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AR15 rifle in 7.62x39 short stroking.

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  • AR15 rifle in 7.62x39 short stroking.

    Yeah, I know, I hate the caliber. Far too many sent in my direction long ago.... But I digress....
    Nice young man associated with the neighbor across the road picked up a parts gun from his cousin. The nice young man said he knew an oldman that fixed guns in the army and as a cop. He failed to say my training ended in the late 80's when my agency went Beretta 92F and I quit working on their guns.

    The weapon is a gadget cluttered misfit typical of redneck mentality involving this platform. Chambered in 7.62x39, its a full 20" barrel with a rifle length gas tube. Has a carbine stock with an unknown spring and a Spyder 2 buffer weighing 4 oz. The weapon was "built" by the cousin's uncle that recently passed away. The cousin had several 10rd 5.56mm mags he was trying to use. Of course that failed. So the weapon came here. And the neighbor across the road said "Let's get Wade over here. He's a freakin' genius with guns." He forgot I stopped working on them, and not trained upon anything past the mid 80's. Oh why am I so popular now?

    Its plain that its short stroking. Will fire and eject, but not feed. I do the standard drill for this in accordance with my training from years ago. It persists. Mind you, this is in that ugly 7.62x39 chicom round that makes me want to throw the weapon in the creek. But I try. Nice guy has a perfect shooting Windom made 7.62x39 carbine of his own.
    His upper functions fine in the cousin's lower. Ok. But the cousin's upper did not function in the Windom lower. Short stroked!

    The allen screws for the gasblock are stripped, and horribly mulated. I can't move them. I no longer have a shop. I'm thinking heavier buffer. Neighbor (after 8 beers) insists on the gas is blowing out the barrel and misses the tube all together. I wasn't going to argue with a drunk that believes he knows everything about weapons.

    The gasblock is in question. There is zero signs of gas leakage. None. Ignoring the drunken neighbor's ranting, it is presumed that it is acting as if undergassed. Cases land closeby at around 2 o'clock. Less than 3 ft from the weapon on a bench.

    So, all you AR whizkids, figure this one out. Its got me stumped. I say heavier buffer. But remember, this i s a 7.62x39 chicom nasty boy! Its beyond my expiration date of training. So pitch in and help on ancient warrior with this perplexing weapon.

    I thank you all.......
    "Don't try to cover up a lack of training with a tool you don't understand."

    John Lovell on upgrades.

  • #2
    I am new to the AR with 2 build kits. Rifle length sys and mid length. First I checked that gas block aligned with port in barrel. And both had to be drilled out a few thou larger. 2 in the mag ,2nd gets picked up and chambered. Last one locks bolt open.
    I was raised in the 50\'s on jackrabbits and gunpowder.salt and pepper wooda made\'em taste better

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    • #3
      I wonder if the gas seals in the bolt are aligned correctly. (That brain fart comes from finding all sorts of strange thinking and ill applied logic.)
      -Remote locations are cheap insurance.
      -There are two kinds of ships: Submarines and targets

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      • #4
        The .39's can be problem children.

        Rifle length gas system is quite unusual, it also would provide the least amount of gas of the four available lengths.

        Another way to check the gas ( you used the brass ejection) is to remove the upper, plug the breech, I use a straw but you can use a rubber tube fitted on the gas tube in the ejection port area then blow. You should be able to hear and feel the air come out the end of the barrel. I have done so many I can just tell. Since you have another there you can compare them. If you get "gopher cheeks" it is most likely constricted.

        The .39 AR also has two other problem areas that cause the issues you are seeing:

        The feed ramps are usually too tight. I have had to open them up and polish them on both of mine. I use a Dremel. You can test this by pulling the charging handle back and hold it. Release the bolt catch then slowly allow the charging handle to move forward. Watch very closely as you may see the other problem I'll address in a moment. You should be able to see where they are getting hung up. Then work on the ramps as necessary.

        The next problem area is the magazine(s) ASC is the only brand that I can get to run and then only the ten and twenty rounders, and they still need work. They are steel as is most of the ammo people shoot out of .39's. If it were brass it could shear off a tiny amount while feeding. Steel on steel it just doesn't give. I file down the area in the front ot the follower to reduce the area where the case mouth contacts the magazine. You can see if it is happening by slowly sliding rounds out with your finger.

        To function test your handy work load up all the mags you have worked on and cycle through each round slowly as described earlier. If that works load them up again and cycle them using the bolt release.

        As for the buffer. It is an oz heavier than a carbine buffer already. I think that is an unlikely culprit. You could always swap with the other gun that runs?
        Endeavor to persevere.

        Call sign: Limp Wrist

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        • #5
          Endeavor to persevere.

          Call sign: Limp Wrist

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          • #6
            Feed ramp enlargement and front edge of magazine where case mouth catches and slows it down.
            Endeavor to persevere.

            Call sign: Limp Wrist

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            • #7
              Thanks guys. There is only one functional mag, and it came with the Windom built carbine. Its a Bushmaster mfg. and functions fine in the Windom upper.
              The feed ramps on both weapons are of the M4 design. Apparently the ramp is functional, as it allows rounds to feed from hand cycling and use of the bolt release.

