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Thread: SureFire Bolt Carrier Group Analysis

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    SureFire Bolt Carrier Group Analysis

    Well, I finally got some more information on this design, and I am less impressed.

    First off, I thought the cam path was lengthened and the cam angle swallowed out. This is not the case, the cam path is merely widened which introduces potential problems. Below is a diagram of the SureFire cam path superimposed on the MIL-Spec cam path:



    You can see all that has been done is move the unlocking cam forward 0.062 inch. This means the cam pin gets to accelerate another 1/32 inch before slamming into the cam path. Further, the cam angle is the same, so the rotational loading on the cam pin is the same.

    Then there are other problems.



    Because the front point of the cam path was moved forward, it pulled the bolt rings forward in the bolt carrier. The rings get pulled far enough forward they exit the 0.500 bore and fall in the chamfer to the 0.530 bore. This will increase wear on the bolt rings and increase the likelihood of twisted bolt rings.

    So, what the reason for the longer cam path to start with? According to the literature it was to increase the length of time from the start of bolt carrier motion to the unlocking of the bolt to ease extraction. So, how does it do in this regard?

    So, by taking a high speed camera with a frame rate of 10,000 frame per second (fps), you can film the bolt carrier motion through the ejection port window. By measuring the movement of a mark on the carrier with a frame-by-frame examination of the film you can figure out the time it takes for the bolt carrier to move backwards the 0.325 inch for a MIL-Spec carrier and compare it to the time it takes to move the 0.387 inch for the SureFire carrier. So, 18 rounds were fired in a freshly cleaned AR with a MIL-Spec carrier and filmed. Then the weapon then cleaned again and the bolt carrier group replaced with a SureFire BCG and the test repeated.

    The average time to unlock for the MIL-Spec BCG was 0.01081 seconds.

    The average time to unlock for the SureFire BCG was 0.01066 seconds.

    With an average difference of 0.000144 seconds (0.14 milliseconds, the observed difference was 1 to 2 frames).

    Iím having trouble correlating this 0.014 milliseconds to a pressure reduction, as Iím not 100% sure the pressure-time curves I have access to, have enough resolution for this, nor due I have a good enough ignition zero-time to sync the pressure curve to the unlocking.

    CONCLUSIONS:

    The widened cam path may delay the unlock until the pressure has been reduced by 15%, but I canít tell, it might be more, it might be less. In any case the unlocking is only delayed less than 1.5%. The most worrying to me is the fact that the gas rings start to leave the 0.500 inch bore. Unfortunately, the length of the 0.500 inch bore cannot be increased due to interference with the bolt when the bolt is fully retracted.

    Then there is the increased carrier velocity and impact load on the cam pin due to the longer dwell track. The cam path should have been designed as below, this would have reduced the cam pin loading.



    It would have delayed unlocking the same as their design, but also would have reduce the energy loss due to bolt rotation. (And, would simplify manuafture.)

  2. #2
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    Quote Originally Posted by lysander View Post

    The average time to unlock for the MIL-Spec BCG was 0.01081 seconds.

    The average time to unlock for the SureFire BCG was 0.01066 seconds.
    Am I reading this right? The regular carrier unlocked later?

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    Quote Originally Posted by lysander View Post
    The cam path should have been designed as below, this would have reduced the cam pin loading.



    It would have delayed unlocking the same as their design, but also would have reduce the energy loss due to bolt rotation. (And, would simplify manuafture.)
    Wouldnít an increasing radius reduce cam pin load more than a reduced angle?

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    Quote Originally Posted by 1168 View Post
    Am I reading this right? The regular carrier unlocked later?
    You were reading it right, but the numbers were reversed.

    Corrected original post.

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    Quote Originally Posted by Todd.K View Post
    Wouldn’t an increasing radius reduce cam pin load more than a reduced angle?
    What radius? The one at the top of the cam path?



    That radius has been increase slightly. If that radius is increased any more, then the cam angle must be reduced. As shown the cam pin load remains more, or less, the same but the bolt is rotated over a longer period of time, so the bolt carrier retains more velocity. This will aid extraction.

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    Not surprised.

    Thanks for the informative post!

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    Outstanding post, Lysander. Fascinating. This needs to be pinned, IMHO.

    Any appetite for the same treatment on an LMT Enhanced Carrier?


    Sent from my iPad using Tapatalk
    "That thing looks about as enjoyable as a bowl of exploding dicks." - Magic_Salad0892

    "The body cannot go where the mind has not already been."

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    Is this the carrier that Jim Sullivan designed for his Arms West constant recoil upper or something? The thing is basically vaporware so I can't say Im disappointed.
    Last edited by vicious_cb; 01-21-23 at 17:55.
    Forward Ascertainment Group

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    Quote Originally Posted by lysander View Post
    That radius has been increase slightly. If that radius is increased any more, then the cam angle must be reduced. As shown the cam pin load remains more, or less, the same but the bolt is rotated over a longer period of time, so the bolt carrier retains more velocity. This will aid extraction.
    Yes. Would a larger radius there reduce cam pin load even if the final angle is not reduced? Because pressure/bolt thrust is higher at the beginning of the cam pin path and lowers towards the end. Or am I overestimating the pressure drop in that time?

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    Quote Originally Posted by Todd.K View Post
    Yes. Would a larger radius there reduce cam pin load even if the final angle is not reduced? Because pressure/bolt thrust is higher at the beginning of the cam pin path and lowers towards the end. Or am I overestimating the pressure drop in that time?
    Increasing the radius will decrease the pin load slightly. Look at the image below That is a force diagram for the pin loading. "B" is the force required to rotate the bolt, and will be a constant due to friction and inertia of the bolt. "A" is the force only due to friction between the cam pin and the unlocking surface*. Since the force from the locking surface is always perpendicular to the locking surface as the radius increases (or the cam angle decreases) the "A" will decrease.



    Now, instead of just that little short radius, you have the entire cam length to work with. You will save a little bit by increasing the initial radius, but you will save more by leaving the radius alone, but shallowing out the entire cam path. Look below and assume the red circle cannot move in any direction but up and down on the page, which cam path will use the least energy moving the red circle down?.



    ________________
    * While the gas pressure is generating lots of force, the only way it can get to the pin is through the contact area between the unlocking surface and the pin. Since the bolt is free to rotate, the only "downward" force is that overcoming the bolt resisting rotation, and the only "rearward" force is the sliding friction between the pin and unlocking surface. The major loads on the cam pin are the loads generated when it stops after unlocking.
    Last edited by lysander; 01-21-23 at 22:42.

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