6.5 Creedmoor AR10 Cycling Problem

Just-some-guy

Private
Minuteman
Dec 10, 2018
31
18
So here is my issue... I built an AR10 (DPMS style) in .308 with a Criterion 18" rifle length gas system, standard weight rifle buffer and spring, and a Toolcraft BCG; this setup runs flawlessly with pretty much all my loadings and with the factory ammunition that I've (sparingly) used.

I decided that I wanted to build another upper in 6.5 Creedmoor. I am using a Ballistic Advantage 22" Premium barrel (rifle length gas) Superflous Arms adjustable gas block, and a Toolcraft HP BCG. Since assembling this upper, and during load development I have had continued issues with cycling. The problem appears to be short cycling.

When I developed a load for this upper I began using SMK 142gr bullets with H4350 powder at 39 grains and worked my way up to 42 grains; I had short stroke issues with the lighter loadings (39.0 and 39.5), but things seemed to be reliable at higher loadings. This was all done with the gas block in the most "open" position. However, I was only testing 5 rounds of each charge weight.

Today I took the upper out to sight it in and initially it seemed to work fine with the load I chose (42 grains of H4350 / SMK 142's); ejection was initially at 3:00. After twenty rounds or so it started to short stroke and the ejection pattern moved to about 4:00. The gas block remained in the most "open" position throughout, I have also verified that the gas port is in alignment with the block.

So, here is what I am wondering about. With the .308 upper the rifle has been super reliable. With the 6.5 upper it has always appeared to be undergassed. Could my issue be the standard .308 rifle buffer and buffer spring? I am temped to change out the buffer spring to a lighter one, but I really don't want to change the parts of the lower; I thought it would be cool to have a large frame gas gun with two uppers in different calibers that could just be push pinned on and things would work...

I could go hotter on my loads, but I am already seeing signs of minor pressure, so I don't think that is the best option. I haven't done a lot of testing with any factory ammo. due to me not having any currently, but I have never had issues with developing functional ammunition across multiple calibers in the past.

Any ideas?
 
  • Like
Reactions: Brown082

celltech

Sergeant of the Hide
Full Member
Minuteman
Jan 9, 2019
426
413
6.5 Creed actually has a higher port pressure than .308...so unless the barrel has an undersized port there should be *plenty* of gas. What does the brass look like? Is there actually too much gas trying to cycle it early?
 

Just-some-guy

Private
Minuteman
Dec 10, 2018
31
18
6.5 Creed actually has a higher port pressure than .308...so unless the barrel has an undersized port there should be *plenty* of gas. What does the brass look like? Is there actually too much gas trying to cycle it early?
That's a good question, the brass looks pretty normal; not dinged up, not scuffed too bad. The primers (CCI 200) are flat with a very slight crater around the firing pin strike. I did have one pierced primer today, which has never happened to me before.

I have been theorizing that there is a lack of gas pressure based off of the ejection patterning. However, I could turn the gas block down / restrict it, and see if that helps.

What it does exactly is: I load 5 rounds in the magazine and then proceed to slow fire. The rifle cycled fine for about the first 20 rounds, then it started to fail to pick up the next round off of the magazine, so the bolt closes on an empty chamber and goes into a type 1 malfunction / "click". After I run the charging handle it will fire the next few rounds fine, then repeat the malfunction. As this happened I noticed the ejection pattern seemingly went from 3:00 to 4:00 o'clock at about 3 - 4' away.

So far, it is not a "major" problem; maybe it just was getting dirty as the wind was blowing the fine grit into the action pretty good. However, the .308 upper runs and runs under the same conditions and never complained once. On the plus side, this upper seems to shoot pretty accurately right out of the gate.
 

Just-some-guy

Private
Minuteman
Dec 10, 2018
31
18
SOAK the BCG in oil during break in
I had it fairly well lubed; there is lots of very fine sand that blows around where I shoot, so I tend to use gun grease on the carrier and BCG and it stays in place and collects less dust and grit. In my AO soaking a BCG in oil just tends to make a big mess.

It may need some break in, as I am just now nearing 100 rounds fired total.
 

bfoosh006

Sergeant
Full Member
Minuteman
  • Jun 13, 2007
    1,201
    467
    PNW
    When you say the adjustment screw is all the way out... how many full turns is that ?

