AR10 issues

Scarlett_Red

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I have a relatively new built ar10 that is probably over 100 rds by now that has started giving me issues. It is 16" midlength gas system with pinned vltor block, armalite parts. It has just recently started pulling the case head enough to deform it as well as starting to shave a peal off the head as well. My diagnosis is something has changed to cause the bolt to unchamber before the case contracts enough to release from the chamber walls. perhaps there is too much pressure? not enough dwell time? I am stumped as it has worked fine until just recently. I have a heavy buffer in it which should have helped in this situation. Any Ideas? thanks
 

FCS

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Re: AR10 issues

Ammunition you loaded or factory ammo?

If factory ammo, is it same lot #?
 

BCP

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    Re: AR10 issues

    What kind of ammunition are you using?
     

    Scarlett_Red

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    Re: AR10 issues

    I have used both reloads and factory ammo. Hornady 165 sp. Have shot federal 168hp as well as hornady tap and superperformance. just showed up on 3 rds of reloads last month and just shot it today with the H 165 sp's.I just noticed the neck mouth is flattened on the side which is new. will too much pressure hold it in too tight? I would like to mention the 2 shots I shot today had a new suppressor on it which of course causes more back pressure, but I saw this problem last month with my reloads and no suppressor.
     

    BCP

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    Re: AR10 issues

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Scarlett_Red</div><div class="ubbcode-body">just showed up on 3 rds of reloads last month and just shot it today with the H 165 sp's.</div></div>

    could be that your reloads are a bit too hot. if you could post up some pictures of the marks on the cases it would help a lot.
     

    Scarlett_Red

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    Re: AR10 issues

    Here are some pics. you can vaguely see the deformed case head and the extractor peal from the bolt trying to rotate a tight expanded case. flattened necks are new. rifle picture included for reference
    IMG_0859.jpg

    IMG_0858.jpg

    IMG_0857.jpg

    IMG_0856.jpg

    IMG_0855.jpg

    IMG_0860.jpg
     

    BCP

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    Re: AR10 issues

    Nice rifle.

    Too hot of a load. Superformance did that in my rifle too.
     

    Scarlett_Red

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    Re: AR10 issues

    Actually the superperformance did not do this.standard loads did. It just started happening. I could take the gas block off and drill and tap a set screw in like JP does and throttle back the pressure a bit. It ain't gonna be easy on a vltor gas block though. perhaps a pri adjustable or a fatboy gas tube would help?
     

    JD42

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    Re: AR10 issues

    Very nice rig. Does it do it with both the supersor on and off?
     

    BCP

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    Re: AR10 issues

    JP makes a low profile adjustable clamping gas block for 70$. Brownells and Rainier have them.

    If it were mine I'd check to make sure that everything was clean including chamber and bolt face, lube it up and shoot some factory ammo without the suppressor.

    I just don't understand why it would go from "works fine" to "not works fine" without anything changing...
     

    bigwheeler

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    Re: AR10 issues

    Did you change powder lots on your reloads? The ejector marks are pretty heavy.
    Not really a good shot of the primer for flow around the firing pin. No bolt swipe
    I can see so it should not be a timing issue, just high pressure imho. Make sure
    your cases are trimmed to the right length and your chamber is clean.
     

    jeepguy242

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    Re: AR10 issues

    deformed case mouth from the case hitting the ground maybe? or did you try and catch it so it wouldnt hit the ground with something? i was trying to stop mine from ejecting them onto the ground using my range bag, and got a few that hit the bag and dented the mouth just like that...

    as for the extractor swipe mark, i had some hot loads that i loaded when i was testing that swiped like that, and i know mine was from pressure as they were up on the high end of the powder charge (45.3 gn varget under a 168smk) so i backed off to a lower charge and they stopped. the primers on those looked flat as hell too and a few of them actually popped out, and the case is too loose to put in another primer.
     

