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Most reliable 16" AR-15 carbine configuration?

Discussion in 'Rifle Country' started by WeedWhacker, Aug 10, 2006.

  1. WeedWhacker

    WeedWhacker Well-Known Member

    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?
  2. Gary G23

    Gary G23 Well-Known Member

    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.
  3. 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.
  4. Gary G23

    Gary G23 Well-Known Member

    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.
  5. Beren

    Beren Moderator Emeritus


    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?
  6. Thin Black Line

    Thin Black Line Well-Known Member

    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.
  7. DMK

    DMK Well-Known Member

    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".

  8. WeedWhacker

    WeedWhacker Well-Known Member

    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!
  9. Bartholomew Roberts

    Bartholomew Roberts Moderator Emeritus

    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.

    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.

    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.

    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.

    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.
  10. benEzra

    benEzra Moderator Emeritus

  11. HorseSoldier

    HorseSoldier Well-Known Member

    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.
  12. MrAcheson

    MrAcheson Well-Known Member

    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.
  13. DMK

    DMK Well-Known Member

    Man, that was a really great post Bartholomew Roberts. That whole post should be in a FAQ somewhere here.
  14. knoxx45

    knoxx45 Well-Known Member

    in a 16" bbl

    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..
  15. taliv

    taliv Moderator

    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?
  16. 10-Ring

    10-Ring Well-Known Member

    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
  17. rangerruck

    rangerruck Well-Known Member

    how about , krebs, robinson, keltec, or maybe robinson arms, for a gas piston driven ar?
  18. Coronach

    Coronach Moderator Emeritus


    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.

  19. Scottso

    Scottso Well-Known Member

    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:
  20. 226

    226 Well-Known Member

    If you can find one, get him a Colt LE6920 and have it sent directly to Specialized Armement Warehouse for their reliability package.


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