10.5 .308 AR pattern SBR issues

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  • Kpquinn9100

    Plinker
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    Aug 22, 2015
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    Camby
    Not sure if this is in the right section? IÂ’m having some issues with my .308 sbr. Of course itÂ’s a .308 variant ar so itÂ’s gonna have issues anyway. These things are extra finnicky. So the main issue is failure to eject, tries to stove pipe following round and jams. It also sometimes has a failure to extract the spent case from the chamber. Just got the bolt to lock back today (suppressed and unsuppressed) with an adjustable gas block but still can only load one round per magazine. Thinking something along the lines of the buffer system/gas system causing this. Plan to upgrade to an slr click type detent gas block next week. itÂ’s also very violent on the second round in the mag. Will try to post pics of what it does.

    Important Specs on the rifle are:
    aero m5 upper and Lower
    aero .308 carbine buffer kit (tube, spring, buffer)
    kak 10.5 carbine gas barrel
    aim surplus (I think) bcg
    seekins adjustable gas block. Old style. Screw on side.

    58E79CE8-1CAE-449A-A2A0-1664CA005519.jpg
     

    jason867

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    Jan 7, 2009
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    New Castle
    I can't help but feel like it'd be a miracle to get an sbr 308win ar, with that short of a barrel, to cycle at all, let alone reliably.

    Try different ammo, I'm certain it will be ammo picky.

    Also, I'm betting you're not getting much more performance than a 300aac blackout, firing a 308 in that short of a barrel...
     

    Kpquinn9100

    Plinker
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    Aug 22, 2015
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    Camby
    I’ve got an 8.5 .300blk as well. Shoots fine. But it’s designed to run on a pistol length gas system so there’s no issues. Most .308 ar platforms have issues period. Sbr or not. People with 16” or even 18” barrels run into these same issues. I’m sure the 10.5 complicates things but that’s half the fun! Google ‘kak 10.5 .308 pistol’ where there’s a will there’s a way! Seems to shoot better suppressed which doesn’t surprise me with what seems to be short stroking going on.
     

    AllenM

    Diamond Collision Inc. Avon.
    Industry Partner
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    Apr 20, 2008
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    Avon
    sounds like it may be under gassed if the bolt desnt want to lock open, Opening up the gas port in the barrel a few thousanths could help a couple of the issues you are having.
    I would pull the gas block off and measure the port and do some research on port sizes. With the short barrel you will need more gas then a stock port will give.
     

    jason867

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    New Castle
    I'm betting you're not getting much more performance than a 300aac blackout, firing a 308 in that short of a barrel...

    Just wanted to followup on this:

    I did some quick research, and comparing 147gr-150gr loads, out of 10"-11" barrels, 300AAC blackout was averaging around 1600-1700 fps, while .308WIN was averaging around 2200 fps.

    In this comparison, you could say that your 10.5" .308WIN SBR is getting slightly better ballistics than 7.62x39mm/300AAC with 16" barrels.

    Obviously, you will get different results if you start playing with the variables.

    It's not as small of a difference as I expected. Whether this matters to you or anyone else reading this is up to you guys.

    Just remember that many bullets are designed to perform within a certain velocity range, and may underperform when you push them outside of that velocity window...
     

    Kpquinn9100

    Plinker
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    Aug 22, 2015
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    Camby
    Thanks Allen. I’ll be swapping gas blocks next week so I’ll measure the port when I have it apart.

    Good info Jason. 2200 is cooking. I’ve tried two 147gr loads and one 150gr load. I’m gonna try a few other things before I rule it to ammo. Thanks guys.

    Anyone on here have experience with building a .308 ar? Did you have any issues if so?
     

    Rookie

    Grandmaster
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    Sep 22, 2008
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    Kokomo
    My only suggestion is to make sure you're running it wet. Soak the BCG and put it in. My 308 (not SBR) has issues you're having suppressed unless I lube it really well.
     

    Slow Hand

    Master
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    Aug 27, 2008
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    West Side
    I built an AR-10 off of an aero upper and lower and a DPMS SASS barrel over the summer. I’ve onky had it out a couple of times and I was having similar issues as you.

    shooting suppressed with 168 FGMM equivalent loads it acted like it was undergassed. Stovepipes mostly.

    168’s unsuppressed it ran fine

    147 NATO ammo suppressed and it ran fine

    147 NATO unsuppressed it ran fine

    adfer doing some research I came to the conclusion that I’m overgassed and the extra pressure pushes the bolt back too fast and it’s closing before the empty can leave the ejection port. My barrelnis .936” so I have limited options in adjustable gas blocks. I bought a JO Enterprise and installed it last week. I was only able to fire a few rounds but it seems to be running better.

    Yours sounds similar but with the failing to lock the bolt back it does sound Undergassed. I’m going to try a flat wire Tubbs spring in it to see if it helps keep it locked up a hair longer as well.
     

    Kpquinn9100

    Plinker
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    Aug 22, 2015
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    Camby
    Good deal. I contacted kak to see what size the gas port on the barrel should be and I was gonna pick their brain on their shop 10.5 pistol they run and see if we can’t get this machine sewing already! They have a new bcg that has two extractors much like the kac system. Think I may try that out as well as I’m also having an issue with extraction sometimes. The stove piping issue isn’t due to bcg but I figured I’d probably change all this stuff while I have it apart and start from scratch. These things are so finnicky one part might throw the whole shooting match off.
     

    1911rocks

    Plinker
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    Nov 15, 2010
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    To the OP: What Gas Tube length? Pistol, Carbine, Mid, Rifle? Change powders (if you're reloading) if not stick to 147gr and lighter.
     

    mammynun

    Master
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    Oct 30, 2009
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    New Albany
    My AR-10 is significantly different, 18" bbl, Syrac Gen1 adj gas block, and a rifle length receiver extension. If I take the time I can adjust the gas to make it very soft shooting and still reliable, but if I change ammo/remove suppressor/etc. reliability goes in the toilet. I generally keep it setup to reliably cycle my weakest unsuppressed ammo and accept the extra recoil when firing suppressed ammo with more balls. I'm sure the 18" bbl and rifle length recoil spring helps smooth out the pressure impulse compared to a 10.5" bbl and a carbine spring.

    My advice would be to see if you can tune it to run with a specific load before adding any other variables.
     

    Kpquinn9100

    Plinker
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    Aug 22, 2015
    63
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    Camby
    Update on this. Put the slr adjustable gas block on it, which was way easier to tune using the click type detent setup instead of the regular set screw type. Also added the kak heavy carbine buffer. Stock carbine spring still being used. Was able to close port off and open 1-2 clicks at a time till bolt locked back with and without can. Found sweet spot for that but still continued to load one round at a time. Still has extraction issue. Didn’t stovepipe on me though. It will fire fine but the bolt will lock back without yanking the case out every now and then. When it does extract the ejection pattern is at about 2:30-3:00. Not ideal but I’ll take it. Some of my 5.56 sbr suppressed guns have a funky ejection around 3 o’clock too. Just mad because I’m losing some of my .308 brass at the indoor range haha. Gonna swap out the bcg with the kak dual extractor set up and hopefully it’ll be good to go!
     
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