AR10/LR308 Failing to extract spent casing?

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  • jamesp15

    Active Member
    Jul 16, 2018
    420
    Southern PG
    A few weeks ago I built an AR10/LR308 (not sure which to call it lol) with the following major parts:
    Aero Precision M5E1 Upper and Lower
    Faxon Match Series 18" Fluted barrel (Link) (Originally 20" but when Faxon swapped the barrel I asked for the shorter, it looks better aesthetically to me.)
    Faxon .308 BCG (and paid for their match/headspace service) (Link)
    Odin Works Adjustable Gas Block (Link)
    Aero Precision Upper and Lower completion kits for M5
    Aero Precision M5 .308 Rifle Length Buffer kit 5.6oz Buffer(Link) (Was originally a carbine with 3.8oz buffer)
    Giessele SSA-E Trigger
    Magpul PRS Gen3 stock and MOE+ Grip
    Midwest Industries Hand Guard (Link)
    Lantac Dragon 7.62 Muzzle Break

    Tried 4 types of ammo:
    Federal Premium .308 165gr
    Hornady SST .308 150gr
    Federal 7.62x51 150gr (XM80CBL I believe is the box label)
    PMC Bronze 7.62x51 147gr

    On ALL of them 85 times out of 90 shots the spent casing will NOT Extract (still in the chamber, though not "stuck")
    A very light tap with cleaning rod down barrel and they come right out, or if I close the bolt and manually cycle it they will extract 100% of the time, also a very light tap of the stock on the ground with the barrel pointing up and the cartridge will come out.

    The spent cartridges have cuts/marks where the extractor tried to grab it. The ones that happen to extract pretty much fall out onto the table right next to me.

    A friend suggested that it was over gassed and I have adjusted the gas block to all of its possible detent settings. (from all the way closed to all the way open, first 6 detents of the adjustment from closed and it doesnt cycle the action enough to hold open on empty magazine, 7th detent and up it will hold open on empty)

    Any suggestions on what to try next?
     
    Last edited:

    357Max

    Active Member
    Feb 28, 2019
    209
    Crownsville
    I can think of 2 possibilities with the first being most probable.

    1. Check extractor on bolt to make sure it has stiff spring pressure holding it closed. Pop a spent round in the bolt to see how well the extractor engages the rim. Faxon advertises the bolt as having double o rings, I suspect one or both were left out (possibly during head space check).

    The only other thing I can think of is improper gas tube length/engagement into gas key on the bolt. If it's to short it would require a bunch/to much gas to get it to hold open on last round, but to short a duration engaged in key. It may be sending bolt back while case is still under pressure
     

    jamesp15

    Active Member
    Jul 16, 2018
    420
    Southern PG
    Thanks for that, I disassembled the BCG and removed the extractor, it has only one O-Ring in it.
    Gonna order some up if I cant get one from a local place tomorrow if thats the case.

    I hope that fixes it.

    Could the buffer also be at fault or contributing? Its a 3.8oz one in their "M5 Buffer Kit" so I assume(d) that it is heavy enough, but that seems a bit light.
    (I am new to the .308 AR "world")
     

    357Max

    Active Member
    Feb 28, 2019
    209
    Crownsville
    I really doubt your buffer is causing this. If everything else is in spec the 3.8 OZ should function fine with the rifle length gas + adjustable gas block.

    3.8 OZ buffer is on the lite side, but your also constrained by the carbine length tube. The link below has the heaviest I'm aware of @ 5.5 or 6.5 OZ
    http://heavybuffers.com/ar10carbine.html

    Did you check the gas tube length? If it's short the light buffer will magnify the problem. Armalite had some odd length gas tubes that can cause trouble if you order an AR-10 vs SR-25/LR 308 tube.
    There is a milled relief in the upper receiver opposite the ejection port usually about 1" long. Your gas tube should stop right in the middle of this relief.

    You didn't mention in your original post if it's trying to jam the next round into the back of the spent case???

    I have the Magpul UBR Gen 2 stock on mine which has a slightly longer tube also known as the VLTOR A5 length so I put the 8.5 OZ buffer + rifle length spring in mine (linked below). I also have an adjustable gas block (SLR sentry 7) on Mid gas 18" barrel.
    First shots out of mine threw brass 20'+ and left a big dent in cases till I got it turned down. Recoil is now about the same as my AR 15 6.8 SPC and brass ejects about 3 feet in the 3 - 4 o clock direction.
    http://heavybuffers.com/xhcar.html
     

    jamesp15

    Active Member
    Jul 16, 2018
    420
    Southern PG
    It was trying to load the next round into the chamber without extracting the prior cartridge so yeah it was trying to double feed/jam the next round into the back of the spent case. After a few of those I loaded up a bunch of 10 and 20rd magazine with 1 round each while playing with the adjustable gas block.

