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Thread: BCG/Buffer/Spring system - Engineering perspective

  1. #31
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    Quote Originally Posted by StainlessSlide View Post
    You're right that attempts to analyse a system more quantitatively often overlook important aspects, even when done by the best engineers. Thus the post to a forum full of heavy AR users, which has borne fruit already.
    The world in general, and the shooting industry in particular, is clogged with engineers who don't understand shit about the needs of the end users, and who try to redesign things they don't understand to begin with. Polling the heavy users is not the same thing as being one.

    and just because you've found a group of fellow eggheads on what you consider a site of heavy users (it's not, BTW) doesn't mean those eggheads are heavy users. You can also find straight women in a gay bar.

    What I'm getting at is, what's the point of what you're trying to do? I can go grab a tape measure and run out to the sidewalk and start measuring the distance between expansion joints from here to the next county, but if I don't have any use for the data then what's the point? What do you think is wrong with the current system and how are you gong to improve it with your data?

  2. #32
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    Quote Originally Posted by Gun View Post

    StainlessSlide, curious to how you approximated some of the values, and the math for integrating pdv.
    A guy on ARFCom said that he measured a typical carbine initial BCG velocity at 180 in/sec, using high speed video. I measured Hooke's constant for the recoil spring using a food scale. These numbers yielded not quite enough energy to fully compress the spring (equation given in OP), so I used 200 in/sec as BCG vel. I guessed that this provided enough energy margin for friction and hammer cocking, about 20%.

    I didn't integrate P dV because I didn't know the function P. Instead I used the (approximate ) fact that the work done on the BCG = integral of P dV = initial kinetic energy of BCG = 50 in. lbs (estimated from its mass and the 200 in/sec).

    The average value of a function P = integral of P between a and b/(b-a). (b-a) is the finite change in volume as that the BCG chamber sees, which a few measurements and some geometry showed to be 0.069 in^3. As said above the integral is estimated at 50. So

    Av P = 50/0.069 = 725 psi

    All of these numbers are rough, just to provide a framework for ideas. The BCG retraction vs. buffer weight and vs. gas port size data to come should be good to a few percent or so.

    Eventually I'll have a high speed camera, and *put a scale* on the receiver in front of the BCG (which nobody who posts high speed video does), and *focus the camera*, and then we'll know something about BCG velocity.
    Last edited by StainlessSlide; 06-18-12 at 14:26.

  3. #33
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    Quote Originally Posted by rob_s View Post
    The world in general, and the shooting industry in particular, is clogged with engineers who don't understand shit about the needs of the end users, and who try to redesign things they don't understand to begin with. Polling the heavy users is not the same thing as being one.
    Truer words were never posted. And yet some engineers manage to come up with nice things that you use every day.

    Quote Originally Posted by rob_s View Post

    and just because you've found a group of fellow eggheads on what you consider a site of heavy users (it's not, BTW) doesn't mean those eggheads are heavy users. You can also find straight women in a gay bar.

    What I'm getting at is, what's the point of what you're trying to do? I can go grab a tape measure and run out to the sidewalk and start measuring the distance between expansion joints from here to the next county, but if I don't have any use for the data then what's the point? What do you think is wrong with the current system and how are you gong to improve it with your data?
    A better understanding of the system allows one to go many ways. One of them is:

    The system sometimes short strokes under conditions of fouling, lack of lubrication, and foreign debris. This may be helped by increasing BCG energy (say with a larger gas port). This might be done *without* increasing initial BCG velocity (and thus making extraction harder)if a heavier buffer is used.

    My preliminary calculations (my next long post) suggest that an increase in gas port radius of 8%, accompanied by an increase of BCG/buffer mass of 33% (say by changing from a stock buffer to an XH), would increase BCG energy by 7% while *lowering* BCG initial velocity by 5%.

    Measuring BCG retraction is a way of determining the validity of these numbers.

    If we can estimate a typical energy reserve, we can see how much proportional change this represents. If it's not worth the trade offs, obviously it's not a good idea. We can't know until we measure and test.

    I haven't heard "eggheads" for a while, it's very Cold War - takes me back. Is it part of your vocabulary for civil conversation?
    Last edited by StainlessSlide; 06-18-12 at 15:25.

