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Will a heavier buffer fix this ?

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  • TexMex247

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    So I've been working my way up with some 308 loads using imr 4064 and Sierra 175gr SMKs. I feel like I'm getting closer to a nice accuracy node as I approach 43.5gr. Groups have shrunk about a half inch. However, at 43.2 I'm getting some fairly heavy CHS although ejection is right at 3 o' clock. In my bolt rifle I get zero pressure signs with the same load. I read in a few places that most AR 10s will do that with heavier bullets. Since I don't want to install an adjustable gas block I was thinking about adding weight to the buffer. Does anyone have any experience doing so and/or do y'all think it will help ?

    I'm going to start by swapping just one steel weight for a tungsten one which will add roughly .9 oz. If it runs well I may even swap the other steel weight too adding almost 2 oz to the buffer. Here is a picture of the CHS marks from the 43.2gr load.
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    Dawico

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    I don't see how a heavier buffer will help but you can try.

    I don't even think turning the gas down will help either.

    Polishing your brass and chamber would be the only thing that would help as far as I can tell.

    The case is stuck in the chamber while the bolt is turning and unlocking. The pressure is too high to allow the case to turn with the bolt.

    In my opinion tone down your powder charge or just live with it.
     
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    What’s your current buffer weight and size? If it’s an AR308 buffer, I think you can only use two weights while the AR15 carbine buffer can go up to three. My LMT 308 comes with an H3 carbine buffer which already has the maximum three tungsten weights. A heavier buffer theoretically will delay unlocking but no idea if it’ll make enough of a difference.
     

    Dawico

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    No, if you are getting overpressure signs, that is an ammo issue. Swapping buffers or gas block isn't going to change anything.
    I don't see signs of overpressure.

    I've had plenty of factory ammo do this without cratered or blown primers.

    I know primers aren't the best gauge of over pressure but his are not flattened at all and the powder charge is a few grains below max.
     

    TexMex247

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    I read on a few other forums that adding mass to either the bcg or buffer will slightly delay the unlock time reducing the case head swipe. Several people posted about how standard pressure soft point hunting ammo will do this too. It seems to be more common with heavier bullets.

    I won't get a chance to test the theory til next weekend but the rifle is an otherwise stock DB 10 w/18" bbl. I don't think this rifle will see many lightweight bullets so I'm more interested it getting it to run what I load for it. That's going to be a steady diet of 165-175 gr pills.

    I have also thought about switching powders which depending on pressure curve would also reduce or minimize the CHS. For now I'll see how it likes the heavier buffer and report back in a couple weeks.
     

    zackmars

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    I don't see signs of overpressure.

    I've had plenty of factory ammo do this without cratered or blown primers.

    I know primers aren't the best gauge of over pressure but his are not flattened at all and the powder charge is a few grains below max.

    My eyes may have been deceiving me, but i swear i saw cratering.

    If not, and its just the brass getting marked from the ejector, then that's normal, brass might be just a tad soft
     

    TexMex247

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    Well just back from the field after a brief range trip this weekend and I truly think it's helping. I basically had bumped the stock ar 10 shorty buffer about .7 oz with tungsten weight. I thought I was gaining closer to .8-.9 but weighing them SXS I'm only seeing a .7-.75 oz difference between the tunsgten and steel weights. The CHS were slightly better even with a .3 grain higher load.

    So, I've decided to replace the other steel weight to bring it up to 5.3oz. I had better groups with the hotter charge and 175 SMKs. Since I think this will be my new happy place load wise I figured I would try to salvage as much life as I can from my brass. I'll post pics after my next test but so far so good.
     

    TexMex247

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    Here's a pic of the last 10 fired at 43.5gr imr 4064 running 4.4oz buffer. The CHS are present but no worse than 16250227087381697547932.jpg
    16250227087381697547932.jpg
    before in spite of increased charge.
     
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    popper

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    You need the adjustable block. Also check head space. Looks like some extractor rim pulls also. Pressure is there early (too much), case is pulled before it 'shrinks'. When the pressure is right, the case releases from the chamber and can be pulled back enough to allow taper on the case to fully release, allowing it to TURN. When the port sees 10-12k psi, the bolt ATTEMPTS to unlock. Cheap fix is moving the block forward to obstruct the hole.
     

    TexMex247

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    Unfortunately it runs a pinned block but that's a great idea. If there's no consequence to running a 5.3oz buffer I'll keep it in regardless. When I fired some 150gr bullets with 45gr IMR 4064 they had almost no ejector marks. I guess with the heavy bullet 43.5gr is about as hot as I can go.

    I plan on a doing another run of these at 43.5 as well as the 150gr load. It may be that this rifle will do better with lighter bullets anyhow. I had 3 of 5 shots clover leafing them although POI was way off from 175gr sight in.
     
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