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Thread: 7.62x25 Bullpup

  1. #21
    Gunco Veteran [486]'s Avatar
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    Quote Originally Posted by xebec View Post
    This operates properly with a normal length recoil spring assembly, but bolt carrier travel is limited by a large HDPE buffer. This prevents the shorter bolt carrier tube from causing the spring to go solid.
    I was wondering how people get over that, especially the guys that make those ultra short pistol builds, and that solution makes a ton of sense.
    On the trigger setup I guess pulling a bar would be less squishy than pushing it, I'll have to add that to the considerations in my conversion, if it ever goes together.

    Thanks again for sharing your build.

  2. #22
    Indian Admin Winn R's Avatar
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    The gas operated conversions previously posted have either reduced the hammer spring or the radial degrees of lockup.

    I'm trying to find the source of the energy, or the efficiencies in your design allowing the avoidance of the above.

    I see how the rear throw only has to be far enough for the hammer to catch the disconnector. The lightened carrier will move quickly. Is the gas tube round without ribs -- it looks like more than the length is different in the piston?

    What really looks good, with the trunnion in the same place and gas operated, is the fit at the trunnion. It appears the geometry for the rear fcg is unchanged and you've avoided the chopped bolt and carrier -- reassuring concerning out of battery and slamfires.

    I'm playing with one that is not yet operational. Thank you, this is very helpful.
    There is no nonsense so errant that it cannot be made the creed of the vast majority by adequate governmental action. -- Bertrand Russell


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  3. #23
    Gunco Member xebec's Avatar
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    Lock up is unmodfied-identical to the AK 74, and hammer spring is unmodified.

    When I tell you about the gas tube Winn, you're going to think I'm full of s__t.

    There is no gas tube...

    The gas produced at about 5" ( I'll post the exact distance) from the breach is plenty to run the full strength recoil system through a .156 port, without any sort of gas tube, and with an unmodified 90 deg gas block. The piston obviously has to fit fairly precisely, while still allowing some tolerance for fouling.

    My experience with this particular weapon is that a number of points in the AK operating system can create resistance which can cumulatively create (sometimes inconsistant) malfunctions. The points which needed special attention were:

    Feed lug of the bolt head. I noticed that, since the PPS 43 Magazine likes to be high in the receiver (touching the lower rails), with feed lips tilted upward to feed the round more directly into the chamber, the lower lug of the bolt dragged across the retained rounds within the magazine, sometimes catching on the rim up the uppermost round. I relieved the sharp edge at the rear of the feed lug, and polished the contact surfaces.

    The angled surface of the bolt carrier that forces the hammer back into engagement with the trigger carriage hook. Different AK fire control parts often have geometries that don't work well together-this is especially true (as you know, I'm sure) of the hammer and bolt carrier. You can have a combination that is hard to "cock" because of an excessively steep angle on the hammer. Similarly, many have experienced a hammer that tends to
    catch a bolt back, or even lock a bolt rearward. I relieved the hammer of enough material that it would reliably engage the disconnector without offering any extra resistance to the bolt carriers rearward travel.

    The angled engagement surface of the trigger carriage hook. This can offer considerable resistance to the bolt carrier. When the hammer hits this surface and rocks the trigger carriage forward, too shallow of an angle can virtually stop the hammer, thus stopping the rearward travel of the bolt carrier. I re-ground this angle to leave ample sear surface for safety and reliability, while minimizing resistance to the hammer displacing the hook forward. Again, all surfaces were well polished after modification.

    I will try to post some pics of these mods when I pull the fire control parts.


    BTW Winn, I read your thread concerning use of PPSH barrels and the amount of the case left unsupported, and I completely agree with yor assessments. You will see that this design allows the 7.62x25 cartridge to fit into the chamber and boltface exactly as does a 5.45x39- virtually completely supported by with an extractor cut in the breachface. This does feel quite safe- fully supported and locked- when the round goes off. It just seemed to me that it was easier in the end to solve the gas-power problem than to try to overcome the numerous difficulties presented by a blowback... especially a bullpup blowback that has a chamber so close to my right molars-hhmmmm.

    xebec

  4. #24
    Happy Camper hcpookie's Avatar
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    Quote Originally Posted by xebec View Post
    When I tell you about the gas tube Winn, you're going to think I'm full of s__t.

    There is no gas tube...

    The gas produced at about 5" ( I'll post the exact distance) from the breach is plenty to run the full strength recoil system through a .156 port, without any sort of gas tube, and with an unmodified 90 deg gas block. The piston obviously has to fit fairly precisely, while still allowing some tolerance for fouling.
    First let me congratulate you on the build - bullpups AND caliber conversions are difficult!

