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Thread: My first AR: 7.62x39 with Superlative Piston kit - Learning from failure...

  1. #1
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    My first AR: 7.62x39 with Superlative Piston kit - Learning from failure...

    UPDATE: learned from my failure and abandoned the piston system in favor of DI for this rifle as you can read about in later posts.

    Hello everyone,

    This is my first post to this forum - I've been lurking for a few weeks now and have learned a lot - thank you to everyone for the knowledge you've shared! I - being an idiot - decided to wait until this year to buy my first gun (a Ruger EC9S) and get my concealed carry permit. I did some shooting growing up, but as an adult never got around to spending the $$ to get into shooting until now. The current societal conditions have finally convinced me that being able to defend myself and my household are a necessary investment...plus I love shooting

    All of which leads me to my AR build. I wanted a general-purpose sort of rifle for home defense/SHTF/plinking, so I started doing research and quickly landed on the AR as the obvious choice. After looking at the various caliber options I settled on 7.62x39 as the only reasonable option - it's the only caliber that I can find ammo for at a price that won't bankrupt me - if the insanity ever dies down I'll eventually build up a .223 AR, but since I can't buy ammo for one right now there isn't much point in building an unusable gun...

    Anyways, here's a quick summary of my build:
    • Aero Precision M4E1 (not enhanced) upper and complete lower
    • Dez Arms 7.62x39 barrel: 18", mid-length gas, fluted HBAR
    • Superlative Arms Piston kit
    • Faxon 7.62x39 bolt with mil-spec firing pin per Faxon's recommendation
    • UTG Pro M-LOK 10" handguard
    • Radian Raptor ambi charging handle
    • Mil Surplus carry handle
    • Generic clamp-on gas block for a front sight base
    • UTG fixed front sight


    Here's a couple pictures and then I'll get on to the "failure" part of this build and ask a couple questions that I'm hoping you all can help me with.

    IMG_20201022_070923134 (Large).jpg

    IMG_20201022_071016135 (Large).jpg

    You might notice in the first pic that there's no front sight...I wasn't paying attention when I ordered the FSB and ended up with a rail-height FSB and a gas block-height front sight. Was also disappointed to see that unlike the US-made UTG handguard, the UTG front sight is made in China. I think I'm going to go with a Daniel Defense front sight to replace the too-tall UTG. For my first test firing, I didn't want to wait, so I made one

    IMG_20201022_135119696 (Large).jpg

    IMG_20201022_135130338 (Large).jpg

    As bailing-wire-and-bubble-gum as it is (I spent a whole 15 minutes on it...), the makeshift front sight worked great, and my first shot (at about 10 yards) was dead center horizontally and about 2" low - good enough for now.

    The minor sight SNAFU notwithstanding, the real issue with this build is my combination of caliber, barrel, and piston kit - it doesn't work. I didn't realize until recently (being a noob...) that the 7.62x39 cartridge produces lower gas pressure than the .223/5.56. I should have contacted Superlative Arms to get their input before ordering a barrel - I now know they recommend a pistol-length gas system for all 7.62x39 builds whereas my barrel is mid-length. I specifically bought this barrel because I wanted the mid-length gas system after reading about how great it is on a bunch of different websites - not realizing that this was specific to the .223/5.56 caliber. My gun will only half-cycle the bolt after each shot - it's way undergassed.

    I've seen a blog online where someone has had success using the Superlative Arms piston kit on a 7.62x39 gun with a 16" carbine-length gas barrel, but that's the only reference build I've been able to find online. My initial hope was that I could open up the gas port on my barrel and solve the issue by increasing flow, but I pulled the piston gas block off and my gas port is 0.125", which is considerably larger than the gas port in the Superlative gas block.

    Here are the possible solutions that I see moving forward:
    1. Enlarge the gas port on the piston kit gas block to increase flow to the piston. Try a lightweight buffer spring.
    2. Buy a new barrel
    3. Buy a traditional DI gas block, tube, and bolt carrier (give up on the piston kit for this rifle)


    I'm leaning towards option #1 because it's the quickest, easiest, and cheapest option. If it doesn't work I've potentially trashed a spendy part of the piston system, but I figure if I drill it out and still can't get proper actuation with my current barrel, the gas block would probably work OK with a carbine length gas barrel. It's all speculation at this point though...

    Options 2 and 3 are difficult as I've spent my budget and don't really have the money for them. I could sell my current parts and make it work, but that'll take a bit of time. For option 2 I haven't been able to find an off-the-shelf 16"+ barrel option with pistol-length gas (don't want to make it an SBR). I could get a custom barrel for $250+ with pistol length gas (my preferred option if I had cash to burn - what I should have done had I known at the start of this build), or get a carbine-length gas barrel for $100-$150 and try to make that work (I suspect it would).

    Any and all input and advice would be appreciated - don't want to ruin my piston kit gas block if that's not going to be a path to success...
    Last edited by Pressingonward; 12-18-20 at 23:11.

  2. #2
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    Option 3 and be done with it !