    The SupArms bleed off GB can be confusing to adjust.

    This video explains it best for me..
     

    rpoL98

    USA
    Full Member
    Minuteman
    Sep 27, 2017
    143
    59
    USA
    ... I thought it would be cool to have a large frame gas gun with two uppers in different calibers that could just be push pinned on and things would work ...
    all of my large-frame lowers have the same buffer (5.3oz), same spring, and I use the adj GB on the uppers. This way I can "push pin" and swap out the uppers: 308, 6.5CM, barrel lengths from 14.5" through 24". That concept does work, and works well. I went with SLR adj GB's. But Superlative's get good reviews too, personal preference.

    I did have one BA 14.5" 308 barrel (carbine), came with a gas port of .055", ended up drilling that out to .0730" to get that one to work with my "standard lower". That was my only instance for undersized gas ports for AR-10 barrels: BA, Criterion, Faxon, Rainier, Anderson, KAK.

    OBTW, my 24" 6.5CM barrel has a gas port of 0.072" (std rifle-gas). if that's any help. Your 22", std rifle-gas 6.5CM, slightly shorter dwell-time, should be about that size, maybe slightly bigger by a few thousandths. Could be slightly smaller, but then the GB would have to be opened up proportionally.
     
    Last edited:
    • Like
    Reactions: Just-some-guy

    mtrmn

    Sergeant
    Full Member
    Minuteman
  • Oct 7, 2009
    1,230
    551
    60
    Louisiana
    I had it fairly well lubed; there is lots of very fine sand that blows around where I shoot, so I tend to use gun grease on the carrier and BCG and it stays in place and collects less dust and grit. In my AO soaking a BCG in oil just tends to make a big mess.

    It may need some break in, as I am just now nearing 100 rounds fired total.
    Let me modify my advice. Put a few drops of oil in the gas exhaust holes in the BCG every 3-4 rounds and see if that helps prevent the malfunction. This will keep the rings lubed during the break-in, and it will help troubleshoot/eliminate this possibility. If it continues to malfunction as before, then you know that's not the problem. Large frame ARs tend to need more bolt lube during break-in than the small frames do for some reason.

    Also, insure the gas tube is the proper length. It should terminate halfway in the cam pin cutout in the upper receiver. Too short and it will cause all sorts of wonky problems that make no sense at all.
     

    Attachments

    • proper gas tube length.JPG
      proper gas tube length.JPG
      32.9 KB · Views: 11
    • Like
    Reactions: Just-some-guy

    Just-some-guy

    Private
    Minuteman
    Dec 10, 2018
    31
    18
    When you say the adjustment screw is all the way out... how many full turns is that ?

    The SupArms bleed off GB can be confusing to adjust.

    This video explains it best for me..
    I'm at 18 clicks, which is supposed to be all the way open; less than 18 clicks restricts the gas, more than 18 clicks starts to bleed off the gas.
     
    • Like
    Reactions: bfoosh006

    Just-some-guy

    Private
    Minuteman
    Dec 10, 2018
    31
    18
    all of my large-frame lowers have the same buffer (5.3oz), same spring, and I use the adj GB on the uppers. This way I can "push pin" and swap out the uppers: 308, 6.5CM, barrel lengths from 14.5" through 24". That concept does work, and works well. I went with SLR adj GB's. But Superlative's get good reviews too, personal preference.

    I did have one BA 14.5" 308 barrel (carbine), came with a gas port of .055", ended up drilling that out to .0730" to get that one to work with my "standard lower". That was my only instance for undersized gas ports for AR-10 barrels: BA, Criterion, Faxon, Rainier, Anderson, KAK.

    OBTW, my 24" 6.5CM barrel has a gas port of 0.072" (std rifle-gas). if that's any help. Your 22", std rifle-gas 6.5CM, slightly shorter dwell-time, should be about that size, maybe slightly bigger by a few thousandths. Could be slightly smaller, but then the GB would have to be opened up proportionally.
    Thank you!
     

    Just-some-guy

    Private
    Minuteman
    Dec 10, 2018
    31
    18
    Let me modify my advice. Put a few drops of oil in the gas exhaust holes in the BCG every 3-4 rounds and see if that helps prevent the malfunction. This will keep the rings lubed during the break-in, and it will help troubleshoot/eliminate this possibility. If it continues to malfunction as before, then you know that's not the problem. Large frame ARs tend to need more bolt lube during break-in than the small frames do for some reason.

