Troubleshooting LR308 (in 6.5 CM)

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  • Fix

    • Extractor

      Votes: 1 25.0%
    • Buffer/spring

      Votes: 1 25.0%
    • Mags

      Votes: 0 0.0%
    • OP is a moron, its something else

      Votes: 2 50.0%

    • Total voters
      4

    KIBarrister

    Opinionated Libertarian
    MDS Supporter
    Apr 10, 2013
    3,923
    Kent Island/Centreville
    Oh wise and gentle 2A comrades, I present to you an annoying difficulty.

    The rifle:

    DPMS LR-308 lower (originally a factory rifle, but the upper was the high top pattern). Swapped to DPMS-compatible barreled upper in 6.5CM (24") that is properly headspaced. Using original 308 bolt, significant pressure signs (case swipe, blown primers, etc.), double feeds, FT Eject.

    Added an adjustable gas block, switched to a high pressure bolt (and upgraded the trigger, but I don't see a relation there). I also polished the chamber. Pressure signs are gone, and the feeding/ejection problems are less frequent (but not that much). Every third or fourth round the carrier appears to be extracting the spent casing, strips a fresh round which ends up full seated in the chamber, but then stovepipes the spent casing between the bolt and the forward edge of the ejection port (literally half in/half out).

    The pre-Creedmoor transplant parts left on the rifle are: lower, carrier & carrier key, carbine buffer tube, buffer and spring. Also, still using the [not quite] 20 round DPMS steel box mags.

    My suspicions for the source of the problems are:

    a) extractor is failing to release (I polished it, but I haven't taken a file to the horns);

    b) buffer/spring is too weak;

    c) magazines;

    d) something I am missing.

    Those options are about in the order of cost/expense/headache to deal with, but I figured I would seek input before heading over to Midway (or Chantilly for option c)... I really want to get this resolved because the accuracy is phenomenal, but that is useless if it isn't reliable...
     

    davsco

    Ultimate Member
    Oct 21, 2010
    8,607
    Loudoun, VA
    it extracts the spent casing, holds onto it without ejecting, and grabs and chambers a fresh round?

    maybe include a pic or two to help diagnose.
     

    billymx48

    Member
    Jun 20, 2015
    96
    re

    so I had a lr308 in 308 win do similar stuff to me. Sent it back to the factory they polished the chamber and replaced the extractor and ejector. I would say your problem is an ejector but since it has jammed a few round on the extractor you might have a burr on it now that catchs the head of the case. I would say try the ejector or/and extractor.
     

    erwos

    The Hebrew Hammer
    MDS Supporter
    Mar 25, 2009
    13,866
    Rockville, MD
    Affirmative. Extracts, ejects, chambers without issue manually.
    I mean, I'm no gunsmith, but when I read your description of the problem, it sounds like short-stroking. Your new bolt fixed your pressure problems, but your ejection problems still remain.

    So, new questions: what buffer/spring are you running? Have you tried dialing your gas back up a little to see what happens? Do you happen to remember how vigorously the cases that ejected were thrown?

    ETA: I see that the high pressure bolt has dimensional changes to better support the primer, so I see why that fixed that problem. If that's the case, you were not necessarily over-gassed, and maybe you should be dialing up your gas block.
     
    Last edited:

    KIBarrister

    Opinionated Libertarian
    MDS Supporter
    Apr 10, 2013
    3,923
    Kent Island/Centreville
    I mean, I'm no gunsmith, but when I read your description of the problem, it sounds like short-stroking. Your new bolt fixed your pressure problems, but your ejection problems still remain.

    So, new questions: what buffer/spring are you running? Have you tried dialing your gas back up a little to see what happens? Do you happen to remember how vigorously the cases that ejected were thrown?

    ETA: I see that the high pressure bolt has dimensional changes to better support the primer, so I see why that fixed that problem. If that's the case, you were not necessarily over-gassed, and maybe you should be dialing up your gas block.