    Gas tube is a Yankee Hill Machine rifle length, looks maybe a bit short to me but almost halfway, looking straight down on it it is probably 40% or so across the relief. (Pic Below)
    (Will pick up a spare gas tube tomorrow and give it a try if you think it might be too short)


    (click any image for full sized version)
    Bolt face after a quick wipe-down to remove oil (couldnt get a clear pic with it lubed up)


    Extractor, with only one o-ring (emailed Faxon customer service about missing o-ring)


    Gas Tube, clearest pic I could get of it.
     

    357Max

    Active Member
    Feb 28, 2019
    209
    Crownsville
    The Armalite rifle length gas tube is about .300 longer then std AR 15/LR 308 tubes. I wouldn't waste my time replacing yours with another std. Mark your gas tube right where it enters the outside face of receiver. Remove the gas block & tube, slide a cleaning rod or whatever will fit into the gas key till it bottoms in the key (bolt in battery) and mark it. Measure the difference. If it's greater then .325 (more then 5/16") then I'd order an Armalite rifle length tube.

    The Faxon link to your barrel just said "barrel gas system rifle" doesn't say if it's Armalite LR 308 or somewhere btwn? It does say LR 308 for the barrel extension. The problem seems to stem from certain AR/LR 308 barrel manufacturers that measure the gas block location on an AR 15, from gas block Step to shoulder where the barrel extension seats and then copy it to a 308 :facepalm: Problem is the barrel extension is longer on the 308.

    The other thing I didn't catch reading your first post is that you have a PRS Gen 3 stock with a carbine length buffer? Confirm you didn't put a longer buffer tube on with the PRS? That stock is really set up for full rifle length tube even though it does say it will work with the carbine length.
     

    jamesp15

    Active Member
    Jul 16, 2018
    420
    Southern PG
    Will measure gas tube tomorrow.

    Yeah its a PRS Gen3 with the Aero M5 Carbine buffer tube/spring/buffer and using the carbine sized "block" that came with the PRS stock. The Gen3 came with a Rifle and a Carbine length plastic piece that goes between the receiver end-plate and the main part of the PRS Gen3 stock. (IIRC the stock "bottomed out" or nearly so on the end of the buffer tube. Assembled the lower almost 6 months ago, so memory isnt 100% on that, will double check tomorrow)

    Thanks for all the help
     

    smdub

    Ultimate Member
    MDS Supporter
    Nov 14, 2012
    4,642
    MoCo
    It was trying to load the next round into the chamber without extracting the prior cartridge so yeah it was trying to double feed/jam the next round into the back of the spent case
    This is an extractor problem. Gas isn't your issue since you've adjusted it from closed to open and the problem still happens. (FWIW, Even wildly overgassed an AR10 will either throw cases a mile or rip the rim off if they have a properly working extractor.) The bolt is moving rearwards w/o the spent case in its grip. Its an extractor problem. Contact Faxon.
     

    jamesp15

    Active Member
    Jul 16, 2018
    420
    Southern PG
    Gas tube is about 1/16 to 1/8th inch shorter than the aluminum tubing I used to measure using your technique, so I dont think thats the problem.

    Contacted Faxon about missing o-ring and they said the extractor may be an older version and that there should be 2 o-rings to function properly. They are sending me the updated/newer extractor and a few o-rings.

    Hope the above fixes it.

    I might replace the PRS Gen3 stock with something else, I have fairly long arms but even at its minimum setting its a bit uncomfortable for me (feels too long). I use most collapsing AR15 stocks at their maximum extension and even put on thicker butt-plates to get more LOP and they are comfortable. Maybe its the extra bulk of it that bothers me.
     

    woodline

    Ultimate Member
    MDS Supporter
    Jan 8, 2017
    1,947
    Extraction issues on large frame ARs can require expertise to properly diagnose, as they may be a result of multiple problems. I had a couple issues with an Aero AR .308 build I did awhile back. It has a faxon barrel and BCG.

    Clandestine sorted it out for me. Highly recommend you take it to him.

    Also check your lower where buffer tube attaches. One of the issues my rifle had was BCG over travel. I didn't bother to check back there. Consequently Chad had to make some minor repairs to my lower from where the BCG was impacting it. I also had extractor issues. And a couple other things.