    Also, insure the gas tube is the proper length. It should terminate halfway in the cam pin cutout in the upper receiver. Too short and it will cause all sorts of wonky problems that make no sense at all.
    Thank you, solid tips; I will look into these things.
     

    Just-some-guy

    Private
    Minuteman
    Dec 10, 2018
    31
    18
    Let me modify my advice. Put a few drops of oil in the gas exhaust holes in the BCG every 3-4 rounds and see if that helps prevent the malfunction. This will keep the rings lubed during the break-in, and it will help troubleshoot/eliminate this possibility. If it continues to malfunction as before, then you know that's not the problem. Large frame ARs tend to need more bolt lube during break-in than the small frames do for some reason.

    Also, insure the gas tube is the proper length. It should terminate halfway in the cam pin cutout in the upper receiver. Too short and it will cause all sorts of wonky problems that make no sense at all.
    I just checked my gas tube; it is a couple millimeters shorter than the one in your picture in relation to the cut out in the upper receiver. Is there a "standard" overall length for a rifle length gas tube that I can referrence?

    Thank you.
     

    rpoL98

    USA
    Full Member
    Minuteman
    Sep 27, 2017
    143
    59
    USA
    I just checked my gas tube; it is a couple millimeters shorter than the one in your picture in relation to the cut out in the upper receiver. Is there a "standard" overall length for a rifle length gas tube that I can referrence?

    Thank you.
    BA (and Criterion) use the std AR-15 length gas tubes. Rifle length should check in at about 15-1/8".

    usually, when it's too short, it looks like this:
    0S79ZsY.jpg


    that usually happens when the barrel gas length uses the Armalite AR-10 standard (such as Faxon DPMS AR-10). But BA doesn't, they use the regular old dime-a-dozen AR-15 gas tube lengths.

    the Armalite AR-10 rifle-length gas tube is 15-1/2".
     
    Last edited:

    mtrmn

    Sergeant
    Full Member
    Minuteman
  • Oct 7, 2009
    1,230
    551
    60
    Louisiana
    BA (and Criterion) use the std AR-15 length gas tubes. Rifle length should check in at about 15-1/8".

    usually, when it's too short, it looks like this:
    0S79ZsY.jpg


    that usually happens when the barrel gas length uses the Armalite AR-10 standard (such as Faxon DPMS AR-10). But BA doesn't, they use the regular old dime-a-dozen AR-15 gas tube lengths.

    the Armalite AR-10 rifle-length gas tube is 15-1/2".
    The AR15 Rifle length gas tube is supposed to be 15-1/4" to 15-3/16". At least 1/4" shorter than the AR10 tube. If your gas tube does NOT reach the halfway point of the cam pin cutout then it is essentially too short. If the new tube reaches PAST the halfway point a little bit you can get by with that. My mid-length (11-3/4" AR15 length) tube was too short on my 308 and I had to locate a longer "Armalite AR10 carbine length" tube (12-1/16") for my Faxon 16" barrel. This simple change solved a world of weird problems I was having.
     
    Last edited:

    bfoosh006

    Sergeant
    Full Member
    Minuteman
  • Jun 13, 2007
    1,201
    467
    PNW
    Gas tube length wise, FWIW, all my PA10's and PA65's use the AR15 length gas tubes... never had any issues with those "shorter" lengths.
    Various gas system lengths and barrel lengths., various barrel manufacturer's .
    Criterion, Ballistic Advantage ( same barrel / caliber you are using ), PSA / DC machine, Faxon, Krieger ( only one with smaller, more practical gas port size than the others )

    Yours might prefer the longer Armalite length... hard to say.

    I do use heavy, for the chosen system, buffers ... 5.3oz 2.5" for DPMS pattern carbine , 5.4oz 3.25" for Armalite pattern carbine , and a 9.3oz 308 rifle length buffer... and Tubbs 308 Flatwire recoil springs in all of them.

    What is the weight of your 308 lower buffer and what brand recoil spring ?

    All mine have SLR Adj. GB's... all are "happy" at 6 clicks open from the available 15 clicks fully open.

