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Thread: Will this blow me up? 7.62x25mm barrel

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    Gunco Member Bullpup's Avatar
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    Default Will this blow me up? 7.62x25mm barrel



    I've been working on this project on an off for a few months. Its a 7.62x25mm chambered AK front end. Its a 16" barrel with a very short piston (about 1.5") on a regular carrier. I haven't put a gas port in yet. The gas port will be just about 7" from the breech end, if you subtract the 1" approx case length, then 6" from the case mouth. I'm starting to worry that if the bullet has not left the barrel by the time the bolt rotates and begins to extract then I will get a case head rupture . I really don't want to blow this up.

    I've seen a few other builds, notably PBB's build here, but that build used an AMD length piston and 5/32" gas port (.156") and a carrier which has been significantly lightened, recoil spring reduced. the other reference point I have is Gnarlytoe's build:
    The AK Forum :: Log in
    which uses a krink piston and an 20" barrel and a .205" gas port, recoil spring reduced. I can't seem to find a reference for the length of a krinkov piston, but it's longer than 1.5" by appearance anyway.

    Is there a calculation for timing an ak barrel? Chamber pressure, port size, port distance from breech, carrier weight, and barrel overall length? there are a ton of variables! Also, bullet acceleration: if a tok round is going 1400fps from a 4" pistol barrel, then at the 6" gas port mark (1400 fps x 12" = 16800" per second. 16800 / 11 inches of remaining barrel = 1/1518th of a second to leave the barrel. The acceleration of the carrier backwards is going to be many times slower, + the spring's resistance, but I have no idea what the carrier weight is or what the pressure is at 6" mark, or how to factor in the gas port size.

    then again I might just be making myself crazy.

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    aka: SDK1968 dutigaf's Avatar
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    thats some pretty good figuring there...

    first thing to throw in to you is that 1400fps is a LOW number on 7.62x25 ammo.. most of it is closer to the 1600+ range to start with on surplus stuff.

    AZHONKEY is a guy who does a lot of this type work and his stuff is excellent and functional you may wanna look up some of his work. but im not sure if he has done the gas piston set up or if all his stuff was just blowback operation.
    say what you mean & mean what you say


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    Gunco Regular Gunter's Avatar
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    "Take time to deliberate, but when the time for action has arrived, stop thinking and go in."

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    No Hope For Me 1biggun's Avatar
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    Is there a calculation for timing an ak barrel? Chamber pressure, port size, port distance from breech, carrier weight, and barrel overall length? there are a ton of variables! Also, bullet acceleration: if a tok round is going 1400fps from a 4" pistol barrel, then at the 6" gas port mark (1400 fps x 12" = 16800" per second. 16800 / 11 inches of remaining barrel = 1/1518th of a second to leave the barrel. The acceleration of the carrier backwards is going to be many times slower, + the spring's resistance, but I have no idea what the carrier weight is or what the pressure is at 6" mark, or how to factor in the gas port size.
    HMMM given the number of differant builds I have seen with pistols that have barrels less than 3" long to my varmit AK with a 26" barrel there seems to be no known fromula for barrel timing. your concerns are valid. MY gut tells me that if a 3" barrel on a 7.62x39 dont blow up then your build will be ok. I doubt there is anyone who will say absolutly for sure. sorry I cant be more helpfull as mentione above AZHONKEY HAS PLAYED WITH THIS STUFF AN BUNCH i WOULD TALKE THAT ADVICE AND SEE WHAT HIS GUT TELLS HIM.

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    Gunco Member Bullpup's Avatar
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    Quote Originally Posted by 1biggun View Post
    MY gut tells me that if a 3" barrel on a 7.62x39 dont blow up then your build will be ok.
    The issue isn't barrel length, but gas port distance from the muzzle. On a 10" pistol, the gas port and muzzle are very close. I'm worried that using pistol length gas system coupled with a full 16" barrel I may have problems.

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    No Hope For Me Coils's Avatar
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    The issue isn't barrel length, but gas port distance from the muzzle.
    I don't think the port distance from the muzzle is a problem either, I'd worry about the port size. To small of a port and it won't cycle properly (recoil spring strength will factor in this too), to large of a port will let to much gas bled off and might not provide enough pressure for the bullet to exit the barrel.
    I wouldn't worry about FPS but how much pressure you'll need to cycle the bolt, as long as you just bleed off enough gas to cycle the bolt the X25 round will provide more then enough to get the bullet clear of the barrel.
    I know this is a simple reply, but I think your looking at it in a complexed way and making it harder on yourself.

    So the big questions are; Port size? & Recoil spring strength?

