AR-10 Short-stroking With Scope Mounted, Zero Issues Without

Snuby642

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  • Feb 11, 2017
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    An agb is not going to give you more gas. If your gas block is not over your gas port strait that is a thing.

    @Yondering has the best idea about checking your buffer / sping and tube to see they match properly. No special tools needed for that just don't trash the detent pin or tube.

    It would be easy enough to get the wrong part in a kit from any manufacturer these days.

    Fast food can't even make a taco properly any longer.
     

    baddoggy

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    Try lubing your bcg. I had similar issues with new builds until I disassembled the bcg and lubed it with oily fingers.
     
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    Eustice

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    It sounds like you’re on the edge of getting too much gas to cycle the action.

    Once you mount the scope, the added mass of the rifle/optic combo resists being moved as fast, and the cyclic rate is out-running the magazine lift, causing bolt-over-base malfunctions. This is common on guns that are gassed too hard.

    Your biggest problem is this:

    * Bear Creek Arsenal 20" barrel

    Replace that with a real barrel with a correct gas port diameter and chamber dimensions. Your gas block is mounted correctly as the shadow shows coverage of the port.

    I would expect to find pretty much every BCA barrel out of acceptable specs in some way, whether it be chamber dims, gas port, bore diameter, gas block journal diameter, thread concentricity, you name it.
    Nice write up. Over-gassed has a way of masquerading as under-gassed like you said. The bolt is outrunning the magazine lift. Extra weight from the scope causes all the bolt motion to be motion without the recoiling rifle counteracting it.

    My shooting partner had the set screw back out of his AGB before a match. He was getting failures to load and sometimes the ejected round would stovepipe. We diagnosed by taking video of the ejection port in slow motion of him shooting. Otherwise we’d have kept going with the under-gassed idea.
     

    LRRPF52

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    Try lubing your bcg. I had similar issues with new builds until I disassembled the bcg and lubed it with oily fingers.
    AR-15s and AR-10s love to run wet, but more lubrication will only exacerbate his problem because it will reduce the friction coefficient and allow an even faster cyclic rate.

    If my limited internet diagnosis is correct (and I’m totally open to the fact that I may be in error), he needs to reduce the cyclic rate. I suspect this barrel is ported too large.

    If the OP is hellbent on holding onto a BCA barrel, adjustable gas might actually be the best band-aid for this problem.
     
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    Mario1961

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    Trial and error shouldn’t be the method of fixing one of these, and doesn’t need to be if you take time to understand the system and the effect that each part has.

    Band-aid fixes like using a hydraulic buffer to fix a cycling issue is cringeworthy. If the rifle cycles sluggishly, figure out what’s wrong and fix that. These are not finicky rifles if you have the right combination of parts and tune it well.

    You might have a short 308 length buffer in a long 308 buffer tube, at a guess. That can cause some weird issues because the buffer doesn’t bottom out in the tube as it should.
    Not my first rodeo and I understand that system quite well. I have built quite a few of these. Is not tuning and finding the right combination of parts a process of trial and error? Every discipline in science has trials to work out any difficulties, take aviation for example. The P51 Mustang became an amazing airplane when they put in a Rolls Royce Merlin engine. Trial and error is basic to any type of learning process. I have the correct size buffer and tube . That is pretty basic. I had this rifle at the range with lots of competitive old timers that have been around guns their entire lives. They were looking at it as well. All the parts I have in this are qood quality and to spec. Lots of people have had issues with AR10’s . They can be finicky. If changing out a buffer and it works how is that cringe worthy? Sometimes the correct weight buffer for that particular rifle works, it can be as simple as that. This forum is supposed to be a platform to share Ideas and help each other out. Using words like cringe worthy and band aid fixes come across as arrogant. I appreciate the response but why the attitude?
     