    Fortunately Faxon took good care of me when I showed them how chewed up my extractor was after a few range uses. They offered to send me an entirely new bolt but that didn't seem fair. They did replace the extractor and included new o rings, free of charge. That said based on Chad's recommendation I switched to a JP .308 extractor.
     

    cobra

    Ultimate Member
    Feb 26, 2009
    2,058
    White Marsh
    For what it’s worth. Had same problem on at 15. Ripping off rim of shell but round was not stuck in chamber and would almost fall out if butt was tapped on ground. Found extracted had the spring and o ring on it. Removed the o ring leaving just the spring in place and it has functioned fine.
    Have about 300 rounds thru it since them with no problems.
     

    jamesp15

    Active Member
    Jul 16, 2018
    420
    Southern PG
    I am going to try out the new extractor and the second (missing, supposed to come with 2) o-ring per their (Faxons) suggestion. Just need to wait for the parts in the mail, good thing that work is going to keep me busy the next week or 2. If the replacement extractor/o-rings doesnt work I may have it looked at.

    I am the stubborn "if I can probably fix it I will try everything I can before having someone else fix it for me" kind... lol
     

    Clem

    Member
    Mar 19, 2007
    99
    Southern MD
    The AR-15 is MIL-SPEC so parts from different manufacturers play well together. The AR308 has at least two different, incompatible standards, Armalite and DPMS. You might want to check that.

    Mine are DPMS spec. I have adjustable gas blocks and 5.5 oz Heavy Buffer buffers. They seem to work great.
     

    jamesp15

    Active Member
    Jul 16, 2018
    420
    Southern PG
    The AR-15 is MIL-SPEC so parts from different manufacturers play well together. The AR308 has at least two different, incompatible standards, Armalite and DPMS. You might want to check that.

    Mine are DPMS spec. I have adjustable gas blocks and 5.5 oz Heavy Buffer buffers. They seem to work great.

    The Aero M5 receiver set is DPMS "High" pattern but with larger/longer takedown pins, I sourced all the parts with that in mind.
     

    bfoosh006

    Active Member
    Aug 19, 2017
    145
    First, Make sure the chamber is clean, and any oily factory preservatives have been cleaned from the chamber. Those "oils" can make extraction difficult and problematic.

    Can you post a photo of a fired case ? Looking for any rough / less the smooth scuff marks on the case body , possibly from the chamber.

    On a different note... and more about ejection issues...
    Make sure the edges of the extractor claw are slightly rounded off, rather then "pointy" ... the pointy edges will "grip" the case rim at those pointy edges, and not allow the case to "roll" out of the extractor claw.
    __________________________________________SuWHaxU-912480.jpg


    If you slow down the cyclic speed, extraction will be at a lower chamber pressure. Your rifle length gas system should be extracting at plenty low chamber pressures... low enough that it shouldn't be an issue, but...

    Adding a heavier buffer and or recoil spring will slow the unlocking of the bolt.. just by milliseconds, but that is enough to allow extraction at lower chamber pressures.

    I think 3.8oz buffers are to light... personal preference. KAK sells a 5.5ish weight 2.5" buffer. And your OEM weight should work... just be sure the chamber is smooth and burr free ( the reason I asked about the pics ) and all oils removed.

    But... since you have a PRS Gen III , I'd swap over the rifle length buffer tube, and try a 308 rifle length kit. ( on sale this Labor Day weekend )
    https://www.aeroprecisionusa.com/m5-308-rifle-buffer-kit

    A rifle length recoil system allows even heavier weight buffers if you decide to go that route. ( KAK 9.3oz 308 rifle buffer)

    And consider the Tubbs .308 Flatwire recoil spring ( available cheap from LaRue ) ...

    And there is a decent chance your extractor is faulty... or needs another O ring...since it is "slipping" off the case rim. ( But again make sure the chamber is clean )

    The case is probably sticking in the chamber at high enough extraction pressure to force / allow the extractor to snap off the case rim... removing any gummy oil will help.

    Possibly polishing the chamber, will help if there are obvious scuff marks on the spent case. Polishing the chamber is very simple to do.

    FWIW... I don't think it is over gassed... especially if you tried a bunch of gas settings. IMHO, Some thing is gripping the case in the chamber and not allowing the extractor claw to do its job at the right moment.

    Just my 2 cents.

    And how many rounds have been fired so far ? ( More of the part of the equation info )
     

    Scottysan

    Ultimate Member
    MDS Supporter
    May 19, 2008
    2,433
    Maryland
    Extraction issues on large frame ARs can require expertise to properly diagnose, as they may be a result of multiple problems. I had a couple issues with an Aero AR .308 build I did awhile back. It has a faxon barrel and BCG.

    Clandestine sorted it out for me. Highly recommend you take it to him.

    Also check your lower where buffer tube attaches. One of the issues my rifle had was BCG over travel. I didn't bother to check back there. Consequently Chad had to make some minor repairs to my lower from where the BCG was impacting it. I also had extractor issues. And a couple other things.

    Fortunately Faxon took good care of me when I showed them how chewed up my extractor was after a few range uses. They offered to send me an entirely new bolt but that didn't seem fair. They did replace the extractor and included new o rings, free of charge. That said based on Chad's recommendation I switched to a JP .308 extractor.

