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Thread: BCG/Buffer/Spring system - Engineering perspective

  1. #11
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    Quote Originally Posted by AR15barrels View Post
    Yes.
    You need to have at least 1/8" of bolt lug face travel beyond the bolt catch for it to reliably pop up and stop the bolt.
    If you have more than 1/4", you run into bolt catch breakage problems where the carrier accelerates too much and the bolt stop can't handle the battering.
    We see this in blowback AR's when you fail to limit the bolt travel with a longer buffer or a spacer behind the buffer.
    My measurements indicate that there isn't 1/4" available after bolt catch release before the BCG is fully back. If I've made some error, I certainly want to know about it.

    ETA: Of course blowback carbines could be a different case, but they don't have bolt lugs.
    Of course the buffer/BCG can bounce if it hits the back of the RE, making for a stronger strike on the bolt catch. I very roughly estimate this bounce as keeping 20% of the energy or 45% of the velocity.
    Last edited by StainlessSlide; 06-17-12 at 14:37.

  2. #12
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    Quote Originally Posted by skullworks View Post
    What's your sample size? Does the extremes show up in any specific order? Have you accounted for temperature changes in the weapon during testing? What about lubrication?
    This was just a sidelight at a stage of development, hard numbers are coming. The sample size was 6 or so in one session, with no pattern that a casual observer (me) could note. Shots were taken more than 10sec apart, as I had to measure the rod displacement with calipers every time. The carbine would not have been cleaned or lubricated for a few hundred rounds, but the buffer was NiB coated.

    You suggest an interesting possibility of seeing whether the lubrication state changes the variation. This (and the other comments) are why I share my thoughts with the hive mind.

  3. #13
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    Quote Originally Posted by StainlessSlide View Post
    I think that increased dwell time will increase BCG pressure...
    Only if it pressurizes the BCG after the bullet uncorks. If it pressurizes the BCG before the bullet uncorks, the pressure will be the same regardless of dwell time
    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. #14
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    I find all of this very interesting. Thank you.

    I'm new to the AR world, but not to shooting. I suspect all of this is do to the fact that the modern AR is much different from the original design. Reminds me of when I was tinkering with my Grand National.
    Very complex design.

    AK's don't got to worry all of this.

  5. #15
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    Same principles apply to the AK as well
    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. #16
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    Quote Originally Posted by MistWolf View Post
    Only if it pressurizes the BCG after the bullet uncorks. If it pressurizes the BCG before the bullet uncorks, the pressure will be the same regardless of dwell time
    You seem to be considering the timing of the pressure wave which moves down the gas tube. And that the parts of the gas system do not all pressurize at once.

    I was neglecting wave speed because I didn't think it made any difference (to the general conclusion). Here's why:

    It's true that the parts of the gas system do not all pressurize at once. But the barrel does not depressurize all at once either. After bullet exit, a rarefaction wave moves down the barrel (supersonically, I imagine, but who knows what the speed of sound is in the hot, dense powder gases). To exhaust the BCG via the gas tube, the rarefaction wave then has to move down the gas tube.

    Even if the pressure wave does not reach the BCG before bullet exit, it still reaches it before the rarefaction wave does. The interval between the arrival of the pressure and rarefaction waves in the BCG chamber will be the time that the pressure acts. A longer dwell time leads to more interval between these waves. If the interval is longer, there will be more change in the volume of the BCG chamber, and thus more work done on the BCG. Remember that the work done is the integral of P dV.

    I am assuming that the BCG does not vent before the rarefaction wave arrives. Maybe this is what you mean by the BCG moving before the bullet uncorks. If we could tell if the BCG was venting 700-odd psi gas, we would know, because it would mean that the rarefaction wave had not arrived at venting time.


    My argument above is simplified, because a pressure wave which strikes a solid surface is reflected as another pressure wave, which leaves even higher pressure behind it. This means that the pressure in the BCG continues to build even after the pressure wave arrives.

    A person with a high-speed camera who knew his mech. eng. and fluid mechanics could shed some light on this. Or maybe just people with lots of experience with the cyclic rates of different barrel lengths with the same gas system and gas port (and buffer and spring).
    Last edited by StainlessSlide; 06-17-12 at 18:04.

  7. #17
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    Quote Originally Posted by StainlessSlide View Post
    My measurements indicate that there isn't 1/4" available after bolt catch release before the BCG is fully back.
    That's a good thing.
    Blowback bolts have 5/8" more travel than DI bolt carriers due to the lack of unlocking travel and lugs.
    The problem with breaking bolt catches is purely a blowback issue that stems from excessive bolt travel.
    Randall Rausch
    AR15 Barrel Guru
    California Precision Rifle Club founding member

  8. #18
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    Quote Originally Posted by MistWolf View Post
    Only if it pressurizes the BCG after the bullet uncorks. If it pressurizes the BCG before the bullet uncorks, the pressure will be the same regardless of dwell time
    Energy transfer is a function of time and pressure.
    So with equal pressure, more energy transfers when there is more time.
    Randall Rausch
    AR15 Barrel Guru
    California Precision Rifle Club founding member

  9. #19
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    Quote Originally Posted by StainlessSlide View Post
    This was just a sidelight at a stage of development, hard numbers are coming. The sample size was 6 or so in one session, with no pattern that a casual observer (me) could note. Shots were taken more than 10sec apart, as I had to measure the rod displacement with calipers every time.
    Can you further explain your measurement process?
    Pictures may help.
    Randall Rausch
    AR15 Barrel Guru
    California Precision Rifle Club founding member

  10. #20
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    Quote Originally Posted by AR15barrels View Post
    Energy transfer is a function of time and pressure.
    So with equal pressure, more energy transfers when there is more time.
    That's what I said.

    The AR is a self regulating system. When it gets enough pressure to operate, the movement of the BCG separates the gas key from the gas tube and the rest of the gases are vented overboard.

    Therefore-

    As it would be a closed gas system until the bullet uncorks the barrel, if the gases were to begin moving the BCG at this time, pressure and time would be the same whether the barrel was 14.5" or 16".

    But with an open system, that is, the BCG does not pressurize until after the bullet uncorks, the extra distance does affect how much energy the gas carries into the BCG as the longer bore increases the amount of time it takes for the gases to vent enough to be a significant event.

    If you study slow motion videos of a firearm discharging, you can see that the gases vent rather forcefully for some time after the bullet uncorks the muzzle. That's the energy that's operating your rifle.

    I saw a writeup where a guy cut the barrel of his rifle length AR right at the front of the FSB. The rifle functions fine although there is virtually no dwell time.

    SS, yes the system does not pressurize all at once. I'm not sure if reflecting pressure waves play much of a role in all of this like they do in automotive internal combustion engines
    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

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