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Thread: Chamber pressure during extraction

  1. #31
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    Here I am, beating my personal dead horse again. This topic is an illustration of why one should lightly lubricate cartridge cases when it really counts.

    As in the stock market, timing is everything here. If carrier movement is a bit early, chamber pressure will be higher than optimum for easy extraction. A higher pressure than normal round can do this. Also, if the chamber or the round is dirty there will be more than the usual resistance to extraction. And, if the case is softer than normal the same will happen causing the brass to stick to the walls of the chamber as the round is extracted. Imagine what would happen if all three happened in unison.

    Lightly lubing your brass eliminates all of these issues.
    Dave

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  2. #32
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    Quote Originally Posted by Blanksguy View Post
    How would the Lewis-Machine & Tool's Bolt-Carrier design play into this where it "delays" opening of the chamber area by a split-second (?).......IE: A slightly delayed opening of the bolt by the cam machined into the carrier so that the chamber-pressure drops slightly over that of a normal Bolt-Carrier.

    Regards, RichardS in MI.
    US Army, Retired.
    There is a slot in the carrier in which rides the bolt cam. It is straight, then makes a turn. When the bolt cam encounters the turn, this turns the bolt to unlock it. The straight section of the cam slot is longer. This allows the carrier to travel a greater distance which increases the time before it begins unlocking the bolt
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  3. #33
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    Quote Originally Posted by MistWolf View Post
    Ah. Still, what does that have to do with the fact the rifle won't extract the spent case without an extractor?
    Wait, I thought residual chamber pressure would blow that spent case right out of the chamber. And then there is the pressure that keeps the case pinned to the bolt face during the recoil phase of operation?
    Last edited by MarkG; 08-19-11 at 14:27.

  4. #34
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    Quote Originally Posted by MistWolf View Post
    There is a slot in the carrier in which rides the bolt cam. It is straight, then makes a turn. When the bolt cam encounters the turn, this turns the bolt to unlock it. The straight section of the cam slot is longer. This allows the carrier to travel a greater distance which increases the time before it begins unlocking the bolt
    I'm tracking with everything you're saying. But I do have a question, and it's based on my following description/assumptions of the breakdown of the extraction cycle. Maybe I've got it wrong, if so, please correct.

    Primer ignites, bullet releases from case mouth and travels down bore.

    Bullet passes port and charges gas system, sending gas to BCG via gas tube.

    Gas is trapped within a volumed defined by the gas rings, which are part of the bolt. the carrier surrounds this, but is not impacted directly by gas pressure, since it's trapped within the volume bounded at the rear end by the gas rings (which aren't moving, see paragraph below), and unbounded fwd from the key, to the gas tube, to the barrel port, and out to the muzzle -- it's all open going fwd.

    Bolt at this instance can't rotate or move to rear because bolt is engaged against lugs. Even if casehead thrust acts on bolt face, trying to push it to the erar, the bolt won't budge as long as it's locked up against the barrel extension lugs. I can envision once the bolt is unlocked, then the entire BCG, as a unit, can travel rearward, but I can't picture how the carrier moves initially rearward for the straight portion of the cam pin slot you mention. Assuming the bolt remains locked and pressure isn't acting on it in the rearward direction (maybe radially between the area/void forward of the rings and against the interior walls of the carrier, but not rearward), I can't picture how the carrier moves rearward that initial bit before the cam pin rides the angled slot and unlocks the bolt.

    I know it has to happen somehow, I know when reassembling BCG, you can slide the bolt a bit in and out of the carrier, and when the bolt locks up in the barrel extension, the carrier is fully fwd, but when reassembling into the upper and inserting the BCG into the upper (and when rebounding during extraction and feeding cycle) with the charging handle, you have to have it so the carrier is rearward so the cam pin doesn't interfere with the receiver. The cam pin has to be centered at 12:00 for everything to go back together correctly.

