AR10 Last Round Hold Open Issue - Updated 02/11/2019

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  • thperez1972

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    So I built my first AR-10. It's mostly Aero Precision for compatibility reasons, upper (including parts kit), lower (including parts kit), BCG, and carbine buffer kit. It has a Ballistic Advantage 14.5" barrel with a pinned muzzle device. Because it was pinned before I realized I hadn't thrown the barrel nut on there, I am using a Yankee Hill Machine Specter 2 piece gas block (YHM-9378). I put a few rounds of 145gr and 150gr to test it out using both PMAGs and HexMags. In any combination of rounds and magazine, the rifle cycles fine but the bolt does not lock back on the last round. I've manually pulled the bolt back and it does reach the bolt release but there seems to be a good bit of added pressure needed to go from the point where the next round will pick up to where the bolt will catch.

    At this point I'm trying to avoid an adjustable gas block. If that's the only true fix, so be it. But I'm civil service and don't make bank so if I can spend that $130 on bills instead of a 2 piece adjustable, I won't complain. I've also read a number of suggestions for a fix. Switch out the 308 buffer spring for a 223 buffer spring. The 223 has tighter coils and I would suspect the buffer tube/bcg wouldn't be able to travel as far back so that would seem like it would make the problem worse. Another suggestion was to take fine sandpaper to the rails on the bcg so it has a bit more room/less friction to travel. But the bcg doesn't seem to have an issue. The other suggestion was to cut a loop off the buffer spring to allow more travel, and slightly less resistance, for the bcg.

    This rifle will host a can once it's finally released (8 months and counting so far).

    *I realize AR-10 is Armalite and Areo uses the DPMS LR-308 pattern. I'm using AR-10 in the generic.
     
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    Saw

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    Wait until you get your can. I have found (and many others) that the AR-10 (generic term) is harder to suppress than a 5.56 gun. Meaning it is tougher to get them to run consistent with/without can and with different rounds. Your can will likely change the dynamic significantly on that rifle.
     

    sportsbud

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    Two things I can think of... Are you sure you have the correct DPMS pattern buffer it should be 3/4" shorter than a standard AR15 buffer.

    Second is that the gas block needs adjustment. Your ejection should be about 2-3 o'clock. 1 o'clock is way over gassed. Also 4-6 o'clock is undergassed. My Ar10's needed some finagling if the barrel was not dimpled....

    If changing position of the gas block does not help change the gas block...
     

    Magdump

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    I’m assuming you’ve got the correct spring and buffer from what you stated about the build, but I’ve seen people use a carbine buffer in a rifle stock and vice versa. Not saying you don’t know what you’re doing but you did state first 08 build. Gas (or lack of) is always a strong possibility when dealing with BCG travel issues. But if the rifle is cycling and peeling the next round from the mag without fail, chances are the BCG is traveling far back enough to lock on the empty mag. It may clear up after break in. Also try shooting the rifle in a sled or hug a post and fire the gun with the post between your shoulder and the butt stock with one round in the mag. I’ve got 2 pistols that a few other people can’t seem to shoot without a failure to cycle the next round and it never happens for me. Once they get a firm grip and bow up to the recoil it functions fine.
     

    thperez1972

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    It is the shorter buffer. And it's a carbine length spring in a carbine length buffer tube. And hopefully one of my cans will come in soon (one ais at 6 months, the other at 8 months). I'm going to pull off the gas block to make sure it's lined up correctly.
     

    Magdump

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    It is the shorter buffer. And it's a carbine length spring in a carbine length buffer tube. And hopefully one of my cans will come in soon (one ais at 6 months, the other at 8 months). I'm going to pull off the gas block to make sure it's lined up correctly.
    Sounds like a plan. Hopefully you’ll get it sorted out before you get your suppressor in case you’re planning on shooting subs. And if you do, try the Sellier&Bellot. They work great in mine.
     

    sportsbud

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    Since you have DPMS pattern lowers your welcome to meet up and swap with my known good DPMS pattern lowers and uppers. Always love an excuse to break out the 10.5" AR10 pistol lol.
     

    SpeedRacer

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    If it's undergassed, which it sounds like it may be, an adjustable gas block won't help. I've had some bad luck with the YHM 2-piece gas blocks, make sure you've got it perfectly aligned over the gas port and the screws properly torqued with thread locker. The suppressor will add a little more backpressure and likely fix the issue, but I'd still try to get it running right unsuppressed as well.
     