    The gas system you're describing sounds suspiciously like the AR piston conversions, with the exception that they use a short-stroke system. I can't see your system being a problem, with the only exception that I wonder how it will behave in cold weather. That may be the only time it causes a problem, but it will easily corrected

  5. #25
    Gunco Member xebec's Avatar
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    hcpookie- Yea, the reason that I pulled this back out and completed it was becasue of the work that some have been doing on AR gas systems, both DI and short stroke types. Williams and others made DI ARs run with this cartridge, so I knew it would eventually work for my design, although the AR has some advantages in terms of buffer choice, etc.

    This thing really runs, and I don't see my version of cold weather (South Texas) ever being an issue... Here is some video (assuming I can figure out how to link it) of the weapon running through a couple of mags (BTW, temp was about 40 deg F- not real cold):



    I do believe, however, that wimpy ammo would be a no-go here. It runs like a swiss watch with Romanian, Polish, and Bulgarian surplus, and all of this clocks in about the 1850 fps range. If milder pistol type ammo were used, I'm sure reliabilty would suffer.

    xebec

  6. #26
    Gunco Member jrenyard's Avatar
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    Excellent, I am looking forward to better pictures of the trigger and gas systems.

  7. #27
    Gunco Member xebec's Avatar
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    Default Rear Fire Control

    Here are some pics of the rear fire control group. To can see that it is pretty much standard AK, but notice several modifications:

    The fist pic (somewhat blurry, but I think it will suffice) shows the trigger carriage and disconnector. Notice that I have reground the angle on to of the hook to smooth the forward movement of the carriage as it is being pushed by the hammer/carrier. This reduces drag on the bolt carrier as it moves rearward. also note that I added an extension to attatch the cable, so it won't interfere with the hammer, or drag on the hammer spring. The trigger is, of course, removed.

    The second pic also shows the standard AK hammer and spring.

    The third pic shows the cable attatched to the trigger carriage.
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  8. #28
    Gunco Member xebec's Avatar
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    Default Front Trigger Pack

    Here are some pics of the front fire control parts.

    The Trigger itself is a rotational type trigger- the plate with 2 screws to the left of the lower retains the trigger axis pin, to which the trigger and cable cam (to the right of the lower is attatched. The cable pulls the cable forward as the trigger is depressed. Note that the cable is under constant tension, being pulled rearward bt the hammer spring in the rear FCG, and is pulled forward by pressure on the trigger. Thus there is never any slack in the cable- a pull/pull system. The screw to the rear of the cable cam limits the distance that the cam can be pulled rearward by the hammer spring and rear FCG, and also allows me to set the amount of sear engagement between the rear trigger carriage hook and the hammer. In this way I can shorten trigger pull. To the front of the cable cam is a stop bar that contacts the safety assembly (that rotating device forward of the trigger). If the safety is moved to the fire position, the cam can move downward as the trigger is depressed which pulls cable and moves the rear FCG hook forward, releasing the hammer. If the safety is engaged, it blocks movement of the cam, and thus the cable.

    The last picture shows a nylon roller that I installed in the receiver, on the left side between the magwell and receiver wall. This gides the cable and allows it to move smoothly, and also keep it clear of the various interior parts of the receiver. One importan consideration here is the length of the cable. This is naturally determined by the distance between the trigger and the rear FCG- you don't want it too long (sloppy, poor trigger function and reliability) or too short (won't work, or may cause safety problems). I allowed for adjustment by using an RC aircraft type connection for the front of the cable- you can see it in the third picture. The cable end is soldered to an appropriate threaded rod, and the aircraft connector simple screws onto this, and so is adjustable. The rear of the cable connects to the Rear FCG trigger carriage, and is simply a short length of music wire stock soldered to the cable and bent as shown in the third picture of my last post.
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  9. #29
    Indian Admin Winn R's Avatar
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    John -- That thing runs like a scalded cat -- oh excuse me, I should say scalded dog.

    And I'm sure you've found the claims that this caliber inevitably splits cases not to be true provided the chamber bears some resemblance to the cartridge and the case is supported.

    I'm happy to hear of the gas tube - that saves all the work of getting the L1A1 tube and piston fit. The carrier must be fit well on yours not to need the help of the tube to get the piston in the block.
    There is no nonsense so errant that it cannot be made the creed of the vast majority by adequate governmental action. -- Bertrand Russell


    "Never attribute to malice that which can be adequately explained by stupidity." Robert J. Hanlon

  10. #30
    Gunco Member xebec's Avatar
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    Default Gas System

    Not much to show here. I really just shortened the carrier, soldered in a stubby gas piston, and moved the gas block back to very close to the rear sight base. As I posted earlier, the port is .156, and the gas block is a 90 degree type without modifications. There is no gas tube.
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