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Thread: Techie Question

  1. #1
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    Techie Question

    Forgive me if this has been answered here can't fine it.

    Does the BCG unlock and start to move rearward (with less force) before the bullet passes the gas port and then after the bullet passes the gas port move (with greater force increasing speed).......or does the BCG stay locked in place and not begin its rearward motion until the bullet passes the gas port and then move at a consistent speed?
    "First gett'n shot, then gett'n married... baaaad habits"

    "If you're gonna subscribe to hero worship, at least worship a real hero."
    M4Guru

    Gal 2:20

  2. #2
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    If I remember right, it's discussed in this thread somewhere: Heat is bad. DI rifles get hot. DI rifles are bad.

    That was a damn good thread with eight pages filled with some really random stuff that shows what can come out of forums like this. I believe someone there said that the bolt gets pushed back against the lugs in the barrel extension until gas enters the carrier (after the bullet passes the gas port). At that point the carrier begins to move backwards, rotating/unlocking the bolt until it moves back as well.

  3. #3
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    The bolt is locked closed by the diagonal slot in the bolt carrier**. The bolt cannot rotate itself free so it stays locked until the bullet passes the gas port. At this time the gas flows through the tube and pushes back on the bolt carrier. The rearward movement of the carrier allows the bolt to rotate and unlock and cycle the rest of the way.

    **and also the lock tabs on the bolt and barrel extension of course
    Last edited by pilotguyo540; 08-17-10 at 00:37.
    "Oh, its a wonderful day! My sun is shining, my birds are chirping, my humongous chicken defeated Elmo." Huxley

  4. #4
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    Thanks. Everytime I think I've got it a new term like "Dwell Time" comes up and the hamster starts "trying" to turn the wheel
    "First gett'n shot, then gett'n married... baaaad habits"

    "If you're gonna subscribe to hero worship, at least worship a real hero."
    M4Guru

    Gal 2:20

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    Probably way more detail than you wanted but part of this post does answer your question.

    http://m4carbine.net/showthread.php?t=94

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    FROM THIS LINK... MY QUESTIONS ADDED IN CAPS

    http://m4carbine.net/showthread.php?t=94

    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.

    ?DOES THE LENGTH OF THE GAS TUBE ITSELF HAVE ANY SIGNIFICANT EFFECT?

    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.

    ?DOES DWELL TIME END WHEN THE BOLT UNLOCKS? OR IS THE BCG STILL CONSIDERED UNDER PRESSURE TILL IT HITS THE BUFFER?

    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.

    ?IF THE CARBINE UNLOCKS FASTER THEN WOULDN'T IT INHERENTLY HAVE A SHORTER DWELL TIME?

    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.
    Last edited by M4Fundi; 08-17-10 at 03:33.
    "First gett'n shot, then gett'n married... baaaad habits"

    "If you're gonna subscribe to hero worship, at least worship a real hero."
    M4Guru

    Gal 2:20

  7. #7
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    Quote Originally Posted by pilotguyo540 View Post
    The bolt is locked closed by the diagonal slot in the bolt carrier**. The bolt cannot rotate itself free so it stays locked until the bullet passes the gas port. At this time the gas flows through the tube and pushes back on the bolt carrier. The rearward movement of the carrier allows the bolt to rotate and unlock and cycle the rest of the way.

    **and also the lock tabs on the bolt and barrel extension of course
    To be clear- The bolt cam does not lock the bolt. What actually locks the bolt is the lugs engaging lug recesses in the barrel extension. The bolt will not unlock until it's turned. If the cam didn't turn the bolt as the carrier moved rearward, the bolt would not unlock and open.

    If you removed the bolt lugs, the cam would do nothing to lock the bolt and does not even act as a safety lug in case the primary lugs should shear

  8. #8
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    Quote Originally Posted by M4Fundi View Post
    FROM THIS LINK... MY QUESTIONS ADDED IN CAPS

    http://m4carbine.net/showthread.php?t=94

    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.

    ?DOES THE LENGTH OF THE GAS TUBE ITSELF HAVE ANY SIGNIFICANT EFFECT?

    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.

    ?DOES DWELL TIME END WHEN THE BOLT UNLOCKS? OR IS THE BCG STILL CONSIDERED UNDER PRESSURE TILL IT HITS THE BUFFER?

    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.

    ?IF THE CARBINE UNLOCKS FASTER THEN WOULDN'T IT INHERENTLY HAVE A SHORTER DWELL TIME?

