduced compared to the longer barreled carbines.

Whatever projectile is used, it is best with a cannelure to prevent bullet set-back in semi-auto/auto weapons. Also, be cautious with the exposed lead on the JSP designs. Often they will run great for up to 200-300 rounds, but then mysterious feeding failures will begin as a result of lead build-up on the feed ramps. I have personally seen this occur with a variety of JSP's including 55 gr, 60 gr, and 64 gr in a recent LE training course. As soon as FMJ or OTM was substituted, all the feeding failures ceased.

Be sure to watch your ammo storage conditions. Temperatures above 150 deg F will degrade the powder and cause pressure spikes. Hint: Think locked metal conex containers in the mid-east, car trunks in the southern U.S., and storage areas near heaters in the northern U.S.


Most LE agencies around here use the Hornady 75 gr TAP OTM, Federal 55/62 gr bonded Tactical JSP, or Winchester 64 gr JSP (it is on the state contract and is VERY inexpensive)--all have worked very well in actual officer involved shootings. I carry the Hornady 75 gr OTM (SAAMI load) in 30 rd mags and a few 20 rd mags of Federal Tactical 62 gr JSP for barrier situations.

Do short barrel 5.56 mm carbines have some limitations? Yes, especially beyond 100 yards, but BFD…learn what they are, train, and drive on. For LE urban work with lots of entries and mounted work I use a 10.5” LMT CQB-R w/Aimpoint and Noveske KFH because, despite the ballistic compromise, for the mission it is the best choice. For GP/Patrol I carry a 16” with a 3.5x TA11 ACOG (the S&B 1.1-4x Short Dot would be an equally good or even better choice) -- pick the right tool for the job.


Mk262 using the 77 gr SMK OTM is built as premium quality ammunition intended for precise long-range semi-auto rifle shots from the Mk12. It is great for its intended purpose and in the military is the best currently readily available option to increase terminal performance with all 5.56 mm carbines. But Mk262 is NOT necessarily the best choice for LE or civilian self-defense purposes, or for that matter for most military combat use from carbines (Mk18, HK416, M4, FN Mk16 SCAR-L).

What is needed is a environmentally robust, thermally stable combat rifle round with improved terminal performance, yet without the need for the 600+ meter precision accuracy of Mk262 in the Mk12. Such a carbine combat load needs to have a crimped and sealed primer, sealed case mouth, cannelure, acceptable accuracy out to 300-400 meters coupled with good soft tissue terminal performance (early yaw, good fragmentation, at least 12 inches of penetration coupled with maximized tissue damage during the first 10 to 12 inches of travel in tissue. The new Hornady 75 gr milspec OTM (#8126N) loading or equivalent BH milspec 75 gr or Nosler 77 gr OTM loads are examples of this type of combat ammo. Note, the Hornady 75 gr OTM load has a proven combat record in use with certain U.S. SOF units for several years.

At this time, given the current ammo choices available via the military supply system, I prefer to load my mags with mainly Mk262Mod1 and if available, a couple of mags of M955 AP for barriers and some M856 tracer (or TB74 dim trace) for marking targets.

October 4, 2007, 01:38 PM
I have an (above) and find it useful. I have not chronographed it yet, but would be interested in what kind of velocities others are getting. For those switching from a pistol to a SBR, I would be careful, you are violating the law. I believe that if you have the ability in your posession are in :what:violation. Just my reading.

September 14, 2009, 08:42 PM
i did some experimentin and short barrels offer better penetration with Hp bullets then longer barrels , i have a 11.5" bushmaster barrel with welded 5.5" useless flash hider on it i got surprising results with 55 gr FMJ , i would use either 55gr FMJ or heavier Hp for short barrels it will provide exceptional fragmentation and penetration for short distances.

my 11.5 " barrel has excellent barrier penetration too. shot thru plywood and and old wooden door still exploded the water jug.

September 14, 2009, 11:46 PM
There are bunches of Colt 6920's and 6940's on Gunbroker.

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