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Thread: After you pull the trigger

  1. #1
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    Carbine Gas System

    I originally posted this on a different forum, but it got emailed around enough that someone actually emailed it back to me! Good reading, I will go through and clean it up later -- tacos wait right now!

    ==============================
    In order to understand this, there are a few things
    about the functioning of the AR that have to be
    defined, I am away from most of my notes and stuff, so
    most of the figures given are from memory... but
    should be pretty close. For this description, the
    standard rifle gas system is with the gas port located
    at 13.0" and having a 20" barrel... the standard
    carbine system is with the port at 7.5" and a barrel
    of 14.5"

    The pressures at the gas ports are: 13.5K for the
    rifle and 26K for the carbine -- or twice as much.

    The dwell time (the time that the gas system is
    charged with high pressure) is determined by the
    amount of barrel after the gas port. These are nearly
    identical between the rifle and the carbine.

    Pressure from the port is regulated only by the size
    of the gas port and the diameter of the barrel.

    These two factors determine the internal bolt
    pressure, the maximum pressure that is obtained in the
    bolt carrier/piston combination -- for the rifle this
    pressure is about 1000psi and for the carbine it is
    over 1500psi, half again as much.

    When the rifle is fired, primer shot sets the bullet
    forward until it contacts the rifling, at this point
    the powder charge ignites and sets the shell case
    fully back, binds the action and start to propel the
    bullet. The bullet jumps slightly again and is etched
    by the rifling... it stops again very briefly as the
    pressures build to a point for the bullet to overcome
    the mechanical advantage of the rifling twist and the
    ! bullet starts to spin, at this point the chamber
    pressure is at max, 50K plus (there are some that
    believe there is another, third stop the bullet makes
    and some testing suggest this may be true).

    As the chamber pressures start to climb, the brass
    case expands and becomes plastic, this is essential to
    seal the case in the chamber -- the correct term for
    this is Obturation, when the case is obturated and
    sealed, it is stuck in the chamber, practically welded
    in really.

    The Lock Time, or the time that the action remains
    locked with no attempt to start unlocking is very
    important... on the rifle, the lock time is about 550
    microseconds, the lock time for the carbine is about
    375 microseconds -- this may not seem like much, but
    it is much shorter of a time, also keep mind that the
    chamber pressures are twice as high in the carbine
    when the unlocking starts.

    What does all of this mean? When the carbine is fired,
    the system attempts to unlock earlier than intended
    and while the case is still fully obtucated... this
    results in the action bind delaying the unlocking and
    stressing the system. As the 5.56N is not drastically
    tapered, "squirting" is not a big problem in most
    guns. When the internal bolt pressures finally unlock
    the bolt, the velocity of the reward movement in the
    carbine is much higher than what the rifle was
    designed for, it also must start extraction of the
    obtucated case... as you know, the AR does not have
    any sort of initial extraction, perhaps the single
    biggest shortcoming of the design. This has been known
    to cause ripped case heads...

    At this point, as the bolt start to unlock, it is
    rotated to unlock... due to much higher velocities
    with the carbine, the rotation of the bolt creates
    some centrifical force and helps to "float" the extractor...
    the extractor on the AR is not balanced and the
    forward part of it weighs more -- some argue that the
    pressure of the extracted cartridge case keeps the
    case head against the bolt face and test have shown
    that the extractor does not generate enough force to
    actually life from the case, but the fact is that the
    extractor does float enough to negate some of the
    spring pressure, and the contact with the case
    rim becomes "soft". For this reason, it is much more
    likely that the extractor will simply pop off, rather
    than actually rip the case.

    Balanced extractors and different designs have been
    developed (LMT), but the best solution to date has
    been stronger extractor springs and spring buffers.
    That about covers the FTE issues...

    Back to bolt velocity. The high speed of the bolt has
    a couple of other detrimental effects, one of the most
    common is that the bolt is cycled so fast that as it
    returns to battery, it actually has enough force to
    "bounce" off of the barrel extension when closing and
    locking... this bounce back is very small, but can be
    enough to cause the weapon not to fire... this "bolt
    bounce" is pretty well known.

    One other problem is that the bolt can cycle so fast
    the magazine spring can not keep up with it and the
    round stack is not properly aligned and forced back
    into place before the bolt returns to batter --
    therefore there is no new cartridge picked up and the
    bolt closes on an empty chamber, this is what some
    call "ghost loading", or bolt-over-base jams... this
    is far worse in full auto fire as the bolt does
    actually move faster in full auto than semi auto; this
    is due to the fact that the top cartridge in the
    magazine does not apply force to the bottom to the
    bolt causing drag.