    LRRPF52

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    Not my first rodeo and I understand that system quite well. I have built quite a few of these. Is not tuning and finding the right combination of parts a process of trial and error? Every discipline in science has trials to work out any difficulties, take aviation for example. The P51 Mustang became an amazing airplane when they put in a Rolls Royce Merlin engine. Trial and error is basic to any type of learning process. I have the correct size buffer and tube . That is pretty basic. I had this rifle at the range with lots of competitive old timers that have been around guns their entire lives. They were looking at it as well. All the parts I have in this are qood quality and to spec. Lots of people have had issues with AR10’s . They can be finicky. If changing out a buffer and it works how is that cringe worthy? Sometimes the correct weight buffer for that particular rifle works, it can be as simple as that. This forum is supposed to be a platform to share Ideas and help each other out. Using words like cringe worthy and band aid fixes come across as arrogant. I appreciate the response but why the attitude?
    I can’t speak for anyone else but I’m genuinely trying to be helpful based on my experiences with AR-10s dating back 20 years now.

    I have to disagree with the statement about quality parts to spec, since BCA is not the company one would associate with those things. They’ve been cranking out tent stakes for years now at bottom basement prices based on a volume business model with minimal attention to detail.

    If you could measure the diameter of your gas port on that barrel, that would be helpful. As soon as you mentioned that it runs fine without optics, but not with optics, that pointed me in the direction of a mass issue on the edge of maximum allowable cyclic rate.

    A lower mass rifle will travel to the rear more, which takes away relative travel distance and velocity from the BCG. If the rifle is on the edge of maximum cyclic rate, that can allow it to still feed.

    Once you add mass to such a configuration, it resists recoiling to the rear as fast, so the BCG travels faster in the raceway and can exceed the magazine’s ability to lift the cartridge stack into presentation.

    That is my best guess as to what’s happening with your rifle.

    If you slow down the cyclic rate by choking the gas, it should fix this problem.

    I would also ensure that you have the correct action spring for your buffer length and RET.

    There are 3 different RET lengths for the AR-10 size rifles.

    AR-15 carbine length with heavier AR-10 carbine spring, and tiny little short LR-308/AP4 buffer.

    ArmaLite A5 mid length RET with heavy/longer carbine spring and AR-15 carbine length buffer.

    AR-15 rifle length RET with AR-10 heavy/longer rifle spring and AR-10 rifle buffer (slightly shorter than an AR-15 rifle buffer).

    Even for those who are well-versed on the AR-15 platform, the AR-10 can be a formidable minefield of parts combinations that will assemble together, but won’t function correctly.
     

    Mario1961

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    I can’t speak for anyone else but I’m genuinely trying to be helpful based on my experiences with AR-10s dating back 20 years now.

    I have to disagree with the statement about quality parts to spec, since BCA is not the company one would associate with those things. They’ve been cranking out tent stakes for years now at bottom basement prices based on a volume business model with minimal attention to detail.

    If you could measure the diameter of your gas port on that barrel, that would be helpful. As soon as you mentioned that it runs fine without optics, but not with optics, that pointed me in the direction of a mass issue on the edge of maximum allowable cyclic rate.

    A lower mass rifle will travel to the rear more, which takes away relative travel distance and velocity from the BCG. If the rifle is on the edge of maximum cyclic rate, that can allow it to still feed.

    Once you add mass to such a configuration, it resists recoiling to the rear as fast, so the BCG travels faster in the raceway and can exceed the magazine’s ability to lift the cartridge stack into presentation.

    That is my best guess as to what’s happening with your rifle.

    If you slow down the cyclic rate by choking the gas, it should fix this problem.

    I would also ensure that you have the correct action spring for your buffer length and RET.

    There are 3 different RET lengths for the AR-10 size rifles.

    AR-15 carbine length with heavier AR-10 carbine spring, and tiny little short LR-308/AP4 buffer.

    ArmaLite A5 mid length RET with heavy/longer carbine spring and AR-15 carbine length buffer.

    AR-15 rifle length RET with AR-10 heavy/longer rifle spring and AR-10 rifle buffer (slightly shorter than an AR-15 rifle buffer).