    I am a fan of Adj. GB's and heavy buffers / Tubbs 308 Flatwire recoil springs

    At this point , maybe the grease is causing an issue ? I would try it with normal rifle oil and see if that helps.

    I suggest triple ( ? ) checking the gas system is all snug, looking for any possible leaks.

    Also , next time it gets sluggish... do the single mag fed, fire , lock back of the BCG test.... maybe repeatedly to see if the BCG is short stroking.

    What mags are you using ?

    And does it do it with factory ammo ? ( sorry if I missed that )
     
    • Like
    Reactions: Earnhardt

    Just-some-guy

    Private
    Minuteman
    Dec 10, 2018
    31
    18
    A
    Gas tube length wise, FWIW, all my PA10's and PA65's use the AR15 length gas tubes... never had any issues with those "shorter" lengths.
    Various gas system lengths and barrel lengths., various barrel manufacturer's .
    Criterion, Ballistic Advantage ( same barrel / caliber you are using ), PSA / DC machine, Faxon, Krieger ( only one with smaller, more practical gas port size than the others )

    Yours might prefer the longer Armalite length... hard to say.

    I do use heavy, for the chosen system, buffers ... 5.3oz 2.5" for DPMS pattern carbine , 5.4oz 3.25" for Armalite pattern carbine , and a 9.3oz 308 rifle length buffer... and Tubbs 308 Flatwire recoil springs in all of them.

    What is the weight of your 308 lower buffer and what brand recoil spring ?

    All mine have SLR Adj. GB's... all are "happy" at 6 clicks open from the available 15 clicks fully open.

    I am a fan of Adj. GB's and heavy buffers / Tubbs 308 Flatwire recoil springs

    At this point , maybe the grease is causing an issue ? I would try it with normal rifle oil and see if that helps.

    I suggest triple ( ? ) checking the gas system is all snug, looking for any possible leaks.

    Also , next time it gets sluggish... do the single mag fed, fire , lock back of the BCG test.... maybe repeatedly to see if the BCG is short stroking.

    What mags are you using ?

    And does it do it with factory ammo ? ( sorry if I missed that )
    All good information!

    I am currently running a standard KAK 5.4 oz rifle buffer and the spring they sell for the AR10, aka nothing special.

    Using oil may be a solution as the grease and the accompying grit that built up may be slowing the carrier down enough to cause problems; I didn't seem to have this issue when I was load testing last time out. I will try oil next time as suggested and see if there is any change.

    The gas system is what I suspect the problem is; I have moved the gas block a few times trying to ensure the port is properly aligned. This has been challenging since I don't have an air compressor to blow through the tube as I adjust it, so trying to get optimal alignment has been challenging. I don't see any evidence of leaking gas at the gas block. I had a similar problem with a small frame AR awhile back and it was a leaking gas block. I will have to re-check this if the other solutions fail to work.

    I am using Pmags and firing my own ammo. I have been looking for some factory ammo to try out on my next outing but it is both scarce and expensive!

    Question: What effect does increasing weight to the buffer have on cycling? Does a heavier buffer cycle with less gas pressure, or more? In other words, if the system is undergassed will increasing buffer weight help, or hinder the operation. Same for buffer spring, I would assume that a lighter spring would allow the system to cycle with less gas pressure.

    Thank you for your advice, it definitely gives me some things to look at.
     
    Last edited:

    TonyTheTiger

    Like a Boss
    Full Member
    Minuteman
    Feb 14, 2017
    2,885
    3,147
    Question: What effect does increasing weight to the buffer have on cycling? Does a heavier buffer cycle with less gas pressure, or more? In other words, if the system is undergassed will increasing buffer weight help, or hinder the operation. Same for buffer spring, I would assume that a lighter spring would allow the system to cycle with less gas pressure.
    The heavier the reciprocating mass and/or spring the more gas you need to run it. So if you're already undergassed you either need lighter components, hotter ammo or a bigger gas port
     

    rpoL98

    USA
    Full Member
    Minuteman
    Sep 27, 2017
    143
    59
    USA
    is your Toolcraft BCG nitride (or phosphate), or nickel boron? a while back I had some problems with multiple NIB 308 BCGs (all the same brand, but not Toolcraft), sticking, that was alleviated by replacement with nitride BCG, same brand. the same phenomena doesn't occur with any of my AR-15 NIB BCG's. it's odd, but, oh well. I think it came down to how the coating was applied.

    but I still think your hole is too small. can you get a measurement of your gas port?
     