    Running standard original DPMS buffer and spring (measured both and they look to still be in spec). The adjustable gas block was checked after the bolt replacement, and is properly tuned pursuant to superlative arms instructions. I polished the extractor more and checked the function on both the extractor and ejector last night and they both seem GTG. I am looking at replacing the buffer/spring with the JP AR10 heavy to help slow down the carrier and extraction...
     

    woodline

    Ultimate Member
    MDS Supporter
    Jan 8, 2017
    1,947
    Running standard original DPMS buffer and spring (measured both and they look to still be in spec). The adjustable gas block was checked after the bolt replacement, and is properly tuned pursuant to superlative arms instructions. I polished the extractor more and checked the function on both the extractor and ejector last night and they both seem GTG. I am looking at replacing the buffer/spring with the JP AR10 heavy to help slow down the carrier and extraction...
    One other thing you can do is check the back of your lower receiver for damage. That can be an indication that your bcg is traveling too far to the rear and impacting your receiver. This can cause issues like the ones you are having. This happened to a DPMS pattern rifle of mine that was having similar issues. Clandestine cleaned up the damage and put about $1.00 in quarters in the back of my buffer tube to remediate that particular issue. Incidentally that and some of his other gunsmithing magic (most of which you've already done from the sound of things) took care of the issues I was having. In my case the fancy pants PWS buffer tube turned out to be too long for the DPMS buffer and spring I was using, so it probably never would have happened if I were following the KISS method of parts selection for that particular build.
     

    erwos

    The Hebrew Hammer
    MDS Supporter
    Mar 25, 2009
    13,866
    Rockville, MD
    Running standard original DPMS buffer and spring (measured both and they look to still be in spec). The adjustable gas block was checked after the bolt replacement, and is properly tuned pursuant to superlative arms instructions. I polished the extractor more and checked the function on both the extractor and ejector last night and they both seem GTG. I am looking at replacing the buffer/spring with the JP AR10 heavy to help slow down the carrier and extraction...
    One last question: if you feed a single round into your rifle with the magazine, does it lock back? My suspicion here is short-stroking causing weak ejection, and the above test will confirm if that's the case.

    If it does lock back consistently, yeah, your bolt is moving too fast. Try a heavier buffer and/or spring in that case. I think some people make 6.5-specific system that would work.
     

    KIBarrister

    Opinionated Libertarian
    MDS Supporter
    Apr 10, 2013
    3,923
    Kent Island/Centreville
    One last question: if you feed a single round into your rifle with the magazine, does it lock back? My suspicion here is short-stroking causing weak ejection, and the above test will confirm if that's the case.

    If it does lock back consistently, yeah, your bolt is moving too fast. Try a heavier buffer and/or spring in that case. I think some people make 6.5-specific system that would work.

    Yes, it locks back on the empty mag. The more I talk this through with everyone the more I am looking at needing to further decelerate the BCG and delay the extraction.
     

    KIBarrister

    Opinionated Libertarian
    MDS Supporter
    Apr 10, 2013
    3,923
    Kent Island/Centreville
    One other thing you can do is check the back of your lower receiver for damage. That can be an indication that your bcg is traveling too far to the rear and impacting your receiver. This can cause issues like the ones you are having. This happened to a DPMS pattern rifle of mine that was having similar issues. Clandestine cleaned up the damage and put about $1.00 in quarters in the back of my buffer tube to remediate that particular issue. Incidentally that and some of his other gunsmithing magic (most of which you've already done from the sound of things) took care of the issues I was having. In my case the fancy pants PWS buffer tube turned out to be too long for the DPMS buffer and spring I was using, so it probably never would have happened if I were following the KISS method of parts selection for that particular build.

    Thanks - I'll give it a look. The buffer/tube/spring are the only stock parts left on the rifle apart from the lower and bolt carrier
     

    woodline

    Ultimate Member
    MDS Supporter
    Jan 8, 2017
    1,947
    Yes, it locks back on the empty mag. The more I talk this through with everyone the more I am looking at needing to further decelerate the BCG and delay the extraction.
    Overgassed and undergassed systems can sometimes exhibit similar symptoms, unfortunately.
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    The superlative arms gas block is something I would remove. They are temperamental.

    SLR would be my advice.

    Ditch the standard action spring and install a Superior Shooting Solutions Flatwire .308 Spring. They add more closed bolt load which helps with extraction and bolt speed issues.

