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Thread: Chamber pressure during extraction

  1. #11
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    Quote Originally Posted by MK18Pilot View Post
    Did you read the PPT that curious1 linked to? Read it again and lets see if you reach the same conclusion.
    From slide 17, "Case extraction and ejection successfully occur as long as the case is held against the bolt face by the residual chamber pressure while the extractor lifts and returns to position."

    It is the same residual chamber pressure that I concluded accelerates the bolt and bolt carrier rearward which I observed in the high speed video I watched. I see their conclusions and mine as complimentary not contradictory.

    Not that I claim any great insight on the matter. It is as simple as watching the high speed video and observing the acceleration. The cylinder / piston mechanism of the bolt and bolt carrier cannot cause acceleration at this point as they are fully extended and can not have relative motion.

  2. #12
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    Quote Originally Posted by Suwannee Tim View Post
    From slide 17, "Case extraction and ejection successfully occur as long as the case is held against the bolt face by the residual chamber pressure while the extractor lifts and returns to position."

    It is the same residual chamber pressure that I concluded accelerates the bolt and bolt carrier rearward which I observed in the high speed video I watched. I see their conclusions and mine as complimentary not contradictory.

    Not that I claim any great insight on the matter. It is as simple as watching the high speed video and observing the acceleration. The cylinder / piston mechanism of the bolt and bolt carrier cannot cause acceleration at this point as they are fully extended and can not have relative motion.


    I am fairly certain the acceleration you are observing is because the BCG is now unlocked from the barrel extension.

  3. #13
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    Quote Originally Posted by Thomas M-4 View Post
    I am fairly certain the acceleration you are observing is because the BCG is now unlocked from the barrel extension.
    F=MA. What is the F if not thrust from the internally pressurized cartridge case?

  4. #14
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    Once the case seal is broken the bore pressure should drop immediately. You now have a bore with both ends open.
    The slow motion videos I have seen to me look like the acceleration corresponds to the point in the BCG travel when the bolt completes the unlocking part.
    But that is just my opinion.

  5. #15
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    Quote Originally Posted by tirod View Post
    I once posted that residual chamber pressure helped power the bolt cycle, and got the impression from posters I could do more to help understand the action cycle if I would not bother. They had all the answers.

    While reading a recent edition of the "Book of the AR15," I ran across a reprint of the Guns and Ammo article from November 68 titled "Is the 'new' M16 debugged?"

    The author, James Mason, takes the time to describe the action cycle, and I quote "The unlocking action happens so quickly that there is still considerable remaining or residual pressure (10,000 to 12,000 psi) in the breech following the passage of the bullet from the bore. This transient residual pressure helps operate the action by forcing the empty case out of the chamber and assisting the rearward movement of the breechblock and its carrier. (The dependence on residual pressure allows for the reliable use of lighter weight operation parts.)" end of quote.
    This article also goes into specific detail about how the action can malfunction and what parts were changed - in the months preceding November 68, the date of it's printing.

    I have a concept that the early writers had direct access to Colt engineers and Army technicians, whereas today, that same data is considered proprietary. There's a lot of stuff in that article that relates directly to the same problems carbine actions have.
    I would think that pressure exists as long as the dwell duration exists. i.e., as long as the gas system is charged. Hypothetically speaking, cut the muzzle right after the port, and you don't have enough dwell to generate/harness adequate gas pressure to initiate unlocking and rearward movement of the carrier, not at least without some type of gas trap device.

    Unlocking is initiated once the bullet passes the port but before it exits the muzzle. Given the ports location and the amount of dwell, enough gas volume is delivered to the carrier that unlocking and carrier rearward movement occurs. By the time the bullet exits the muzzle, pressure drops to zero, this can be seen on various graphs for barrels with pressure gauges installed. But the unlock has completed and enough rearward velocity has been initiated to ensure there's adequate momentum to complete the cycle. At least that's my semi-educated guess.