    My DPMS LR-308 AP4 had a similar issue, for which Clandestine did a chamber polish and extractor upgrade. Turned out the rough polish was "grabbing" any brass with a patina pretty firmly.
    Once freed up it did require a heavier buffer to compensate, but never a problem since.
     

    jamesp15

    Active Member
    Jul 16, 2018
    420
    Southern PG
    First, Make sure the chamber is clean, and any oily factory preservatives have been cleaned from the chamber. Those "oils" can make extraction difficult and problematic.

    Can you post a photo of a fired case ? Looking for any rough / less the smooth scuff marks on the case body , possibly from the chamber.

    On a different note... and more about ejection issues...
    Make sure the edges of the extractor claw are slightly rounded off, rather then "pointy" ... the pointy edges will "grip" the case rim at those pointy edges, and not allow the case to "roll" out of the extractor claw.
    __________________________________________SuWHaxU-912480.jpg


    If you slow down the cyclic speed, extraction will be at a lower chamber pressure. Your rifle length gas system should be extracting at plenty low chamber pressures... low enough that it shouldn't be an issue, but...

    Adding a heavier buffer and or recoil spring will slow the unlocking of the bolt.. just by milliseconds, but that is enough to allow extraction at lower chamber pressures.

    I think 3.8oz buffers are to light... personal preference. KAK sells a 5.5ish weight 2.5" buffer. And your OEM weight should work... just be sure the chamber is smooth and burr free ( the reason I asked about the pics ) and all oils removed.

    But... since you have a PRS Gen III , I'd swap over the rifle length buffer tube, and try a 308 rifle length kit. ( on sale this Labor Day weekend )
    https://www.aeroprecisionusa.com/m5-308-rifle-buffer-kit

    A rifle length recoil system allows even heavier weight buffers if you decide to go that route. ( KAK 9.3oz 308 rifle buffer)

    And consider the Tubbs .308 Flatwire recoil spring ( available cheap from LaRue ) ...

    And there is a decent chance your extractor is faulty... or needs another O ring...since it is "slipping" off the case rim. ( But again make sure the chamber is clean )

    The case is probably sticking in the chamber at high enough extraction pressure to force / allow the extractor to snap off the case rim... removing any gummy oil will help.

    Possibly polishing the chamber, will help if there are obvious scuff marks on the spent case. Polishing the chamber is very simple to do.

    FWIW... I don't think it is over gassed... especially if you tried a bunch of gas settings. IMHO, Some thing is gripping the case in the chamber and not allowing the extractor claw to do its job at the right moment.

    Just my 2 cents.

    And how many rounds have been fired so far ? ( More of the part of the equation info )

    Around 100rounds total in trying to troubleshoot.
    Faxon is sending me an "updated" extractor, apparantly I was shipped with an old style extractor. Still waiting on it in the mail.

    Will post up a few casing pictures when I get back home this evening.

    Chamber is clean and grease/oil free, put a cotton thing like dentists use to hold your cheeks back off your teeth/gums (cant recall the name of it) and spun it around in the chamber and nothing "caught" so no larger/obvious burrs.

    Hopefully I will have the replacement extractor and missing o-ring early next week so I can get back onto troubleshooting this.
     

    bfoosh006

    Active Member
    Aug 19, 2017
    145
    Around 100rounds total in trying to troubleshoot.
    Faxon is sending me an "updated" extractor, apparantly I was shipped with an old style extractor. Still waiting on it in the mail.

    Will post up a few casing pictures when I get back home this evening.

    Chamber is clean and grease/oil free, put a cotton thing like dentists use to hold your cheeks back off your teeth/gums (cant recall the name of it) and spun it around in the chamber and nothing "caught" so no larger/obvious burrs.

    Hopefully I will have the replacement extractor and missing o-ring early next week so I can get back onto troubleshooting this.

    Very good, looking forward to the pics.
     

    jamesp15

    Active Member
    Jul 16, 2018
    420
    Southern PG
    Sorry forgot about the pictures I was going to take of the spent casings, and now im not sure which is which in my shop.. grr

    Apparently Faxon had a bad batch of extractors, mine is/was one of them. They were going to send me a fixed/updated one and all thier stock on hand were "bad". So I am stuck waiting on one of thier new batch they are making/having made which might be 2 to 6 weeks. Ouch.

    Would any DPMS pattern extractor work on this bolt? I might bite the bullet so to speak and get a JP extractor just to get the rifle functional. (I am patient but this has been too long, I want to get it functional and sighted in before winter so its ready for next spring when I should have more convenient access to a 200+ yard range than I do right now.)
     

    woodline

    Ultimate Member
    MDS Supporter
    Jan 8, 2017
    1,947
    Any DPMS pattern extractor should work on your bolt. I have a JP on my Faxon bolt and it works just fine.
     

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