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Thread: 300 Blackout & Vltor's A5 system?

  1. #31
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    300 Blackout & Vltor's A5 system?

    You might also ask over at http://www.300blktalk.com/forum/.

    -john

  2. #32
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    I was searching over there, but no luck so far. I may have to post up.

  3. #33
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    Quote Originally Posted by PortDawg View Post
    Brand new 300 blk I put together ( I had a spare lower sitting around, next thing I know, it turned into a rifle! ).
    A5, standard buffer
    BCM upper receiver
    BCM bcg
    16" AR stoner SS barrel
    Vltor SS lowpro gas block
    SS Battlecomp
    Geissele SSA
    Old doublestar stripped lower I had collecting dust
    Troy Alpha 15"
    Magpul FDE Ceracote
    It would eject beautiful, but would not lock back on last round. I swapped in another spare lower I have with a standard carbine buffer setup to see how that worked. Same issue. No bolt lockback.
    I had 2 spare standard A5 buffers ( I use the A5 on my 556 & 6.8), so I took them apart and swapped 1 steel weight with 1 tungsten weight. This gave me a lighter than standard A5 buffer.
    It works great! Bolt locks back everytime, cycles beautifully, ejects the brass into a neat little pile.
    I now see Vltor makes a lower weight A5 buffer, I do not know what the weight difference is between it and what I did with parts on hand. Hell, probably the same as what I did.
    This is what worked for me and my setup.
    Quote Originally Posted by Corse View Post
    I'm trying to figure out how a heavier buffer would get the bolt to lock back.
    To recap here
    3.0 oz CAR buffer short stroked.
    4.56 oz A5H1 buffer worked beautifully.
    5.33 oz A5 buffer short stroked.

    16" CAR gassed 300 BlK

    It appears there is more to cycling than just buffer weight.

    This is not unexpected.

    There are several competing forces at work during cycling.

    1)
    The gas system delivers an overall impulse or total momentum to the BCG, regardless of how heavy it is.

    A heavier mass will have a lower energy, while a lighter mass will have a higher energy, given the same momentum.

    It takes energy to compress a spring, so in general, given two buffers with the same momentum, the lighter buffer will compress the spring more than a heavy buffer.

    2)
    The work required to extract a case changes with time.

    In general, the longer you wait, the easier it is.

    I believe there is also a "snap action" effect that occurs when the case is cooled, contracted and "done".

    Extraction work is lowest just after this point in time.

    3)
    Whatever total energy the BCG has must be split up to extract the case, recock the hammer, compress the action spring and fight friction.

    If one of those tasks takes too much energy, there may not be enough left over to complete the others.


    My take in this 300 BLK scenario, is the barrel/gas port/cartridge had overall gas drive that was a little low.

    Too low to fully cycle the heavy A5.

    Due to the CAR gas system that starts the cycle early, the lightweight CAR buffer was trying to extract too early, using us too much energy to extract the case.

    This did not leave enough energy to compress the spring.

    The mid weight A5H1 with biasing spring held the action closed long enough for the case to be "done" and was able to extract is using very little energy.

    It had enough energy left over to fully compress the spring and cycle.
    Black River Tactical
    BRT OPTIMUM Hammer Forged Chrome Lined Barrels - 11.5", 12.5", 14.5", 16"
    BRT EZTUNE Preset Gas Tubes - PISTOL, CAR, MID, RIFLE
    BRT Bolt Carrier Groups M4A1, M16 CHROME
    BRT Covert Comps 5.56, 6X, 7.62

  4. #34
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    I'll try swapping some different weights and see what happens. I might get luck and get this thing running like a top with a buffer switch.

  5. #35
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    A guy at my local range was running the A5 system with a 10.5" barrel that he made at work, he was also running a Gemtech Sandstorm I believe and that thing was running like a f'in champ. I believe he said he was using a standard H buffer, he said the H3 was slowing the gun up a lot. You could try opening up the gas port if you can't find any resolutions with various buffer weights.

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