    Expert684

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    I have a DPMS AR-10 (18") I bought new, it had "feeding problems" thought it was under-gassed, turns out it was over-gassed. I drilled out the gas port in the barrel. Seems as thought port size changes with location due to barrel length. I have two weapons, one with an 18" and one with a 24" barrel. If I remember 18" had a .063" hole and 24" maybe like a .078" hole. I drilled the .063" out to .078" and installed a JP industries adjustable regulator. Shut it off, put one round in the mag fired it, opened the the regulator until it locked back. Never had problems with since. Here is the funny part, since I put the regulator on the gun It literally tripled the accuracy of the gun. I have made multiple trips to the range over the last few years with it thinking the groups the gun shot was a fluke. Summer, winter, gun clean or dirty it will shoot .750" (.375 MOA) groups all day at 200 yards. This is no B.S. I have never seen any thing like it?
     
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    thperez1972

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    Since you have DPMS pattern lowers your welcome to meet up and swap with my known good DPMS pattern lowers and uppers. Always love an excuse to break out the 10.5" AR10 pistol lol.

    Sounds good. The next time I'm in NO, we can go out to the lower 9th ward and shoot at the burned out cars.

    If it's undergassed, which it sounds like it may be, an adjustable gas block won't help. I've had some bad luck with the YHM 2-piece gas blocks, make sure you've got it perfectly aligned over the gas port and the screws properly torqued with thread locker. The suppressor will add a little more backpressure and likely fix the issue, but I'd still try to get it running right unsuppressed as well.

    That's the ultimate goal.

    I have a DPMS AR-10 (18") I bought new, it had "feeding problems" thought it was under-gassed, turns out it was over-gassed. I drilled out the gas port in the barrel. Seems as thought port size changes with location due to barrel length. I have two weapons, one with an 18" and one with a 24" barrel. If I remember 18" had a .063" hole and 24" maybe like a .078" hole. I drilled the .063" out to .078" and installed a JP industries adjustable regulator. Shut it off, put one round in the mag fired it, opened the the regulator until it locked back. Never had problems with since. Here is the funny part, since I put the regulator on the gun It literally tripled the accuracy of the gun. I have made multiple trips to the range over the last few years with it thinking the groups the gun shot was a fluke. Summer, winter, gun clean or dirty it will shoot .750" (.375 MOA) groups all day at 200 yards. This is no B.S. I have never seen any thing like it?

    So the idea it to purposely over-gas the rifle so it can be regulated by the adjustable gas block?

    What gas block are you using? They make one of the only 2 adjustable 2 piece gas blocks I could find.
    https://www.jprifles.com/buy.php?item=JPGS-9D
     

    dougstump

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    I installed that adjustable gas block on a friends AR-10. He's trying to find the one magic setting that will allow him to shoot:
    1. Supersonic un-suppressed
    2. Supersonic suppressed
    3. Subsonic suppressed

    "I don't think so Tim!"
     

    Gator 45/70

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    Thanks but I have to have a 2 piece. The muzzle device is pinned. In retrospect, I should have just gone with a 16" instead of a 14.5" barrel. That extra 1.5" is turning into an expensive pain.

    That's the one I'm using,Had the same problem with the can attached,Severely over-gassed so much that the casing would do a 180 in the chamber!
     

    Saw

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    I installed that adjustable gas block on a friends AR-10. He's trying to find the one magic setting that will allow him to shoot:
    1. Supersonic un-suppressed
    2. Supersonic suppressed
    3. Subsonic suppressed

    "I don't think so Tim!"

    I (and many others) have chased that unicorn on an AR-10. My recommendation would be to find a better use of your time. Plenty of recipes that work on 5.56 and 300BLK...not so much on the big boy. I thought I had a sweet spot that works for suppressed and un-suppressed supers and that turn out to be round specific (168 HPBT match...good rounds for it to work).

    Perez, I dont know what can you are getting but make sure it is warrantied and rated for 14.5* barrel on 308s. FWIW mine is 16* min for that round.
     

    thperez1972

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    I (and many others) have chased that unicorn on an AR-10. My recommendation would be to find a better use of your time. Plenty of recipes that work on 5.56 and 300BLK...not so much on the big boy. I thought I had a sweet spot that works for suppressed and un-suppressed supers and that turn out to be round specific (168 HPBT match...good rounds for it to work).

