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Thread: BCG/Buffer/Spring system - Engineering perspective

  1. #41
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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.
    Yeah, we were talking about a reserve in the initial BCG kinetic energy, which comes from initial BCG mass and velocity. Initial velocities are often nonzero, and I'm estimating 200 in/sec

    Of course some of this kinetic energy goes into the spring as spring potential as the BCG moves rearward (see the equation in the OP), and then gets returned to the BCG as it moves forward.

    I always thought that this ebb and flow of energy was one of the beauties of simple harmonic motion, which this is akin to.

    Your "inertial energy" could only be interpreted as rest energy, and I suuure don't want to get relativistic in this system.

  2. #42
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    Quote Originally Posted by StainlessSlide View Post
    Yeah, we were talking about a reserve in the initial BCG kinetic energy, ...

    No, you distinctly stated stored kinetic energy in post 39. The BCG is not a flywheel.

    The system you are trying to pick apart has too many variables which come into play in real world use. You can postulate on what the ideal dimensions might be so that an AR15 would function perfectly, but translating this into what I previously stated, as real world use, has been beaten to death like that poor horse.

  3. #43
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    Quote Originally Posted by StainlessSlide View Post
    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%.Av P = 50/0.069 = 725 psi
    The action spring isn't supposed to fully compress.
    If it does its due to being the wrong spring like rifle in a carbine extension or wrong buffer like carbine in a rifle extension.

    Part of the purpose behind the buffer specific length and addition of the neoprene bumper at the end is to help ensure that the spring never fully compresses.
    Yes, the bumper softens the impact should the buffer travel too far aft, but it also physically stops the spring from ever reaching a fully compressed state.

    A fully compressed spring is for most intents and purposes a solid object, and getting one to that point creates excessive travel motion as springs are never supposed to deliberately be fully compressed.
    Additionally it creates too much wear and tear on the spring and the objects its contained in due to impact when the spring 'bottoms out' and creates early weakening of the spring due to excessive flexion.

  4. #44
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    I just had a stroke my F'n brain hurts from reading all if that!!!

    Great read though, I learned a lot...I'm still a JR engineer, and if I'm half the engineer some of you are...it will be a bright future for me

  5. #45
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    Quote Originally Posted by GTifosi View Post
    The action spring isn't supposed to fully compress.
    If it does its due to being the wrong spring like rifle in a carbine extension or wrong buffer like carbine in a rifle extension.

    Part of the purpose behind the buffer specific length and addition of the neoprene bumper at the end is to help ensure that the spring never fully compresses.
    Yes, the bumper softens the impact should the buffer travel too far aft, but it also physically stops the spring from ever reaching a fully compressed state.

    A fully compressed spring is for most intents and purposes a solid object, and getting one to that point creates excessive travel motion as springs are never supposed to deliberately be fully compressed.
    Additionally it creates too much wear and tear on the spring and the objects its contained in due to impact when the spring 'bottoms out' and creates early weakening of the spring due to excessive flexion.
    In this context, I was using "fully compressed" to mean the maximum compression possible in this system, which is when the buffer bottoms in the RE.

    You're certainly correct that coil clashing is undesireable.

  6. #46
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    Quote Originally Posted by Gun View Post
    No, you distinctly stated stored kinetic energy in post 39. The BCG is not a flywheel.

    The system you are trying to pick apart has too many variables which come into play in real world use. You can postulate on what the ideal dimensions might be so that an AR15 would function perfectly, but translating this into what I previously stated, as real world use, has been beaten to death like that poor horse.
    Any mass in motion contains kinetic energy, which can be thought of as stored within it. Human intent (as in a flywheel) is not necessary

    But in this case the BCG is intended to store energy from the expanding gases, and give some of it to the spring, so the spring can give it back for the forward cycle.

    If you can present any specific difficulties with my ideas, I'd love to hear them.
    Last edited by StainlessSlide; 06-19-12 at 00:54.

  7. #47
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    i have a question...

    what exactly are you trying to fix with your study or make better?

  8. #48
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    Don't forget to include all the various friction forces and force required to cock the hammer.

    They're not insignificant.

    The hold of the rifle is also important. A loose hold allows the whole weapon to accelerate in recoil, which tends to hold the BCG shut.

    Also, if you get your test system up and running well, you could look at the effect of the A5 buffer system on the consistency of the stroke.
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  9. #49
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    Quote Originally Posted by Gun View Post
    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...
    Yes, I see your point

    Mk18, Rob, I don't consider the information useless. It helps me understand not only the AR, but the world around me better. It also exercises my mind and improves my critical thinking by applying scientific principals which helps me to understand those principles by looking at them from different angles. It's entertaining, expands my understanding and is much better than watching re-runs of Gilligan's Island or wondering how America got fat
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  10. #50
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    Quote Originally Posted by sinlessorrow View Post
    i have a question...

    what exactly are you trying to fix with your study or make better?
    Post #34 may shed some light on this.

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