    Jumper

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    Re: AR10 issues

    It looks like your bolt is unlocking while the pressure is too high. I wouldn't say your load is overpressure because your primer doesn't look flattened. I would try an adjustable gas block. In fact, my AR 10 had the same problem and it was fine after I adjusted the gas.
     

    bigwheeler

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    Re: AR10 issues

    If the timing was out/high pressure upon extraction, there would be extractor marks
    bent rim, bolt swipe. I see none of that. Just the case getting hammered into the
    ejector really hard.
     

    Jumper

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    Re: AR10 issues

    The third pic shows a curved mark ending in a raised burr. I would interpret this to mean that the case is still being forced into the bolt face as it rotates to unlock. The sharp edge of the ejector plunger hole shaves this mark and raises the bur. Since there is no extractor mark on the case groove, the pressure must be low enough upon extraction not to bend the case lip. Usually the two problems are seen together. Why they don't both appear here I don't know. Maybe it has something to do with the can.
     

    Scarlett_Red

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    Re: AR10 issues

    Actually the picture doesn't apparently show the deformed case head very well. It is very definitely pulling too hard on a tight case. It is very bulged out. The extractor is making the peal cut as well, as the bolt is turning while the case is still expanded in the chamber. I wasn't thinking about the fact that more pressure means more speed, hence unlocking too quick. Suppressors do cause back pressure which of course means more pressure which means more speed. Suppressed shots are faster than non suppressed. Last month my last hand loads that caused this were unsuppressed, but also a trial run of lee factory crimped ammo. This also causes higher pressure albeit earlier in the bullet travel, but higher nonetheless. I am going to have to drill a hole in the side of the gas block and put in a set screw to throttle back the flow. Hopefully this will solve it.
     

    Casey Simpson

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    Re: AR10 issues


    Polish the chamber.
    Polish the bolt face.
    Lubricate the bcg with grease. Oil is for grandma's sewing machine.
    Get off the gas pedal, i.e.; go easy on the propellant.
    Gas gun loads are not bolt gun loads.
    Read good books about AR's like Zediker.
     

    FCS

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    Re: AR10 issues

    Looks like a UBR stock, what buffer are you running?

    What powder in your loads and how much?

    Armalite barrel? Port is probably a bit on large side though without a measurement that is speculation.

    Adjustable gas block is never a bad thing IMHO.
     

    Grump

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    Re: AR10 issues

    Before you try anything else, what are your velocities?

    Rim lift is definitely a sign of either overpressure at the port or a timing problem or both. High port pressure may or may not equal high chamber pressure.

    Any other brands of brass? I don't have the instrumentation to check, but Hornady brass seems pretty soft-headed to me by experience, and also from multiple reports of odd behavior here on the 'Hide.

    Flattened case mouth is just getting dinged on ejection. Even .45 ACP Government models will do that, and their slide velocity is not exactly uber-fast. Pressures are also somewhere around 20KPSI, IIRC. I wouldn't worry about that, especially at that mild level.

    Also, an ejector hole which is not finished properly can have a sharper edge than is proper, resulting in shaving as you are seeing when it really should be only a bright spot. I have seen hundreds of rounds of .223 factory ammo show ejector marks on the first firing...at expected velocities and temperatures below 110F, so it's presumably safe.

    I'd try some different brass before making any conclusions here. Lapua or LC. Federal might still be soft as became the story 15-20 years ago, but I have some of that on reloading #4 and no ejector marks and no loose primer pockets from MY loads.
     

    Scarlett_Red

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    Re: AR10 issues

    I am pretty sure I have it figured out at this point. thanks all. the handguard is a Troy extreme rail. Very nice and small diameter for an ar10. Rifle has perfect balance even with a suppressor. I plan on using it for dual purpose hogs/recreational distance shooting.It has a quick detach Bobro mount and I plan to get a trijicon RMR for close range hogs. I am not sure I will get back to shooting it much til spring so I have time to work out the bugs.