  4. #34
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    Quote Originally Posted by rob_s View Post
    The world in general, and the shooting industry in particular, is clogged with engineers who don't understand shit about the needs of the end users, and who try to redesign things they don't understand to begin with. Polling the heavy users is not the same thing as being one.

    and just because you've found a group of fellow eggheads on what you consider a site of heavy users (it's not, BTW) doesn't mean those eggheads are heavy users. You can also find straight women in a gay bar.

    What I'm getting at is, what's the point of what you're trying to do? I can go grab a tape measure and run out to the sidewalk and start measuring the distance between expansion joints from here to the next county, but if I don't have any use for the data then what's the point? What do you think is wrong with the current system and how are you gong to improve it with your data?
    Beautiful...

    Information you can't do anything with is USELESS.

  5. #35
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    Quote Originally Posted by MistWolf View Post
    Ok, one more time- If the AR used a closed system (bullet still in barrel when BCG gets pressurized and begins moving) the dwell time of an extra 1.5 inches will make no practical difference. Pressure will be the same, when the system dumps excess pressure overboard will be the same, energy to the BCG will be the same.

    The only way the difference of 1.5 inches of bore could change the energy of the gases that pressurizes the BCG is if the BCG pressurizes after the bullet exits the barrel completely (uncorks the muzzle)
    Sorry dude, but I never mentioned when the BCG moved in either system described. Only explained where the extra energy came from, and your reference to venting, from the post I quoted, is ambiguous, but I took it to mean venting in the BCG, not out the muzzle.


    SS, still would like to see those calculations. (didn't see your response until now, but not sure what you stated is going to help in your effort in making the wheel rounder)
    Last edited by Gun; 06-18-12 at 17:18.

  6. #36
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    Quote Originally Posted by StainlessSlide View Post


    Which is why I'm trying to understand the BCG energy reserve -


    There is no energy reserve in the BCG. The BCG does not store energy. The BCG gets bounced around by external forces.

  7. #37
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    Quote Originally Posted by MK18Pilot View Post
    Beautiful...

    Information you can't do anything with is USELESS.
    Sure, information that no one can ever use is useless.

    But some whack job mathematician messes with Hilbert spaces, and ten years later some marginally saner physicist does some quantum mechanics which uses his results, and a century later (filled with engineering labor, the vast majority of it up blind alleys) you have a quantum computer.

    Does that seem useless to you?

    Good technical people have a strong drive to better understand systems, to have a mental model for the interactions which can't be seen except by careful, (somewhat) quantitative analysis.

    And it's a leeeetle hard to tell in advance which kinds of understanding (for example, a more or less quantitative understanding of the mechanism by which buffer weights affect BCG retraction) can help make small, evolutionary improvements in one aspect of one small mechanical system.

    But I just do it for fun

    Does your screen name mean that you're a pilot? If so, what would you rely on if an emergency came up that nobody had ever heard of and that wasn't in the book? I'm sure it would be your internal mental model of the system, and the more complete the better.



    But I digress...
    Last edited by StainlessSlide; 06-19-12 at 01:36.

  8. #38
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    Quote Originally Posted by Gun View Post
    There is no energy reserve in the BCG. The BCG does not store energy. The BCG gets bounced around by external forces.
    It stores kinetic energy, an excess of which can come in handy as a reserve against marginal conditions.

  9. #39
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    Quote Originally Posted by StainlessSlide View Post
    It stores kinetic energy, an excess of which can come in handy as a reserve against marginal conditions.
    Wow. It has inertial energy, because it is at rest in the upper receiver. You are thinking potential energy, which is where the spring comes to play, not the BCG. Good luck.

  10. #40
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    Quote Originally Posted by Gun View Post
    Wow. It has inertial energy, because it is at rest in the upper receiver. You are thinking potential energy, which is where the spring comes to play, not the BCG. Good luck.
    An object in motion possesses kinetic energy, if I remember my highschool physics properly.

    I'm pretty sure SS is talking about when the BCG is in motion.
    Last edited by BufordTJustice; 06-18-12 at 18:10. Reason: Add info

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