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Thread: Chamber pressure during extraction

  1. #21
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    I took some videos shooting an M4 and a Sig 556 and if you look AR it frame by frame, you will catch a frame about 20% of the time that shows the gas ejecting out of the vent holes (leakage?) in the bolt and muzzle but the carrier hasn't even budged yet.

    Makes me think the bullets out of the barrel before anything happens but the high pressure charge from when the bullet was still in the barrel (past the port) still has enough force to cycle the weapon.

    Also, I thought that I read in this thread that pressure at the GP is 15k and pressure at the key is about 1k. If the whole system was charged to it's max before the bullet left the barrel wouldn't the pressure at the key be about the same as the port???

  2. #22
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    That would be dependant on port size I'd imagine.
    Just because the whole system is charged to max doesn't mean it's actually in balance throughout.

    Even the bleed off happens at a different rate between barrel and tube due the differences in diameter holes it has to escape through.
    Barrel leaks out one big one fast, gas tube leaks out a few small ones more slowly because the smaller diameter restricts rapid bleed down.
    This would apply even if the cumulative of the gas tube leaks were = to the size the barrel represents.

    Take an old air hose on a tank, pressurize it then drill three or four 1/8" holes in it about a foot back from the end.
    While that is leaking, cut the end of the hose right off with a really sharp knife or axe so that it opens up pretty much all at once.
    Everything in there was under equal pressure to start, but pressure drop still went out the easiest route.
    Last edited by GTifosi; 08-11-11 at 09:40.

  3. #23
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    Quote Originally Posted by jmart View Post
    I never said or meant to imply the gasses immediately filled the tube/chamber in the BCG, I was simply stating that carrier movement, and subsequent unlocking, is initiated when gas is routed back to the BCG. Initial rearward movement occurs prior to the bullet exiting the muzzle. I suspect unlocking is completed prior to this event as well, but that's just a guess, but I'm making an educated guess that unlocking certainly is initiated prior to muzzle exit. Continued rearward movement is a product of the initial force to get it moving -- there's enough force and momentum to allow the BCG to continue its rearward movement, compressing the spring, and hitting the end of the extension before rebounding and returning to battery.

    Agree that port diamter acts as a restrictor or valve to control the rate of gas entering the gas tube. The amount of dwell then controls the duration that the system gets charged. It's an engineering feat to get the right volume of gas back to the carrier to get it to cycle with the optimum velocity. Volume is a function of port diametr and dwell.



    I don't see why it wouldn't. Once the system is charged sufficiently to overcome the resting state, the BCG will begin to move rearward. Bolt unlocking won't be initiated until the carrier moves rearward to the point the cam pin engages/rides the slot and begins unlocking as the BCG continues rearward. Once unlocked, momentum continues the cycle. But I'm not understanding why initial motion would be delayed until after the bullet exits the muzzle.



    I can imagine that it takes some amount of time, but given the bullet's transit time down the bore of a carbine is about ~ 0.75 of a millisecond, as modeled above, I can only imagine that remaining time for gas exiting is a fraction of that. Gas velocity is quite high compared to bullet velocity. And as the gas exits, the residual pressure drops like a rock. Maybe not instantaneously when talking about ultra slow motion measuring devices, but much quicker than it takes for the BCG to extend fully rearward I would think.

    Do you have any data which shows how a carbine cycling at 650-750 rpm, how much time it takes for the BCG to complete it's rearward movement? At 750 rpm, a shot occurs every 0.080 seconds if my math is correct. That's 80 milliseconds. Subtract 5-6 milliseconds for hammer fall and that leaves ~75 milliseconds inbetween shots. Now you have that entire time for the BCG to complete its back and forth cycle compared to the ~0.75 of a millisecond it takes for the bullet to travel the length of a carbine barrel. Assuming carrier travel consumes the largest portion of the time budget, the bullet is long gone, pressure has long ago fallen to zero, and remaining rearward movement is a function of initial force applied while the gas system was charged (that time between passing the port and exiting the muzzle) and momentum. That's the way I envision it anyway.

    ETA: K.L. Davis' stickied thread "After You Pull the Trigger" states that unlocking occurs at the 375 microsecond point in a carbine. That's .375 of a millisecond. I'm not sure what event that's referenced from (primer ignition, bullet release from the neck or bullet passing the gas port), but if the latter, then unlocking would begin after the bullet exits based on the above graph. If one of the former, then unlocking is initated by the casehead pushing against the BCG, before the system is pressurized -- essentially it would be blow back induced at first, followed by a gas assist once the bullet passes the port. I still don't see what gas is available within the gas system after the bullet exits the muzzle, maybe it's due to inertia, and it takes several microseconds to get everything moving and for pressure to evacuate the muzzle.
    The distance from gas port to piston is very short in the M1/M14 gas system compared to the distance from gas port to piston of the AR. It will take the pressure wave longer to reach the bolt carrier. If the bullet is several feet from the muzzle of the M1/M14 before the piston begins moving, with the greater distance the gas needs to travel in the AR, this would be true as well.

