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Thread: SBR malfunction

  1. #1
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    SBR malfunction

    I have an SBR built on Midwest industries upper and lower with a 10.5 inch barrel. I use the weapon as my daily patrol rifle. Upon taking ownership of the gun it ran flawless. I qualified with it and practiced with it all without a hiccup. After approximately 750 rounds I started experiencing failure to feed and failure to eject. I stripped it down and cleaned it very well. This fixed the problem for approximately 60 rounds, then it returned. I have tried different ammo with no effect. I have noticed that it tends to rear its ugly head once the gun is warmed up a bit. It will run fine for the first mag or so then starts the failures. To be more specific about the failure, it will fire and eject but then fail to strip the next cartridge out of the magazine. (Yes I have tried different mags) a few things I have read says my buffer spring is too weak, some say it’s too strong. Some literature points to undergassing. Some points to overgassing.

    PLEASE help. The guy who built it was of no help and has since shut his doors. I know the collective knowledge here has to have an answer or a solid suggestion.

    Thanks

  2. #2
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    What is the BCG?
    I'd check into extractor spring replacement, or try a different BCG of known quality.

  3. #3
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    What weight buffer? It should be marked on the front of the buffer H1, H2, etc. Measure your action spring (buffer spring) relaxed, out of the rifle. Should measure 10 1/16” to 11 1/4” in length (carbine).

    Check for markings on the barrel, etc to try to let us know more specifically what kind of rifle we’re looking at.

    Does it reliably lock back on empty mags? A lock back test will help us nail down the difference between too much spring or weight and not enough.

    Make sure your carrier key isn’t loose.
    Last edited by 1168; 11-25-17 at 08:00. Reason: Add

  4. #4
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    I'd just add two things to 1168's post:

    1) Short-stroking can also be caused by gas ring wear. You can test for gas ring wear by removing the bolt from the carrier and visually inspect the gas rings. Then insert the bolt all the way into the carrier without installing the cam pin. Turn the bolt carrier so the bolt is facing downward, if the bolt falls out then the rings need replacement. Given the history you related, worn gas rings wouldn't be my first suspicion, but the check is so easy you might as well cross it off the list.

    2) I would guess that your rifle will run on either a carbine (generally unmarked, weight about 3.0 oz) or H (generally marked H or H1, about 3.8 oz) buffer. If it has an H buffer and your spring measures 10 to 11, I'd try a carbine buffer (and mistrust the rifle until I was SURE - .8 oz is not that much). If it has an H2, drop back to an H and check function.

    3) If everything seems to check out with no problems, try the SBR using another BCG.

  5. #5
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    I would try a new buffer spring. If that doesn’t fix the issue, if you have a carbine buffer then I would try an H or H2. Maybe try new gas rings if nothing else works. 10.5’s tend to be pretty gassy.


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  6. #6
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    Quote Originally Posted by Mr. Goodtimes View Post
    I would try a new buffer spring. If that doesn’t fix the issue, if you have a carbine buffer then I would try an H or H2. Maybe try new gas rings if nothing else works. 10.5’s tend to be pretty gassy.


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    Gas port spec is key for sure


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  7. #7
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    Quote Originally Posted by Mr. Goodtimes View Post
    I would try a new buffer spring. If that doesn’t fix the issue, if you have a carbine buffer then I would try an H or H2. Maybe try new gas rings if nothing else works. 10.5’s tend to be pretty gassy.
    I started experiencing failure to feed and failure to eject.

    I stripped it down and cleaned it very well. This fixed the problem for approximately 60 rounds

    To be more specific about the failure, it will fire and eject but then fail to strip the next cartridge out of the magazine.

    We seem to have reached opposing conclusions, these are my thoughts:

    Without a good description of the fail to eject, it is hard to determine if the extractor is losing control of the case, or if the action isn't cycling far enough to allow the case to eject.

    The action not completely cycling and failing to eject kind of meshes with not picking up the next round from the mag.

    So that leads me to believe over buffered/sprung or undergassed as the simplest explanation.

    What do you think?

  8. #8
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    First thing to do is a lockback check to see if it's short stroking.
    The number of folks on my Full Of Shit list grows everyday

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  9. #9
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    I would read this first and then come back with some additional info.

    https://www.m4carbine.net/showthread...leshooting-101



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  10. #10
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    Quote Originally Posted by 26 Inf View Post
    I started experiencing failure to feed and failure to eject.

    I stripped it down and cleaned it very well. This fixed the problem for approximately 60 rounds

    To be more specific about the failure, it will fire and eject but then fail to strip the next cartridge out of the magazine.

    We seem to have reached opposing conclusions, these are my thoughts:

    Without a good description of the fail to eject, it is hard to determine if the extractor is losing control of the case, or if the action isn't cycling far enough to allow the case to eject.

    The action not completely cycling and failing to eject kind of meshes with not picking up the next round from the mag.

    So that leads me to believe over buffered/sprung or undergassed as the simplest explanation.

    What do you think?

    This is my suspicion. But more info required, for sure. That lockback test may be the key.

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