              I strongly believe that the issue is the low gas pressure. I'm well acquainted with the old M16 and it's behavior. My "spyder senses" keep telling me its the gas system. But here's the thing.
              1. This weapon was said to have been a parts gun. However, it appears to actually had been assembled in a factory at one time.
              2. The original owner switched out many items. Stuck many thins both on it and in it. The cousin was said to remark that the weapon never functioned from the beginning. And functioned even worse after his now deceased uncle started to "work" on it.
              3. The only mags the original owner used were 10 rd, 5.56mm aluminums. He was adamant that his mag choice was correct, as well as the many things he did to it. In short, a "Bubba Gun" abused by the ignorant.
              4. The gas block appears to be a copy of the original style with the upper post removed. Held in place by 4 badly damaged screws. They appear to have once been Torx but Bubba stripped out the center of them wil an ill fitted allen wrench.
              5. Drunk Neighbor said he shot brake cleaner down the gas tube in one of his many attempts to fix it by tearing it all apart and speculate on it with Coors light infused thinking.

              I'll try one more time with it, and then I'm going to walk away from this "project" from the deceased Bubba.
              Drunk Neighbor doesn't help things either with his self-invented knowledge where he makes up stuff as he rambles on.

              Again, many thanks for the assist. I'll apply what I can. Inwardly, I have the distinct impression that this is another doomed Bubba problem that I should have avoided.
              "Don't try to cover up a lack of training with a tool you don't understand."

              John Lovell on upgrades.

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              • #8
                I will not disagree that you could possibly get a 5.56 magazine to run with .39’s I can assure you they are not the same. Since the .39 is a fatter case the ribs on the side of the mag are smaller so the inside of the mag can accommodate the extra width. The feed lips are a different angle to work better with the case taper that the 5.56 does not have to worry about.

                These are both ASC 10 rounders. Bottom is 5.56 top and left are 7.62x39.
                Last edited by deaddog; 07-28-2019, 13:02.
                Endeavor to persevere.

                Call sign: Limp Wrist

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                • #9
                  Either the hole in the barrel, gas block or both are / is too small to get enough gas into the workings to blow the bolt back. Or in technical terms: Aint enough puff for a shove. Drill out the screws for the gas block and take the thing orf. if need be, biggen it a bit and try again. Full length gas systems are the softest shooters when they work and i like them but some things they tend to get a bit persnickety with. Fast powders for example are a no no.
                  The H in ENGINEERING stands for Happiness

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                  • #10
                    Yesterday's long exposure to our generous Texas heat in the building at Drunk Neighbor's place was not conducive to my health. Age, current conditions, and medications I take prohibits being in the heat. I overdid it and was flat on my back most of today sleeping for hours. Around 1330 and again at 1400 I was stirred by rapid gunfire from the building across the road some 75m distance from me. He just steps outside by the road and fires as fast as he can in the dirt. With the unmistakable staccato of the rounds, Drunk Neighbor took it upon himself to carry on.

                    I called him a couple of times, but no answer. Oh well, I was really too tired to talk with him anyway. And judging by the manner of the gunfire, he got those stripped screws off and cleared up an obstruction in the gasblock, tube, or port in the barrel. Good for him.
                    Thus I'm calling this cased closed. Even a guy that graduated from the University of Coors can occasionally get something right.

                    And I barely escaped again from a complete run of Bubba Goes Wild once again.

                    Yet another reason I should never get involved in these things.
                    "Don't try to cover up a lack of training with a tool you don't understand."

                    John Lovell on upgrades.

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                    • #11
                      I think the AR is simply rebelling at being chambered for a Russkie caliber.
                      "Beware the fury of a patient man." - John Dryden

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                      • #12
                        "Don't try to cover up a lack of training with a tool you don't understand."

                        John Lovell on upgrades.

                        Comment


                        • #13
                          A passing note.

                          It appears that the cousin, who was helping out another, and introduced the overweight clunker of an AR15, went YouTube surfing while off on a job this week. He forwarded a video to D.N. , who heard the word of .100" would help. Ok. He had a .125" drill bit. Yup, he really bored out the barrel vent! Then he used the light carbine weight buffer with this. He said "It hammered some" when he shot it. Fortunately, he did remember the old man [me] speaking of a heavier buffer for this weapon and the deceased owner had one in it after his many modifications. D.N. put it in and said "That kinda tamed down that hammerin' a whole bunch."
                          Ah, through his inebriation comes forth "cold filtered" inspiration.

                          Thus once again the adventures of the modern day forms of Heckle & Jeckle continues, no doubt with yet another announcement of "Hey, y'all watch this. Here, hold my beer."
                          "Don't try to cover up a lack of training with a tool you don't understand."

                          John Lovell on upgrades.

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                          • #14
                            For future reference,

                            The AR is designed around a 55,000psi cartridge, the .223Remington.

                            The 7.62 Soviet round (x39mm), is a 45,000psi cartridge.

                            The rifle gas system on the AR provides 20-25,000psi gas to the carrier piston assy.

                            The .30 Cal. Barrel has 50% greater volume, and drops pressure faster.

                            The 7.62 only uses about 15% more of the same powders that .223 uses, thus, 7.62 starts with less chamber Pressure, and is unable to support that pressure so what Pressure it makes drops faster, thus I expect the rifle system probably only feeds 10-15,000psi, on a good day, to drive the AR systems.

                            Poor gas Port Pressure, thus it short strokes.

                            Yes, drill the barrel port out bigger than {censored} and make up the drive energy on volume.


                            I'm glad YOU have Interesting Neighbors and I don't. Lol.

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