    Just-some-guy

    Private
    Minuteman
    Dec 10, 2018
    31
    18
    I think your hole is too small.
    This is what I am starting to believe. Since I have an adjustable gas block, opening the barrels gas port slightly shouldn't have any negative effect because I can ultimately control the gas flow with the gas block. Correct? In other words, there really is no downside to opening the port slightly for experimentation purposes in this instance.
     

    TonyTheTiger

    Like a Boss
    Full Member
    Minuteman
    Feb 14, 2017
    2,885
    3,147
    This is what I am starting to believe. Since I have an adjustable gas block, opening the barrels gas port slightly shouldn't have any negative effect because I can ultimately control the gas flow with the gas block. Correct? In other words, there really is no downside to opening the port slightly for experimentation purposes in this instance.
    Correct. Don't overdo it.
     
    • Like
    Reactions: Just-some-guy

    rpoL98

    USA
    Full Member
    Minuteman
    Sep 27, 2017
    143
    59
    USA
    This is what I am starting to believe. Since I have an adjustable gas block, opening the barrels gas port slightly shouldn't have any negative effect because I can ultimately control the gas flow with the gas block. Correct? In other words, there really is no downside to opening the port slightly for experimentation purposes in this instance.
    please make sure you have the wire-gauge drill bits. just having the fractionals just doesn't ...
     

    bfoosh006

    Sergeant
    Full Member
    Minuteman
  • Jun 13, 2007
    1,201
    467
    PNW
    Should you decide to drill... make sure you don't over drill into the rifling on the opposite side.

    And drill bits that small are easy to break.... so go slow at the drill press.

    Drill bit size chart...
    https://www.custompartnet.com/drill-size-chart

    You might consider e-mailing BA , and asking what your gas port size is on that specific barrel.

    I looked to see where I wrote my same barrels gas port size.. but can't find it.
    Don't hold me to it... but I want to say it was around .088- .093

    If need be.. I could loosen the gas block and measure it again for the sake of just doing so.

    The gas system is what I suspect the problem is; I have moved the gas block a few times trying to ensure the port is properly aligned. This has been challenging since I don't have an air compressor to blow through the tube as I adjust it, so trying to get optimal alignment has been challenging. I don't see any evidence of leaking gas at the gas block. I had a similar problem with a small frame AR awhile back and it was a leaking gas block. I will have to re-check this if the other solutions fail to work.
    Also... while this sounds cheesy.. consider the dry piece of uncooked spaghetti noodle trick for a reasonable gas block alignment.

    MAKE SURE you get the 2 pieces of spaghetti out !!



    Drill bit size chart...
     
    • Like
    Reactions: Just-some-guy

    Just-some-guy

    Private
    Minuteman
    Dec 10, 2018
    31
    18
    Update:

    So here is what is new to this thread; I removed the gas block and found no evidence of gas leaking.

    The gas tube measures 15 1/8" long and appeared to be lined up properly, gas block looked to be in proper alignment as well.

    The gas port was difficult to get an accurate measurement as I don't have an unlimited amount of tools, but it was originally smaller than .0076" as that size drill bit would not fit into the port. I estimate that the port was originally from the factory somewhere around .0072 or .0073" I then successfully enlarged the port to .0076".

    The bolt carrier did have some stiction due to my lubing it with grease so I re-lubed with light gun oil and it travels smoothly and the bolt locks up to the barrel extension lugs smoothly.

    The bolt carrier travels well back into the extension tube as normal and travels smoothly.

    I re-aligned the gas block using the "spaghetti noodle" trick and reassembled everything. All that is left to do is test it out and see if things have improved or if I ruined the barrel... Upon reflection, with as hot as the loads that I was firing last trip out were (41.8 gr. of H4350 under 142 gr SMK's at 2800 fps; near max loading) I shouldn't have had any sluggish bolt conditions. The primers showed that there was high pressure; the system should not have needed the gas block wide open to run that hot of a load. Hopefully now I can have a bit more gas and bleed off some excess with the adjustable gas block. I will report back with results after I get a chance to test, hopefully in a few days.
     
    Last edited: