full auto and semi bcg

Discussion in 'AR-15 Discussion' started by Kdub, Feb 23, 2015.

  1. Kdub

    Kdub New Member

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    I recently finished a 10.5 SBR build. Its a spikes lower and upper with a low profile gas block. When I shot it I was having a lot of short stroke problems. I have a "frankenbolt" full auto bcg in it (which I'm less than pleased with). It has a psa lpk. I weighed the buffer and its 2.9 ounces. I'm wondering if the extra weight from the bcg could be causing the short stroke. The upper was complete from Mas Defense. I'm aware the gas port could be misaligned, but I'm wanting to rule out everything else before I tear the upper apart. Any suggestions on what to look at. A heavier buffer would just create greater problems as it would increase resistance when cycling correct? Is there ever a problem with the action cycling too fast?
     
  2. fsted2a

    fsted2a Active Member

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    I would check the gas key for leakage and good staking first if you are looking at the bcg. Then check to see if the barrel nut is touching the gas tube. Either one of those issues could be giving you problems. In a SBR, you want a heavier buffer and a M16 carrier to slow the cyclic action down. I would say 80% odds you have a gas problem somewhere.
     

  3. Kdub

    Kdub New Member

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    I thought the same thing about a heavier buffer, but then when I started having short strokes and failure to lock open issues I figured the bcg wasn't being pushed all the way back. So in my head a heavier buffer would just make the problem worse.
     
  4. JonM

    JonM Moderator

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    Best place to start is make sure everything is properly put together. Short stroke is caused by lack of gas. Three causes, misaligned gas port (not common), the bolt gas rings are miss or worn, or improperly installed gas key (very common). If the gas port is misaligned it will restrict gas flow and rub on the gaskey. Its also the easiest thing to fix. Check the sticky on aligning a gasblock.

    The gas key has to seal the carrier so both conract surfaces must be dry and squeeky clean. The gaskey bolts are torqued 50 to 60 inch pounds i torque to 60. Then stake them by using a center punch and drift a bit of the key material into opposite sides of each screw.

    There should be three gas rings on the back of the bolt. With the bcg out and the bolt extended stand the bcg on the bolt. If it collapses the rings need to be replaced.
     
  5. fsted2a

    fsted2a Active Member

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    You have a malfunction. If everything is working properly, the heavier buffer is necessary.