    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.
    I think the second to the last paragraph partly answers one of your questions

  9. #9
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    Quote Originally Posted by MistWolf View Post
    To be clear- The bolt cam does not lock the bolt. What actually locks the bolt is the lugs engaging lug recesses in the barrel extension. The bolt will not unlock until it's turned. If the cam didn't turn the bolt as the carrier moved rearward, the bolt would not unlock and open.

    If you removed the bolt lugs, the cam would do nothing to lock the bolt and does not even act as a safety lug in case the primary lugs should shear
    Right. All the cam does is rotate the bolt. But without the can the lock tabs would never engage. It actually never occurred to me that this would need clarification. Now everyone knows

    To the OP:

    The length of the gas tube would have to affect the dwell time. I think it would be important to know exactly when dwell time ends, but either way we know it takes longer (about 2x) to unlock the bolt on a rifle gas system. I would GUESS about half of the difference is due to the distance from the chamber and the other half time to build enough pressure to push the bolt carrier back.

    As far as when dwell ends, I don't know for sure, but the second half of that question shows a little confusion you may have. The bolt does not contact the buffer. The bolt carrier does. For all intents and purposes, it is in contact with the buffer at all times.

    Does the carbine have a shorter dwell time because it unlocks faster??? Again we would need to know when dwell ends. If dwell ends when the bullet. Leaves the muzzle, the dwell time for both rifles would be close with the rifle having a minuscule amount less dwell due to the increased muzzle velocity in the 20 inch barrel. If dwell time ends when the bolt opens. The rifle has twice the dwell time.

    I don't think it really matters. The important thing to keep in mind is how much more violent the carbine action is in comparison to the rifle length gas system.

    GO MIDDY!!! YAY!!!
    "Oh, its a wonderful day! My sun is shining, my birds are chirping, my humongous chicken defeated Elmo." Huxley

  10. #10
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    Quote Originally Posted by pilotguyo540 View Post
    Right. All the cam does is rotate the bolt. But without the can the lock tabs would never engage. It actually never occurred to me that this would need clarification. Now everyone knows

    To the OP:

    The length of the gas tube would have to affect the dwell time. I think it would be important to know exactly when dwell time ends, but either way we know it takes longer (about 2x) to unlock the bolt on a rifle gas system. I would GUESS about half of the difference is due to the distance from the chamber and the other half time to build enough pressure to push the bolt carrier back.

    As far as when dwell ends, I don't know for sure, but the second half of that question shows a little confusion you may have. The bolt does not contact the buffer. The bolt carrier does. For all intents and purposes, it is in contact with the buffer at all times.

    Does the carbine have a shorter dwell time because it unlocks faster??? Again we would need to know when dwell ends. If dwell ends when the bullet. Leaves the muzzle, the dwell time for both rifles would be close with the rifle having a minuscule amount less dwell due to the increased muzzle velocity in the 20 inch barrel. If dwell time ends when the bolt opens. The rifle has twice the dwell time.

    I don't think it really matters. The important thing to keep in mind is how much more violent the carbine action is in comparison to the rifle length gas system.

    GO MIDDY!!! YAY!!!
    "See, Capt'n, that's why, although you fly it, it's my airplane. You ride her hard, use her, abuse her and at the end of the day, it's this lowly airman that sweats and bleeds over her until she's put back to rights. I understand her. And trying to translate what you told me is wrong into what's really wrong adds time to the troubleshooting. You'll tell me the hydraulics don't lock the landing gear (when I know the hydraulics weren't designed to lock the landing gear that they lock mechanically) when the real trouble is the the landing gear light didn't go out because the position switch is bad"

    They are locking lugs not tabs. Not everyone knows that all the cam does is rotate the bolt or somebody wouldn't have made the claim that the cam along with the locking lugs locks the bolt.

    The dwell time is measured from the time the bullet clears the gas port until it exits the muzzle. Once the bullet clears the muzzle the gas pressure drops. Self loading small arms are designed that the bolt does not extract the spent casing until the pressures in the barrel have dropped to safe levels.

    Carbine length gas systems have their ports closer to the chamber where pressures are much higher than they are further down the bore. In a 14.5" barrel, the distance from port to muzzle is shorter than it is with a 16" barrel. The BCG is exposed to these raised pressures for a longer time. Again, that's because the bolt must stay locked until pressures in the barrel drop to safe levels. If you used a carbine length gas system with an 18" or 20", dwell times are even longer.

    Sure, Go Middy, YAY! But all you have to know capt'n is how much more violent the middy is compared to the rifle length system because you can see it doesn't really matter. Except to those who have made an effort to understand how it all works so they can be kept running
    Last edited by MistWolf; 08-22-10 at 20:18.

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