    The distance from the AR gas port to muzzle is less than the distance from gas port to piston. The volume needed to filled before operating pressure is reached is greater with the AR than the M1/M14. Of course, the operating pressure of the AR could be lower as well and the 5.56 generates less volume of gas than the 7.62x51.

    While it takes .75 milliseconds to travel the length of a carbine barrel it takes even less time to travel from the gas port to the muzzle. Remember: The gases cannot enter the gas tube until the bullet passes the gas port. By the time the carrier begins moving, the bullet has cleared the muzzle.

    After the bullet clears the gas port, the gases pour through the port and rushes down the gas tube. (For mathematical purposes, I have seen the velocity of the expanding gases listed as a constant. If I recall correctly, it was 4750 fps.) The gas port restricts the flow of the gases. That means it restricts how many cubic feet per minute can flow into the gas tube. This increases how much time is required to bring the gas tube and expansion chamber inside the carrier up to operating pressure. Once it does reach operating pressure, it needs time to begin accelerating the carrier.

    The pressure inside the bore drops as the gases exit the muzzle, rapidly compared to human comprehension but slowly compared to the time needed to operate the system. The bore, like the gas port acts as a restrictor. It takes time for the gases to evacuate and for the system to drop to atmospheric. It's during this time the pressure inside the carrier reaches operating levels and accelerates it rearward. Enough momentum is imparted to the BCG to unlock the bolt, extract the spent case and compress the action spring.

    The blowback system uses recoil energy and gas pressure in it's operation. With some, the bolt begins moving before the bullet clears the muzzle. Spring pressure and bolt mass is used to slow down bolt movement so it doesn't open completely before pressures have dropped to safe levels. When the bolt begins moving before the bullet clears the muzzle, cases bulge at their base from the pressure. This limits the use of low pressure cartridges in the straight blowback system. The use of high pressure rounds would cause case ruptures. The design of the CETME/HK solves this problem by using roller locks to delay any bolt movement until the bullet clears the muzzle and pressures have dropped to safe levels, allowing the use of high pressure cartridges.

    That's why it's necessary that the carrier does not begin moving until the bullet clears the muzzle. If it began moving before, it will begin extraction too soon, placing greater stress on the extractor because the case is still expanded and sticking to the chamber wall. It will also bulge the case if extraction is begun before pressures drop and ruptures could result. There is a delay between the time the carrier begins moving and the bolt begins unlocking, but it's of a short duration.

    Part of the problem with this discussion is we do not have time stamps for each event. If we did, the sequence could be easily understood
    Last edited by MistWolf; 08-11-11 at 12:09.
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  4. #24
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    Evidence suggests gas pressure in the barrel hasn't dropped to zero when the case relaxes, as gas residue marks the side of the case as it extracts, and even builds up deposits on the bolt lugs. That gas residue isn't coming from the carrier, gas key, or gas tube, as it's positioned to the top and behind the lugs. That leaves the chamber.

    As described, gas pressure in the barrel isn't dropping to zero instantaneously after the bullet exits - it's in a long column with a bore sized aperature, and exerts pressure equally on the cylindrical wall and chamber. At whatever pressure exists at the most points in the barrel, it will exist at the throat, and gradually decrease as the cross section of the chamber - and case - change during extraction. It will also bear pressure inside the case - usually below the plastic limit of brass - which is a figure that would be nice to know. That pressure will also bear against the inside of the case head, and that will push the case against the bolt face. It's no different than the bullet that likely already exited the barrel, it's just on the other end and it's been delayed.

    Basically, a high speed piston at one end, and a low speed one that is apparently pushing against the bolt face on the other, and all it has to do is push a fraction of a foot per second faster than the carrier is moving to be extracted. That means it's ejected from the barrel with the force to move it 22fps plus whatever it takes to keep it against the face of the bolt. Initial pressure when the lugs completely unlock would be likely much higher - but the math could still be done. Since we know the case weighs about 95gr, and it's accelerating up to a speed of 22 fps, then the initial gas charge at a minimum could be calculated.

    The graph pictured has the same annoying feature most others I've found share - a specific lack of information about what happening in the operating cycle. It's all about the bullet.

    And whether we have a need to know.
    Last edited by tirod; 08-15-11 at 23:58.

  5. #25
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    The residual pressure may be enough to help push the case, but I doubt the gases are enough to keep the case pressed against the face of the bolt without the extractor. If I recall, the rifle will not function reliably without an extractor
    Last edited by MistWolf; 08-15-11 at 23:57.
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  6. #26
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    That is because the extractor is the pivot point, the fulcrum.
    My brother saw Deliverance and bought a Bow. I saw Deliverance and bought an AR-15.

  7. #27
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    I'm not trackin' you
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  8. #28
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    It gives the ejector somethig to work against.
    My brother saw Deliverance and bought a Bow. I saw Deliverance and bought an AR-15.

  9. #29
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    Ah. Still, what does that have to do with the fact the rifle won't extract the spent case without an extractor?
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  10. #30
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    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.

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