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Thread: SMALLEST GAS PORT SIZE??

  1. #11
    Gunco Regular Gunter's Avatar
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    The longer stuff needs a longer piston or carrier. I just put the gas block on the 308 barrel with the PSL gas tube and with a std Yugo carrier/piston, the piston is about 3 inches too short.

  2. #12
    No Hope For Me 1biggun's Avatar
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    Gunter how long is the original barrel?? The other thing That may come into play in all of this is that the 17-223 has a MV of 4000+fps compared to the 2300 of the 7.62x39 this means that the bullet will be out of the barrel sooner almost twice as soon. this kind of throws things the opposite as far as lock time, making me think that a shorter piston may work. I think the gas tube and piston on a 223 sagia and the 223 SARs are standard length so I am going to go with a standard length piston on bolth builds.I am still going to try the tighter fitting piston riding in a custom gas block for 1.75 inches this will be acomplished by using a AMD gas tube and a standard length piston that protrudes 1.75 out of the amd tube. the the protruding amount of piston will be fitted into a custom gas block were it will run tighter clearances than normal hopefuly working with a smaller gas port. I relize that this may affect the reliabilty of the weapon after a lot of shooting due to carbon build up but it gets cleaned about every 25 shots anyways I am also going to to have clearance rfrom the tube to the block so the barrel will be completly floating still. this all needs to be done plus the entire 17-223 build (still no reamer) by the end of the month as that is when I take it to South Dakota. I have been told that one of my buddies has a new DPMS AR 15 varmit build that he plans to out do me with (I am guessing the new 204 Ruger). we will se about that

  3. #13
    Gunco Regular Gunter's Avatar
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    The barrel is the same, just taken at different heights which makes it look longer in the top insert.

    I think the barrel is around 22 inches now without the brake. I would have to measure it again. Shows the size next to a std Romy.

    Sorry for the crappy pic. Had to take it indoors.

  4. #14
    Happy Camper hcpookie's Avatar
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    Yes I'm with Winn - I think the gas port location may need to be worked through. The issue will be the "burn time" if the port is too close to the chamber. I think.

    It could be reasoned logically that the close to the chamber, the more gas will bleed away from the bullet while in the barrel. That, I think, translates to lower muzzle velocity. That could probably affect accuracy in the long-distance area.

    Now, I don't think you'd get a squib effect by having a "short" piston... I really don't think that would behave that way. If it did, then your load is REALLY light!

    Turn this around. The impulse on a Dragunov, PSL, and other heavy-caliber "squad sniper rifles" is being pushed by a significantly higher load of gunpowder in the 7.62x54r and therefore it really needs a longer piston if you follow the logic that, at a certain point, the pressure in the barrel will be at the optimal point in-between high-pressure beating of the gun, and low-pressure poor performance (as if the gas block were at the end of the barrel).

    Thus, a longer piston design may be counter-productive for a smaller round. Consider yet again the AKS-74U design. That cool cone flash hider has a purpose - it is an expansion chamber that serves as a muzzle BOOSTER which helps maintain velocity and permits the AKSU to cycle properly.

    I wouldn't even consider AR designs since they don't use a piston. :/


    The 24" RPK barrels are a full 8" longer than the 16" AK barrels. The muzzle velocity is noticeably different on my RPK. In fact, it is noticably different on my Chinese NHM-91 with its 20" barrel... amazing that 4" can make that much difference... however the point is that the RPK has the same piston as an AK.

    An AMD-65, with its 12" barrel, has a piston that is only slightly shorter than a normal AK. 120MM IIRC. That puts us where? 16"-12" = 4", so there is a diminishing return there somewhere.

    There is also a point where the barrel "whip" harmonics encountered during the firing cycle would let you optimally place the gas block to minimize barrel oscillations, however this is conducive only to accuracy and not function, if one were to follow the logic.



    So where does this leave you? I'm not sure!

    If I were to follow my logic (perhaps flawed since it only considers function!), then I would imitate an RPK-74 design and use a normal piston. An RPK-74 is probably closest to your caliber as can be had. The Czechs use 5.56 in their VZ-2000 (I forget - is it actually called a VZ-2000?) which is also a close model for you to follow. Their RPK version has a standard piston vs. the "standard" model. Food for thought.

    HTH,
    - Jerry
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  5. #15
    Gunco Regular Gunter's Avatar
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    Not to highjack 1biggun's thread.

    You guys might like to check this video. Not about AR vs AK, but for the slo-mo AK footage. If you look close you will even see a left handed AK.



    Another thing to ponder is that it is trying to push a large amount of powder/gas down a tiny hole. The pressure curve would be different than say a 7.62x39.

    I just copied the PSL design since the 7.62x54R and the 7.62 NATO are close.
    Last edited by Gunter; 05-07-2007 at 04:16 PM.