    To the OP, in order to diagnose why extraction isn't happening, I think it's important to understand the extraction cycle and timing of events. That could lead fix actions such as heavier buffers and/or carriers to slow down BCG velocity and/or delay initial rearward movement, addition of a HD extractor spring and/or donut to increase extractor tension, a new extractor., investigation of the gas port diameter to see if it has eroded and enlarged which leads to overgassing, etc. There could be many things at play, and understanding how it's all supposed to work helps in diagnosing root causes of problems when it doesn't work.

    Thanks in advance.
    Last edited by jmart; 08-19-11 at 20:34.

  5. #35
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    You've outlined the process correctly. Gas enters the BCG behind the gas rings. Since the bolt's linear motion is restricted until it unlocks, the only thing that can move is the carrier. The carrier is forced to the rear by gas pressure, unlocking the bolt, then the entire unit commences its rearward travel.

    Please excuse the crudeness of this picture but I hope it helps.



    Quote Originally Posted by jmart View Post
    I'm tracking with everything you're saying. But I do have a question, and it's based on my following description/assumptions of the breakdown of the extraction cycle. Maybe I've got it wrong, if so, please correct.

    Primer ignites, bullet releases from case mouth and travels down bore.

    Bullet passes port and charges gas system, sending gas to BCG via gas tube.

    Gas is trapped within a volumed defined by the gas rings, which are part of the bolt. the carrier surrounds this, but is not impacted directly by gas pressure, since it's trapped within the volume bounded at the rear end by the gas rings (which aren't moving, see paragraph below), and unbounded fwd from the key, to the gas tube, to the barrel port, and out to the muzzle -- it's all open going fwd.

    Bolt at this instance can't rotate or move to rear because bolt is engaged against lugs. Even if casehead thrust acts on bolt face, trying to push it to the erar, the bolt won't budge as long as it's locked up against the barrel extension lugs. I can envision once the bolt is unlocked, then the entire BCG, as a unit, can travel rearward, but I can't picture how the carrier moves initially rearward for the straight portion of the cam pin slot you mention. Assuming the bolt remains locked and pressure isn't acting on it in the rearward direction (maybe radially between the area/void forward of the rings and against the interior walls of the carrier, but not rearward), I can't picture how the carrier moves rearward that initial bit before the cam pin rides the angled slot and unlocks the bolt.
    Last edited by Eric D.; 08-19-11 at 21:07.
    B.A.S. Mechanical Engineering Technology

  6. #36
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    Quote Originally Posted by Eric D. View Post
    You've outlined the process correctly. Gas enters the BCG behind the gas rings. Since the bolt's linear motion is restricted until it unlocks, the only thing that can move is the carrier. The carrier is forced to the rear by gas pressure, unlocking the bolt, then the entire unit commences its rearward travel.
    Not trying to be a pest, but I'm still not tracking. Empirically speaking, I know the carrier has to move rearward a bit independently of the bolt, I just can't logically figure out how it happens.

    The only possible source of gas pressure affecting rearward movement that I can postulate is that gas leaks fwd along the bolt body within the carrier, vents into the area bounded by the rear of the lugs of the barrrel extension and the radial face of the carrier at the front (the vertical circular portion), and that moves the carrier rearward that first 1/6th - 1/8th of an inch before the cam pin engages the angled portion of the slot and rotates up to the 12:00 position.

    I can't see how gas trapped within the area bounded by the rings, the carrier's internal walls (against which the gas acts in a radial, outward vector), and the rear of the bolt body (which is essentially fixed) acts with a rearward force vector that allows the carrier to move rearward.

    I'm envisioning that some force has to act on it and push it to the rear. That means a vertical surface. The only three vertical surfaces I can envision are the radial face of the carrier, the bolt wasp section with the rings, and the buffer face. Everything else is radial, or outward.

    We've already agreed that the bolt can't move to the raar because the lugs are butted up against the barrel extension lugs, so it's essentially fixed in place for this phase. That leaves the face of the carrier and the buffer face.