    Even for those who are well-versed on the AR-15 platform, the AR-10 can be a formidable minefield of parts combinations that will assemble together, but won’t function correctly.
    I agree, the AR10 is definitely a minefield of parts combinations lol
     

    Des_Draws

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    If the rifle is truly short stroking ( last round not locking back ) and Since this is a new build, Then Check the gas port diameter

    here's a table of gas port sizes
    2CD1D5E3-59B5-4CCC-9230-E46B46307B69.png

    If the port dia is undersized or if all other indications (buffer, bcg, gastube, gas block etc )check out ok Then consider drilling out gas port toward the max dia and install a AGB.
    Most likely the gas system is marginally under gassed if scope (short stroke) no scope ( acceptable cycling )

    I have run into the undergassed issue with lower quality barrels. Drilling out the gas port and using a AGB fixed it to
    95% reliable cycling - which is good enough for my purposes

    don't attempt if you do not feel 100% confident Or do not have the proper tools ie drill bits
     

    LRRPF52

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    If the rifle is truly short stroking ( last round not locking back ) and Since this is a new build, Then Check the gas port diameter

    here's a table of gas port sizesView attachment 7610009
    If the port dia is undersized or if all other indications (buffer, bcg, gastube, gas block etc )check out ok Then consider drilling out gas port toward the max dia and install a AGB.
    Most likely the gas system is marginally under gassed if scope (short stroke) no scope ( acceptable cycling )

    I have run into the undergassed issue with lower quality barrels. Drilling out the gas port and using a AGB fixed it to
    95% reliable cycling - which is good enough for my purposes

    don't attempt if you do not feel 100% confident Or do not have the proper tools ie drill bits
    I think It’s doing the opposite of short-stroking and is already gassed too hard, otherwise it wouldn’t function when lighter, then bolt overrides when heavier (with optics).
     

    2aBaC̶a̶

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    Either way an adjustable gas block is beneficial. If its over gassed, an agb will likely fix it. If its currently under gassed finding the fix will likely raise it to a point it requires some tuning.
     

    2aBaC̶a̶

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    6.5cm
    16" Wilson Combat
    .75" Dia
    .075" port
    5.125" from Muzzle
    13.125" gas tube
     
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    TonyTheTiger

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    Please elaborate
    What caliber is it for? Intended ammo? What about the endless combinations of barrel length and gas system length, such as the super common 16" mid gas barrels, that aren't listed? What about .936 gas journal barrels? Does it take into account any spring rates, buffer weights, bcg weights other than mil spec? There's literally a million variables, and that table lists possibly ideal gas ports for 9 of them and misses all the most common ones.
    And again, if it's for 5.56, which is likely, it probably means nothing in a 308 thread.
     

    Des_Draws

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    I hear ya
    never trust one source If it's important to you check around. There are recommended port dia for different calibers on the web
    The one I showed was for .308 but I used it for a 6.5 CM build( similar pressure )
    Worked for me and solved a vexing problem but its only a guide. I worked up slowly to just slightly below max dia for 20 in barrel.
    It took me a lot of troubleshooting and actually drilling out the port was a last resort. It taught me what a lot of folks already know: for cycling issues start at the gas port and work backwards instead of randomly changing buffers, springs bcgs
     

    msgriff

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    Normally I would have gone with over-gassed, as that condition is often confused with under-gassed, but I remember similar issues (not mine) with the 16" Recon Tactical being solved by upping the port after consulting Wilson. As a note, a de-burr tool is typically used on the bore side of the port by those that do it correctly.
     

    shafjess

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    Interesting that I recently started a AR10 build and just before I start putting it all together I get an email from the Hide with this as a popular post. I have pretty much the same style build with it being a Aero Precision upper and lower and a Bear Creek Barrel. And sure enough, the gas tube that I had originally purchased, short. The way the gas block lines up on the barrel is exactly as stated, like it needs to be held off the stop of the barrel just a bit. So just wanted to say thank you to you all. I have since purchased and have ready for install the Wilson Combat rifle length AR10 gas tube and an adjustable gas block from Texas AR. So glad I was able to come across this thread prior to putting everything together, would have probably been in the same boat as the OP. Thanks again.