  3. #3
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    Go option 3 and sell the piston kit to recoup some of the money. You may even be able to get someone to trade you a DI setup for your stuff.
    “You have made us for yourself, O Lord, and our heart is restless until it rests in you.” -Augustine

  4. #4
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    I was able to make a piston system work for my 7.62x39.

    Carbine length barrel and bolt from Bear Creek, Adams Arms adjustable gasblock piston setup. Adams Arms Low Mass Bolt Carrier and A5H1 Buffer Setup.

    I have gotten it to run quite reliably. Are you sure you'd drill the gasblock itself? What about expanding the gas port. Is it already too large for the gas block hole?

    If so, that might be best to just get a DI kit for the time being to see if the midlength barrel will run at all.

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    Thanks for the input everyone. I tend to agree that going the DI route is probably the most straightforward...but it's hard to give up on the idea of the piston gun.

    HeruMew - yes, the hole in the barrel is 0.125", and the hole in the gas block is smaller. I need to measure it, but I'd guess it's around 0.090"

  6. #6
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    You seem like a really bright individual, and the probably serves you well in your niche.


    Unfortunately you seem to have gone down the rabbit hole and into the weeds with your AR build. Here's a few of my 'observations' from being around AR's.

    1. If its not PWS or HK; dump the piston. Period.

    2. The AR magwell will not accept a magazine designed for the larger diameter round. Good luck finding anything close to a reliable magazine that holds a decent amount of rounds. My buddy tried a 7.62x39 AR; and mags were the biggest blocker.

    3. Don't build your first gun. Buy one.

    Pick an AR and feed it 556. If 7.62x39 is the only available round.... you are now an AK guy!! Go hang out with Brandon Herrera.


    Ironically my favorite online website has M855 but no 7.62x39


    M855:

    https://www.sgammo.com/product/223-5...ty-ammo-wm8551
    Tactical Nylon Micro Brewery

  7. #7
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    Agree 100%


    Quote Originally Posted by turnburglar View Post


    Unfortunately you seem to have gone down the rabbit hole and into the weeds with your AR build. Here's a few of my 'observations' from being around AR's.

    1. If its not PWS or HK; dump the piston. Period.

    2. The AR magwell will not accept a magazine designed for the larger diameter round. Good luck finding anything close to a reliable magazine that holds a decent amount of rounds. My buddy tried a 7.62x39 AR; and mags were the biggest blocker.

    3. Don't build your first gun. Buy one.

    Pick an AR and feed it 556. If 7.62x39 is the only available round.... you are now an AK guy!! Go hang out with Brandon Herrera.


    The standard DI system is actually a "coaxial piston", rather than an offset piston and has many advantages, especially in longer barrels.

    The 7.62x39 barrel will mostly likely cycle just fine in a DI configuration, other than the obvious issue of difficulty feeding 7.62x39 reliably from the standard magwell.


    Is the piston system adjusted properly for maximum pressure? i.e. 4.5 turns out?


    Quote Originally Posted by Pressingonward View Post

    Thanks for the input everyone. I tend to agree that going the DI route is probably the most straightforward...but it's hard to give up on the idea of the piston gun.
    Black River Tactical
    BRT OPTIMUM Hammer Forged Chrome Lined Barrels - 11.5", 12.5", 14.5", 16"
    BRT EZTUNE Preset Gas Tubes - PISTOL, CAR, MID, RIFLE
    BRT Bolt Carrier Groups M4A1, M16 CHROME
    BRT Covert Comps 5.56, 6X, 7.62

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    Thanks guys. I definitely agree I'm off in the weeds a bit with this build...I tend to do that...

    I've got some 20 round and 30 round C-products Duramags for it - I've read that they feed well and are a big improvement over some of the older mag options. We'll see...

    Clint, good question. The piston system is adjusted at 4.5 turns out, and I verified visually that the port to the piston is fully unblocked at that adjustment position (verified it after pulling it off the barrel to check my port size)

  9. #9
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    Quote Originally Posted by Clint View Post
    The standard DI system is actually a "coaxial piston", rather than an offset piston and has many advantages, especially in longer barrels.
    I'd be curious to learn more on the advantages for DI in longer barrels if you've got time to elaborate.

  10. #10
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    My first AR: 7.62x39 with Superlative Piston kit - Learning from failure...

    Quote Originally Posted by Pressingonward View Post
    Thanks guys. I definitely agree I'm off in the weeds a bit with this build...I tend to do that...

    I've got some 20 round and 30 round C-products Duramags for it - I've read that they feed well and are a big improvement over some of the older mag options. We'll see...

    Clint, good question. The piston system is adjusted at 4.5 turns out, and I verified visually that the port to the piston is fully unblocked at that adjustment position (verified it after pulling it off the barrel to check my port size)
    Don’t listen to the AR-47 nay-sayers. The Duramags are good to go. FYI, the 28 rounders will fit in NATO Mag holders.

    I can’t comment on pistons. I will say my DI AR-47 hasn’t missed a beat and is more accurate than any AK I’ve ever shot. There’s no reason to be concerned about not having a piston.

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