    All of this happens in a few tenths of a millisecond from what I've seen on graphs, and it take 5-6th tenths of a millisecond to reach the port and IIRC, the bullet exits the muzzle at ~ the 1 millisecond point in a 20" barrel, so it happens quicker in a carbine-length barrel. But when the bullet exits the muzzle, the gauge registers zero, it just falls off from the pressure measured at the muzzle.

    Last edited by jmart; 08-09-11 at 23:53.

  6. #16
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    is there anyway to view that powerpoint on an Apple iMac??? Like some type of download or plugin.....

  7. #17
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    Quote Originally Posted by Suwannee Tim View Post
    From slide 17, "Case extraction and ejection successfully occur as long as the case is held against the bolt face by the residual chamber pressure while the extractor lifts and returns to position."

    It is the same residual chamber pressure that I concluded accelerates the bolt and bolt carrier rearward which I observed in the high speed video I watched. I see their conclusions and mine as complimentary not contradictory.

    Not that I claim any great insight on the matter. It is as simple as watching the high speed video and observing the acceleration. The cylinder / piston mechanism of the bolt and bolt carrier cannot cause acceleration at this point as they are fully extended and can not have relative motion.
    There is NO residual chamber pressure to accelerate the bolt group. The bottom carbine received only a gas charge through the gas tube from the top carbine yet completed the cycle of operation except that it failed to extract.

  8. #18
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    Quote Originally Posted by jmart View Post
    ...Unlocking is initiated once the bullet passes the port but before it exits the muzzle...
    This is incorrect. The gases do not instantly fill the expansion chamber of the BCG. They must travel the length of the tube and must fill the BCH with enough pressure to overcome it's inertia. The gas port also acts as a restrictor which increases the time needed to bring the gas tube and expansion chamber up to pressure. In the meantime, the bullet continues to accelerate down the bore.

    The Army did studies on the M1 Garand and the M14 back in the early 60s. They found that the bullet was several feet from the muzzle before the piston & op-rod began moving. It would be no different with the AR. The BCG would not begin motion until the bullet has cleared the muzzle.

    Also, the pressure inside the bore does not instantly drop to atmospheric when the bullet clears the muzzle. It takes time for the volume of the gases to rush out. This can be seen when studying high speed videos. The gases can be seen steadily jetting from the muzzle some time after the bullet has cleared the muzzle. Check out R. Lee Ermy's Lock & Load series. It uses plenty of high speed film to show the exiting of projectiles from various weapons in slow motion. It's available for Instant Viewing on Netflix
    Last edited by MistWolf; 08-10-11 at 11:04.
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  9. #19
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    Quote Originally Posted by MistWolf View Post
    This is incorrect. The gases do not instantly fill the expansion chamber of the BCG. They must travel the length of the tube and must fill the BCH with enough pressure to overcome it's inertia. The gas port also acts as a restrictor which increases the time needed to bring the gas tube and expansion chamber up to pressure. In the meantime, the bullet continues to accelerate down the bore.
    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.

    The Army did studies on the M1 Garand and the M14 back in the early 60s. They found that the bullet was several feet from the muzzle before the piston & op-rod began moving. It would be no different with the AR. The BCG would not begin motion until the bullet has cleared the muzzle.
    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.

    Also, the pressure inside the bore does not instantly drop to atmospheric when the bullet clears the muzzle. It takes time for the volume of the gases to rush out. This can be seen when studying high speed videos. The gases can be seen steadily jetting from the muzzle some time after the bullet has cleared the muzzle. Check out R. Lee Ermy's Lock & Load series. It uses plenty of high speed film to show the exiting of projectiles from various weapons in slow motion. It's available for Instant Viewing on Netflix
    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.
    Last edited by jmart; 08-10-11 at 22:24.

  10. #20
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    The bullet is waaaaayyyy out of the muzzle before the bolt completes unlocking. The cam path has a straight section to allow residual pressure to drop before unlocking is commenced.

    I think even in a carbine, the bolt carrier velocity does not go more than a bit over 22fps. It still takes many milliseconds for the bolt to completely unlock, the bullet is many, many dozen of feet away before this occurs.
    My brother saw Deliverance and bought a Bow. I saw Deliverance and bought an AR-15.

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