    Perez, I dont know what can you are getting but make sure it is warrantied and rated for 14.5* barrel on 308s. FWIW mine is 16* min for that round.

    I have 2 cans, a Dead Air Sandman S at 8 months with the ATF (and pending as of a call today to check the status) and a SilencerCo Omega at 6 months. The Omega has a restriction od 16" for a 308. The Sandman has no barrel restriction. I recently reached out to Dead Air on that issue before this build:

    "I have a Sandman S set to be released from NFA Jail any day now. I'm currently starting an AR10 build and will be getting a Dead Air muzzle device for the build. Being relatively new to the world of suppressors, I have a question about barrel length. I know the Sandman S has no barrel length restriction but how does wear and tear relate to the barrel length? This will mainly be a range/farm use rifle. I don't have plans for long range shooting with this rifle so I don't need a longer barrel for accuracy at distance. The talk is a pin and welded 14.5" 308 barrel is too harsh on cans. I'd prefer to stay 18" and go as short as possible while still considering the life of the can. Any help in this matter would be greatly appreciated."

    Their reply:

    "We use stelite for our baffles on the Sandman series of silencers. We have multiple silencers with 30-40k rounds through them with no wear on the baffles at all. We also have never had a silencer come back to warranty for baffle erosion. You should be good to go with either barrel length and we do mean no barrel length restriction. I hope this helps and please let me know if you have any other questions!

    Thanks,
    Tasson Nique
    Dead Air Silencers"
     

    Saw

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    Good for Deadair. I want one next. Seriously.

    My Omega (stellite baffles) has maybe 3000 rds thru it. Maybe 2-300 .308s from a 16* barrel with brake and couple thou of 5.56 thru a 10.5* barrel with FH and I can see some erosion. It isn’t bad and doesn’t effect a thing, but I can see it.

    If you don’t want to chase unicorns all day with the 14.5* AR-10, pick your primary configuration and get that 100% and then go backwards to the next. Getting a big block to run reliable both with and without can and without adjustable gas is going to be a challenge. Add in subs and you have your work cut out for you. Gas, spring, BCG moving mass. Only adjust one at a time.
     

    Expert684

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    So the idea it to purposely over-gas the rifle so it can be regulated by the adjustable gas block?

    What gas block are you using? They make one of the only 2 adjustable 2 piece gas blocks I could find.

    I goofed a little, JP is on another gun, I'm pretty sure it's something else. I had to machine it down a little to fit inside my rail, so it has been blasted and re-finished, but it is a 1 piece low profile block. The gun went from one of my most hated guns to one of my most effective overnight.
     

    thperez1972

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    While the government was shut down and Form 4's were on hold, I went ahead and ordered an adjustable gas block from Witt Machine. I finally went by Jim's to pick up a gas tube so I wouldn't have to unpin the one from the other gas block and got the thing installed. I closed it off and opened it a half turn at a time until the rifle would eject. It had to be opened quite a bit before that happened. I then tried to open it until it loaded the next round. I got to the point where the screw was loose and almost out and it still would not cycle. Can I now assume it is undergassed rather than overgassed? If so, should I open up the gas port a bit and try again?

    https://www.wittmachine.net/750-two...profile-gas-block.html?id=22804960&quantity=1

    750-two-piece-adjustable-low-profile-gas-block.jpg
     

    sportsbud

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    While the government was shut down and Form 4's were on hold, I went ahead and ordered an adjustable gas block from Witt Machine. I finally went by Jim's to pick up a gas tube so I wouldn't have to unpin the one from the other gas block and got the thing installed. I closed it off and opened it a half turn at a time until the rifle would eject. It had to be opened quite a bit before that happened. I then tried to open it until it loaded the next round. I got to the point where the screw was loose and almost out and it still would not cycle. Can I now assume it is undergassed rather than overgassed? If so, should I open up the gas port a bit and try again?

    https://www.wittmachine.net/750-two...profile-gas-block.html?id=22804960&quantity=1

    750-two-piece-adjustable-low-profile-gas-block.jpg

    Yep sounds undergassed, time to open up that gas port a tad... This reminds me that I need to get a extra power hammer spring for my AR10... 7.62 surplus seems to have one to two a magazine that are stubborn and require a second try... May try and make the firing pin protrude a little more as well...
     
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