    The common solution to this issue is to use a stronger
    recoil spring and a heavier buffer... this works, but
    is treating the symptom, not the problem.

    PigTail and expansion chamber gas tubes attempt to
    fool the rifle into thinking that the gas port is,
    located further away that it really is, but they are
    not as good of a solution as actually moving the gas
    port out...

    I guess that about covers it for a quick rundown, of
    course all of this is not nearly as simple as it
    sounds.

  2. #2
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    K.L.,

    Thanks for posting this, as I had not seen it before and am truly intrigued about the effects that the different component selections have on the weapon's operation and, particularly, on its reliability and durability.

    I'd much appreciate it if you could take it a step further and elaborate on the so-called "mid-length" gas system and the advantages/disadvantages of its application with various barrel lengths (including specifics on gas-port and bolt pressures, etc.).

    Perhaps it's the subject of another thread, and I'm sure it's been discussed ad nauseam before, but I'm very curious what the optimal combinations of buffer weights, carrier mass, and recoil-spring rates are for the various barrel/gas-system lengths.

    Again, thanks for sharing.
    Aubrey<><

  3. #3
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    Aubrey,

    Sorry for missing this earlier... I am actually about to leave for a short deployment and don't have a lot of time right now, if this works out like most though, I will have time to address it during the week (if we have internet).

    So, for a quick answer... I am a big fan of the midlength system and have advocated it for a few years now, the pressures are tamed down a lot and on a 16" barrel it makes for probably the best all around rifle package.

    When we first did the midlength 14.5" in 2002, I did a lot of testing with port sizes and buffer weights -- the only thing that I would caution against using with a 14.5" midlength and telestock, is not to use the real heavy buffers (9mm et al) as we experienced some issues in cold weather with this setup.

    I will write up more later, but if I were to recommend one rifle for the most practical "do all" setup, it would be a 16" barrel and midlength system.
    Please let me know if I have offended you...
    I would like to move on to my next task.


    I provide legal consultation and training - specializing in the Law of Gravity.

  4. #4
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    Quote Originally Posted by K.L. Davis
    the AR does not have
    any sort of initial extraction, perhaps the single
    biggest shortcoming of the design.

    Can you elaborate on this?

    Thanks.
    Austin Nichols = Wild Turkey

  5. #5
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    Quote Originally Posted by K.L. Davis
    Aubrey,

    I will write up more later, but if I were to recommend one rifle for the most practical "do all" setup, it would be a 16" barrel and midlength system.
    Is there an ideal buffer weight for the above setup? I received an "H" buffer with my MagPul CTR kit. Just wondering what your thougths were.

    ETA: Found what I think is an answer in antoher thread. Seems like some people prefer to run an H2 buffer, but an "H" buffer also appears to be "just fine." Either way, something heavier than a "standard" weight buffer is preferred. However, having a heavier buffer appears to MORE of an issue in a CARBINE length system, where the sytem takes more of a beating due to the shorter gas tube.

  6. #6
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    Slow Initial Extraction

    Quote Originally Posted by Austin_Nichols View Post
    Can you elaborate on this?

    Thanks.
    In bootcamp, we were taught that the M14 had "slow initial extraction." That was because the M14 extractor was designed to "bite" the case rim as it rotated as the bolt was unlocking. Must have been the same way on the M1 Garand. And I guess every Mauser bolt gun ever made.
    Later, when we were introduced to the M16 before deploying to Viet-Nam, the Instructors said it did not have slow initial extraction. That was one of the reasons they made us clean the crap out of the M16, especially the chamber.
    ColdBlue sends...
    (CB is David A. Lutz, Lt. Col. USMC (Ret'd) (1968-1991)
    Former (now retired) VP MilOps @ Knight's Armament Company (KAC) (1994-2012)
    "...if you can read this, thank a Teacher,
    if you are reading this in English, thank a Veteran..."

  7. #7
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    Bump...

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    Cool Re: Detail description of the cycle

    I've had that picture in my head for years and at times tried to put it to paper. I did fairly well, but not nearly as good as your description.

    woofe

  10. #10
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    Quote Originally Posted by K.L. Davis View Post
    PigTail and expansion chamber gas tubes attempt to
    fool the rifle into thinking that the gas port is,
    located further away that it really is, but they are
    not as good of a solution as actually moving the gas
    port out...
    Do you know why this is?
    "Intelligence is not the ability to regurgitate information. It is the ability to make sound decisions on a consistent basis "--me

    "Just remember, when you are talking to the average person, you are talking to a television set"--RDJB

    One Big Ass Mistake America

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