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Thread: Teach me about Direct Impingement

  1. #11
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    Quote Originally Posted by 5pins View Post
    You’re completely correct, the animation is wrong and the M4 works just as you described.

    The animation also screwed-up the buffer tube depth.
    My brother saw Deliverance and bought a Bow. I saw Deliverance and bought an AR-15.

  2. #12
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    Quote Originally Posted by rifleman2000 View Post

    I am an expert, so I am respectfully asking: Base on the above, does a piston driven AR15 have more wear and tear on the bolt (minus the extra gas that allievates pressure on the bolt/chamber lock up)?
    In my understanding, yes to a degree...especially in systems that begin turning the bolt while there is still anything much more than atmospheric pressure in the barrel (e.g., barrels with a long dwell time, suppressed systems, etc....)

    Also to my understanding, just increased wear is not so much the problem per se...it is much more the increased shear forces on the rear of the bolt lugs.

    As explained to me, as the bolt turns while being pressed against the lugs by residual barrel pressure, the forces will be increasingly concentrated on the trailing side of the rear of the bolt lugs. Those forces can over-stress a weak and/or fatigued lug causing it to fracture and flake off a chip potentially creating a malfunction or leading to more lug failures and catastrophic bolt failure as the remaining unbroken lugs have to carry more of the load.


    MikeN
    Last edited by MJN99999; 05-05-10 at 02:48.

  3. #13
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    Quote Originally Posted by Heavy Metal View Post
    The animation also screwed-up the buffer tube depth.
    Screwed up for sure the bolt doesn't strip the round out the chamber it jumps out the mag before the bolt even begins to move forward.

    The bushmaster animation is better.
    http://www.bushmaster.com/anatomy_bushmaster.asp

  4. #14
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    The gas that is sent down the gas tube into the gas key of the BCG is what sends the BCG rearward. Rather than the gas moving a piston which in turn moves the bolt to the rear (like a connecting rod on a rocker arm or piston in a car engine) the gas is directly (hence DI) forced against the BCG.

    Pressure on the bolt face by the cartridge should be minimal at the time of unlocking and extraction. Like others have posted if there was significant pressure at the time of unlocking the locking lugs would get a good deal of wear.

  5. #15
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    I wish I had all the pictures from ADCO, but the details of what physics are occuring during unlocking and the transition to extracting the case are poorly understood by a lot of folks.

    What makes the DI system a bit unique is that the gas being directed through the gas tube and gas carrier key enters the chromed chamber inside the bolt carrier (functions as a piston), which not only drives the bolt carrier back, but applies pressure forward on the bolt (the bolt tail is in this area, the gas rings on it provide a seal, which is why they are so important to cycling), this force being applied is important, and off-spec systems will cause the rear face of bolt lugs to wear even faster - those failures are usually a combination of this, and mostly suspect metallurgy.

    The bolt face remains forward while the carrier travels back, and during this motion the cam pin functions to begin unlocking the bolt - any suggestion that case pressure is require simly isn't the case, as the amount of force required to cam the bolt out of locked position in the amount of time it does means there is enough momentum in the bolt carrier group to cycle the action.

    Once the camming motion of the bolt and cam pin is completed, the cam pin reaches the end of it's helical track, and the bolt is accelerated rearwards. This quick bit of impulse is why bolts tend to break right around the cam pin hole, and why cam pins demonstrate so much wear in so many distinct areas. The channel the cam pin follows is secondary in function - the bolt will unlock through a combination of the cylindrical part of the cam pin in it's helical channel as well as the top part contacting the upper receiver.
    Further evidence to me that case pressure isn't involved is that when stripped cases occur in the chamber, the BCG generally cycles and loads the next round creating double feeds.

    As the bolt reaches its fully forward position, the holes (two, or three in the case of the LMT enhanced BCG) then allow excess gas pressure to be vented out via the ejection port, and as the bolt carrier group continues rearward travel, the spent case is ejected.
    عندما تصبح الأسلحة محظورة, قد يملكون حظرون عندهم فقط
    کله چی سلاح منع شوی دی، یوازي غلوونکۍ یی به درلود
    Semper Fi
    "Being able to do the basics, on demand, takes practice. " - Sinister

  6. #16
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    The extractor will rip the rim off a case stuck in the chamber. Gas pressure alone is enough to operate the action of an AR.

    Cut the gas off and you have a manually operated rifle. If anything, the back thrust of the case on the bolt face serves to keep the bolt from unlocking until the carrier can perform that function.

    A question- how does the gas get from the carrier key to the rings? Is the carrier key hollow all the way through? Or is it from spillage as the key separates from the gas tube? If it's spillage, how is the gas contained until it gets to the rings? Unfortunately I don't have my rifle with me to study
    Last edited by MistWolf; 05-05-10 at 21:14.

  7. #17
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    Quote Originally Posted by MistWolf View Post
    A question- how does the gas get from the carrier key to the rings? Is the carrier key hollow all the way through?
    Yes the key is hollow all the way through.

  8. #18
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    The gas key directs the gas down into the bolt carrier (the front cylindrical hole in it is hollow, with just gas vents on the starboard side). The bolt tail with the gas rings fits into this cylindrical hole, the gas rings make a seal around the outside, so when you disassemble the bolt carrier, you'll see this as a lot of open space, but the tail of the bolt usually fills it, and it's chromed because the bolt reciprocates and rotates inside it, with the gas rings making the seal against the bolt carrier, and it's chromed because the majority of gas used to cycle the action is routed through here.
    عندما تصبح الأسلحة محظورة, قد يملكون حظرون عندهم فقط
    کله چی سلاح منع شوی دی، یوازي غلوونکۍ یی به درلود
    Semper Fi
    "Being able to do the basics, on demand, takes practice. " - Sinister

  9. #19
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    http://www.dtic.mil/ndia/2003smallarms/din.ppt

    I found this on vuurwapen.blog. Microsoft offers a free viewer for ppt files.

    1) There is residual pressure on the case forcing it against the bolt face. It some instances it's the only thing that extracts a case.

    2) DI operates the cycle all by itself. This has been the accepted dogma long before I served, but like a lot of proprietary data, never demonstrated. Thank you TACOM.

    This certainly amplifies what many of you were saying.

  10. #20
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    For those of you who haven't read these two articles these are very good reads which will clarify some questions asked.

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

    http://www.ar15barrels.com/prod/operation.shtml
    Last edited by Robb Jensen; 06-30-10 at 08:02.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

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