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Thread: SMALLEST GAS PORT SIZE??

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    No Hope For Me 1biggun's Avatar
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    Default SMALLEST GAS PORT SIZE??

    I am starting on my 17-223 varmit build #2(waiting for reamer) It has occured to me that the gas port hole on a 223 is almost as big as the .17 bore.
    What is the smallest gas port hole any one has gotten away with on a 223 build?? I am thinking of trying a custom GB with tighter piston to block clearances and possably having the piston run an inch or so in this block with tighter clearances to try to get the bolth my 223 and the soon to be done 17-223 to cycle with a smaller hole. was also thinking of a lightest possable recoil spring. was also considering a smaller piston as well thinking it would take less volume to move it. was going to try one the same as a FAL

    I also need a few AMD 65 pistons to play with If anyone has a few originals laying around CHEAP I could use a few to play with. I gave mine away a year or so ago.

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    Happy Camper hcpookie's Avatar
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    (cross-posting from GNA)

    Biggun, all I can give you on this is the size for .223 builds (based on my Saiga and Galil barrels) -



    http://pookieweb.dyndns.org:61129/AK...dimensions.htm



    5.56x45 Saiga 90 degree .067" # 51 bit 11.9063" (11 29/32") / 302.420mm




    That's the smallest from the "factory" stuff. Since I have only a good idea what a 17-223 is, I wonder if there are any AR builds in a similar caliber? 17 hornet or HMR or whatever it is called? What would that use? I wonder if Varmint Al has anything on his web page.



    .177 - .067 = .011 so you would still have "enough" barrel around the hole, IMO.
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    Administrator pirate56's Avatar
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    the gas port hole is about .110" smaller than the bore. the main thing to consider is the distance the gas port is from the muzzle. the force delivered to the piston is a combination of pressure and time. if the .17 round delivers simillar chamber pressures and the gas port is far enough back it should work fine with a sdt gas system.

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    Indian Admin Winn R's Avatar
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    I've always wondered -- Once it unlocks is it the recoil and barrel pressure that drives the carrier back (ala blowback) or is it the pressure in the gas tube?

    Figuring the overkill in function design, I wonder if the little .070" hole would operate a 7.62. Next one drilled I may try that.
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    Administrator pirate56's Avatar
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    It is basically the momentum from the initial force of the gas. the gas tube on an ak is vented where the tube meets the gas block. on an ar the gas tube is only into the bolt carrier a short distance. on an m1 carbine it is just the push from the gas piston that cycles the gun.
    there are tremendous forces at work, considering the head of an ak piston is about .500" the surface area is about .2 squ in. and chamber pressure is about 50,000 cup you have about 10,000 cup of pressure acting on the face of the piston for a couple of milliseconds.

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    Happy Camper hcpookie's Avatar
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    Ditto what pirate said - think about other short-stroke rifles - AR-18/180; SKS; VZ-58; G-36/SL8; etc. etc. etc. Once that initial thrust is achieved, it is all momentum!
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    No Hope For Me 1biggun's Avatar
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    thanks for the input. I want to keep the port as small as possable because accuracy is the main goal here. I am afraid that the relativly large port compared to the very small bullet will negitivly affect the accuracy, and possably shear lead off the bullet.
    I am buildng a gas block for my 223 varmit build that will use a a tighter fit between the piston and the block for about 2 inches, after the piston travels back the 2 inches it will reach the tube and also have some venting. I am going to start with a very small hole and work up until it cycles. if it works out I will duplicate it on the 17-223. my thinking is the tighter fit will allow it to work with a smaller hole because it will initally work due to pressure and not the force of the blast. once it has started to move the venting will keep it from shoving the carrier out the back of the gun.( I will deffinatly be string testing this set up. the last thing I want is a full 60,000- 100,0000 psi on the piston face) I am also incorperate a shut off valve in the block of some sort most likely a cross pin through the hole.
    I figure it is best to work it all out on the cheaper and more worn barrel first. I have a fair amont of money invested in the stainless 17-223 barrel and I want to get it right the first time. It looks like the pressures in the 17-223 will be around 50,000 psi. I have been warned to keep the pressures and velocity down on the .17 to prevent barrel erosion and rapid fouling, so it may be in the 45,000
    originaly I was going to use a adjustable FAL style block but after thinking about it it would require an even larger hole in order to give adjustabilty as they work by bleeding off excesive cases when hotter loads are used.

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    Indian Admin Winn R's Avatar
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    If I'm following what's being said;

    The piston has barely moved (but is at max velocity) when the barrel is clear and pressures have dropped to nothing.

    The little movement has not even unlocked the bolt by the time the bullet is out.

    The remainder of extraction, ejection, and spring compression is done by the carrier momentum.

    Thank you gentlemen.
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    No Hope For Me 1biggun's Avatar
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    I am starting to be concerned that on my 26" barrel with over 16" in front of the Gas block that it may unlock to soon. I am starting to rethink using a amd length piston and thinking a longer than normal one may be in order. my original reason for going with a short piston was mainly looks and trying to balance the gun out some as it is pretty nose heavy now. I was also thinking there may be some accuracy gains from having the bloch mounted further back in the heaveir prt of the barrel. My next question is on the very long barreled AK style weapons is they all use a standard length piston CORRECT??

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    Indian Admin Winn R's Avatar
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    Not true for the heavy cartridge sniper stuff.
    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

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