    As far as the headspace did you check it with the ejector and Extractor removed? If not, that is a must.

    Is the ejector flat faced or rounded? It should have a beveled edge on it. Again this helps with ejection.

    FBIvan answered some other concerns. Bolt travel is a serious issue with large pattern AR's. You need to limit bolt travel. The carrier should not be traveling too far back, and the bolt face should be even with the back of the ejection port on it most rearward travel, but still have enough clearance for the bolt catch to work. It's a fine balance.

    Another issue is the ejection port opening in the upper. DPMS uppers are notorious for having a slightly too small of an ejection port. I have opened many up to resolve the exact issue you are having.

    The extractor and extractor spring could be an issue as well but that isn't something I can explain. I need a gauge and to have it in my hand.

    Don't use JP buffer parts. Just don't.
     

    KIBarrister

    Opinionated Libertarian
    MDS Supporter
    Apr 10, 2013
    3,923
    Kent Island/Centreville
    The superlative arms gas block is something I would remove. They are temperamental.

    SLR would be my advice.

    Ditch the standard action spring and install a Superior Shooting Solutions Flatwire .308 Spring. They add more closed bolt load which helps with extraction and bolt speed issues.

    As far as the headspace did you check it with the ejector and Extractor removed? If not, that is a must.

    Is the ejector flat faced or rounded? It should have a beveled edge on it. Again this helps with ejection.

    FBIvan answered some other concerns. Bolt travel is a serious issue with large pattern AR's. You need to limit bolt travel. The carrier should not be traveling too far back, and the bolt face should be even with the back of the ejection port on it most rearward travel, but still have enough clearance for the bolt catch to work. It's a fine balance.

    Another issue is the ejection port opening in the upper. DPMS uppers are notorious for having a slightly too small of an ejection port. I have opened many up to resolve the exact issue you are having.

    The extractor and extractor spring could be an issue as well but that isn't something I can explain. I need a gauge and to have it in my hand.

    Don't use JP buffer parts. Just don't.

    Lots of good information!

    Headspace checked without ejector/extractor.

    Ejector edge is slightly beveled.

    If addressing the buffer/spring is not effective, my next step is going to be to look you up (someone above already mentioned you helped them out with an AR issue). Two questions if you don't mind: why not the JP buffer? Is that because of the large pattern AR or in general - I have a couple buddies that have them in 556 AR's and sing their praises. Second question, what would be your first recommendation for for a buffer weight? I want to stick with the carbine size tube just to take off a few inches when moving about and storage.
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    Lots of good information!

    Headspace checked without ejector/extractor.

    Ejector edge is slightly beveled.

    If addressing the buffer/spring is not effective, my next step is going to be to look you up (someone above already mentioned you helped them out with an AR issue). Two questions if you don't mind: why not the JP buffer? Is that because of the large pattern AR or in general - I have a couple buddies that have them in 556 AR's and sing their praises. Second question, what would be your first recommendation for for a buffer weight? I want to stick with the carbine size tube just to take off a few inches when moving about and storage.

    AR's are what I do. If you need help in person let me know.

    Keep the carbine .308 buffer, just replace the spring with a Tubb/Superior Shooting Solutions Flatwire Spring.


    The JP Silent System is what I despise. They add unnecessary complexity to a simple design.

    In my experience they allow faster unlocking when compared to a standard buffer/spring combination. It makes a D.I. rifle unlock faster like a gas piston AR. Not good.

    IMO, they are a gimmick. All they do is remove noise, but the tradeoffs are not worth the reward.

    The Superlative G.B. is another that I see for issues. They are very inconsistent. The only adjustable that I have come to trust is SLR.

    It's also possible that you have issues with your gas key or bolts. But that requires an IN# torque wrench to test. You set the wrench to 30 IN# and try to remove them. If they loosen before the tool clicks then the bolts are loose or broken.
     

    j_h_smith

    Ultimate Member
    Jul 28, 2007
    28,516
    I'm not a gunsmith, but I am a Smith, so I have an opinion. Doesn't this sound like a rough chamber? It would seem to me that must be looked at.
     

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