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    Gunco Member dunn6902's Avatar
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    I don't know if this helps at all but I built an ak pistol in 7.62x39 based on one of slamfire's designs and used a 13/64 bit to drill the gas port hole like he recomended.I wanted to see how much backpressure could be lost and still cycle the bolt so I took the gas tube off completely and fired 2 rounds.The bolt cycled fine even without it.

  8. #8
    No Hope For Me 1biggun's Avatar
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    THE TUBE HAS NOTHING TO DO WITH IT. IT ALL HAPPENS WHEN THE PISTON IS STILL IN THE GAS BLOCK. THE BLAST OF THE GAS BLOWS BACK THE PISTON. IT DOSENT PUSH IT BACK WITH A SEALED PENUMATIC TYPE DEAL.

    AS FAR AS BULL PUPS CONCERNS, HE HAS A VALID POINT THE SLOWER BULLET IN A LONGER BARREL WITH A VERY SHORT PISTON "MIGHT" BE A ISSUE. IF THE BULLET IS STILL IN THE BARREL WHEN THE ACTION UNLOCKS THEN THE BOLT WILL SEE ALMOST THE FULL CHAMBER PRESSURE. iN THE CASE OF A AK IT LIKELY WONT BE THE END OF THE WORLD AS THE CARRER IS LIKELY HEAVY ENOUGH FOR A BLOW BACK ANY WAYS (DO NOT QUOTE ME ON THIS) A GOOD EXAMPLE IS A SHOOT GUN SHOOTING A SLUG WITH A LONG BARREL THE SLUG HAS NOT LEFT THE BARREL BEFORE THE RECIOL IS FELT AND THE BARREL RISES THROWUNG OFF AIM. hORNADY HAS DESIGNED A ROUND FOR USE IN A SHORTER BARREL OF ALL THINGS THAT IS MORE ACCURAET BECAUSE THE SHORTER BARREL ALLOWS THE BULLET TO BE OUT OF THE BARREL BEFORE THE MUZZEL RISES THUS IT IS BETTER ON TARGET. 1400 fps IS 1000 FPS SLOWER THAN A 7.62X39 SO THERE IS A POSSABLITY THAT THE PISTION MIGHT RELEASE THE ACTION WHILE THERE IS STILL FULL PRESSURE IN THE BARREL. LIKELY THE ONLY TRUE WAY TO TEST IT IS A LONG STRING. I SEE A LOT MENTIONED ABOUT BARREL TIMING IN REGARD TO AR,S MAYBE THOSE EXPERTS COULD SHED SOME LIGHT ON DITANCE FROM BREACH TO BARREL LEGNTH??? WHEN i DID MY VARMIT BUILD i WANTED A VERY SHORT PISTON AS WELL TO HELP BALANCE OUT THE GUN BUT TALKED MY SELF OUT OF IT.

  9. #9
    aka: SDK1968 dutigaf's Avatar
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    hey bub why are you yelling?

    are you mad at someone?

    theres nothing wrong with the point your making.

    yes x25 is slower than x39. its also about 50 grains lighter...

    and yes we all know that blow back AK's work just fine with 9mm & x25...
    say what you mean & mean what you say


  10. #10
    Gunco Member SW44's Avatar
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    Your barrel and carrier are very similar to the one I am working on, but I am ahead of you in construction. It is a gas operated 7.62x25 based in the 2XMT mag well. I am using a 10" PPSH 41 barrel with the necessary sleeving to fit it up to a AMD-65 cut barrel kit. My piston extends only one inch past the bolt carrier and I have removed 1" of the bolt carrier front tube section. That puts the gas port about 4" to 5" in front of the chambered round.

    I have cut 2" off of the recoil spring and made shorter wire retaining loops for the recoil assembly. This allows the recoil assemble to fit in the shortened bolt carrier at full recoil.

    I made my own hammer spring out of 0.045" music wire because the stock spring is too strong. It is modeled after the stock hammer spring, but I cut off one leg to reduce the pressure even more.

    Yes it does fire without blowing up! It strips a round from the PPSH drum and feeds it into the chamber pretty good when the bolt carrier is pulled back and released. Every round chambered so far (about 20) has fired inspite of the weaker hammer spring. I started the gas port at 0.125" and have opened it up to 0.1718" in the quest to cycle the action.
    It has fully cycled the action once! I am now making a new gas piston for it with tighter clearances. The current piston is 0.009" smaller than the ID of the gas block. That is about standard AK clearances.

    No pictures yet to post. I am waiting to acheive more reliable feeding before posting a detailed photo spread.

    Good Luck!

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