    Even if gas leaked past the rings it would then fill the void between the carrier's walls, the buffer face, and the rear of the wasp/ring section on the bolt. If the buffer face moved rearward and momentarily broke carrier contact, it's not attached, so it's movement would be independent of the carrier's movement.

    That leaves the carrier's face as the only remaining vertical surface against which a force could be applied to trigger initial rearward movement. Unless someone can explain how a force applied radially against the interior walls of the carrier results in a rearward movement vector.

  7. #37
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    Quote Originally Posted by jmart View Post
    Not trying to be a pest, but I'm still not tracking. Empirically speaking, I know the carrier has to move rearward a bit independently of the bolt, I just can't logically figure out how it happens.
    Come on it happens the same way that a piston works in a engine.

    The only possible source of gas pressure affecting rearward movement that I can postulate is that gas leaks fwd along the bolt body within the carrier, vents into the area bounded by the rear of the lugs of the barrrel extension and the radial face of the carrier at the front (the vertical circular portion), and that moves the carrier rearward that first 1/6th - 1/8th of an inch before the cam pin engages the angled portion of the slot and rotates up to the 12:00 position.

    I can't see how gas trapped within the area bounded by the rings, the carrier's internal walls (against which the gas acts in a radial, outward vector), and the rear of the bolt body (which is essentially fixed) acts with a rearward force vector that allows the carrier to move rearward.
    The gas pressure is working in 360 degrees not just on the side of the carrier bore
    I'm envisioning that some force has to act on it and push it to the rear. That means a vertical surface. The only three vertical surfaces I can envision are the radial face of the carrier, the bolt wasp section with the rings, and the buffer face. Everything else is radial, or outward.

    The carrier receives a impulse movement to the rear , the inertia
    of the BCG allows it to continue fully rear ward if every thing is working correctly.

    We've already agreed that the bolt can't move to the raar because the lugs are butted up against the barrel extension lugs, so it's essentially fixed in place for this phase. That leaves the face of the carrier and the buffer face.

    Even if gas leaked past the rings it would then fill the void between the carrier's walls, the buffer face, and the rear of the wasp/ring section on the bolt. If the buffer face moved rearward and momentarily broke carrier contact, it's not attached, so it's movement would be independent of the carrier's movement.
    if the gas leaked pass the rings it would not go there the rings seal on the opposite end leaking gas would go through the carrier vents.
    That leaves the carrier's face as the only remaining vertical surface against which a force could be applied to trigger initial rearward movement. Unless someone can explain how a force applied radially against the interior walls of the carrier results in a rearward movement vector.
    Gas pressure works 360 degrees.
    The carrier pushes of the bolt/gas rings.
    Last edited by Thomas M-4; 08-19-11 at 21:39.

  8. #38
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    We'll get you learned yet

    Maybe this kindergarten scribble will help

    EDIT* This drawing was done in paint and was not meant to be to scale or to accurately represent the geometry of bolt and carrier. It is only meant to show that gas enters the carrier behind the gas rings and how that gas applies rearward force to the carrier.



    The purple arrows show how the gas pressure acts on the interior surfaces of the carrier. There may not be a vertical surface per se but there is a horizontal component to all the arrows acting at an angle. Those components are what generate the rearward force on the inside of the carrier. Make sense?
    Last edited by Eric D.; 08-20-11 at 02:26.
    B.A.S. Mechanical Engineering Technology

  9. #39
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    LOL yea that's it.
    The BCG only requires the initial impulse movement to achieve full rear ward movement [providing everything is working correctly] then the buffer spring takes over after full rear ward travel.

  10. #40
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    High speed video of a AR cycling.
    http://www.youtube.com/watch?v=dUXpESqNv28


    AH I found it !http://www.ar15.com/forums/t_2_130/1..._AR_works.html

    It just took me a couple of times.
    Last edited by Thomas M-4; 08-19-11 at 22:34. Reason: Adding important info

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