  6. #16
    No Hope For Me 1biggun's Avatar
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    I have seen that video before and was thinking about it last night ( I need to get a life) it occured to me that since there is basicly no barrel in front of gas block on my AMD 65 that that there is no whip between the port and the end of the barrel since there is basicaly no barrel. this may account as to why my AMD 65 is the most accurate AK I have in 7.62x39 with a factory barrel. .75 -1.00 groups at 100 yards pmc ammo and a big scope. the gas tube is ridigidly clamped to the barrel and it takes a hammer to close the gas tube latch the reason is that the scope mount is on top of the tube and it must not move at all. I think that it is possable that the tube is actually reinforcing the barrel and reducing the harmonics and whip. I had heard of guys claiming that 16" barrel AK,s shot better with the tube clamped to the barrel. before seeing the slow mo photos I dismissed it, but now it has me rethinking how to acursize a light barreld AK.

    as far as my 223 and 17-223 the barrel is aprox 1.00" were the gas block will go so I am hoping that barrel harmonics will not be to much of an issue. I have been giving this a lot of thought and I have ideas from shorter pistons to running a flexiable tube from the muzzel brake back to were the piston ends. I guess with out a degree in some type of enginering (I got a D in high scool remeadial math) the only thing I can do is make an educated guess and drill a smaller than normal hole and start from there. I believe that in a 223 all the powder is burnt at aprox 20-22" and that no real gains in velocity are made by having a longer barrel. I am going to go with a standard length piston. since I have to make the GAS block anyway to fit the 1" barrel, I am going to have it ride in the block for aproximently 2" in a tighter than normal bore this should allow for a smaller gas port. If it dont work I can always open it up and port it to act like a standard system. I am going to have a gas shut off as well this way I can compare the accuracy of SA versus cycle by hand. I have acess to a chronigraph and will be able to compare velocity as well.
    My main concern right now it that the bolt will unlock to soon due to the long barrel and blow up. the 17-223 has a a 22" barrel so hopefuly all this timing will not be an issue.
    another way to do this would be buy 10 barrels and and keep drilling untill I get the best accuracy! All I know is on the AR15 build I am planing I am buying the stuff already done with no guess work.

  7. #17
    Administrator pirate56's Avatar
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    for the time factor consider that a bullet going 4,000 fps will go 1' in .00025 seconds, one going 2,300 fps will travel 1' in about .00045 second, if the chamber pressure is the same I don't think the time factor will mean much in the transfer of energy, as long as sufficient energy is imparted on the piston to operate the action. don't forget the energy is only on the piston when it is in the gas block.

    as afar as bbl length vs velocity I think I remember readin that there is a 10% loss in velocity for every inch of bbl less than the optimal length the load was designed for

  8. #18
    No Hope For Me 1biggun's Avatar
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    I remember reading that there is a 10% loss in velocity for every inch of bbl less than the optimal length the load was designed for
    Thats what I recall as well. but it might have been 100 fps per inch. it is hard to bellive that a 12" amd 65 would be 40% slower than a 16 inch AK. I think it was 300-400 fps slower how ever. I need to get on a thompson /encore sight and see what those pistols show. I thing optimum in a 223 is aprox 22" for most loads.

    don't forget the energy is only on the piston when it is in the gas block.
    this my thinking as well that is why I think having the piston in a tighter bore for a longer distance will work with a smaller gas port. once it travels the 2" in the bore it will be ported were it enters the tube. I have some concerns that if I get the piston bore clearance to tight it may build up high pressure and break the block.
    I hope to shoot it this week end one last time with out the gas bloc to fine tune my hand loads. then I plan on tearing the gun down to fit the gas block and drill the port hope to have a answer to some of this in a few weeks. also adding a adjustable comb and a adjustable rear rest to the fiber force stock. working on a shell catcher and bolt hold open selector as well. this time I will get some good pictures to post.

  9. #19
    Gunco Veteran panaceabeachbum's Avatar
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    one thing to remember is that even though pressure chamber might be 50k CUP it will be diminished somewhat by the time it reaches your gas port. I dont remember the exact numbers but I read an article where they had ported diff AR bbls at diff distances from the chamber and the pressure had dropped by drasticly by the time the bullet reached the gas port on a CAR length bbl.

    I have a 16" galil with the gas port 6" from the breach face (16" micro galil) that works like a dream with a .040 gas port. If you happen to drill a hole slightly to large you can always turn the head of the gas piston down slightly to creates some blow by and soften the unlocking, same principle as adjustable gas systems that vent excess gas . my .02

  10. #20
    Happy Camper hcpookie's Avatar
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    You are correct on the AMD muzzle velocity - only about 400 FPS slower. The latest issue of G&A's Combat Arms annual lists the AK-102 from Arsenal (called an SLR-106 or something) has about the same loss of velocity - about 200-400 FPS depending on ammo and use of the muzzle booster.


    I really don't think you are going to have a problem. Much of this is academic since the AR platform can shoot so accurately with its gas-feed system, I don't see any problem with where you put it. I believe the impulse would be a problem only if you put it close to the muzzle, like an AKSU design. The AK-102/4/5 SBR design uses a muzzle booster as well. However, both systems can be fired without the booster in place, suggesting that in all but the worst of conditions, you should have no problem with cycles.

    Another thought, was that the K-Var 16" barrel AKSU kits have the gas block at the normal AKSU spot (closer to the chamber), and the 9x39 rifles based on the AKSU also have the gas port in the "normal" AKSU location at slightly over the 6" mark. That puts 10" of barrel - 2/3 total length - ahead of the gas port.

    I think it will work
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