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  1. #1
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    Default Extending gas system length

    I'm looking for some experienced knowledge to validate a few things I think I need to account for in extending a gas system to a full rifle length. I've upped a carbine length 308 to a rifle length with an adjustable gas block, 18 inch barrel. Have found I needed to shorten the buffer spring to allow the action to fully operate (due I assume to the greater volume and tube distance the gas has to fill and travel) but I'm not sure that's the only consideration. It seems to me that now it could be playtime with combinations of buffer, spring (strength and length) and possibly buffer tube to find the sweet spot for this new gas system. I know one thing, Physics IS physics and I'm not gonna change it.

    I'd appreciate any guidance or rules of thumb.

  2. #2
    Join Date
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    Quote Originally Posted by Froghalla View Post
    I'm looking for some experienced knowledge to validate a few things I think I need to account for in extending a gas system to a full rifle length. I've upped a carbine length 308 to a rifle length with an adjustable gas block, 18 inch barrel. Have found I needed to shorten the buffer spring to allow the action to fully operate (due I assume to the greater volume and tube distance the gas has to fill and travel) but I'm not sure that's the only consideration. It seems to me that now it could be playtime with combinations of buffer, spring (strength and length) and possibly buffer tube to find the sweet spot for this new gas system. I know one thing, Physics IS physics and I'm not gonna change it.

    I'd appreciate any guidance or rules of thumb.
    I am just beginning to learn about the AR system and so I am just speaking generally. Relative to gas pressure, gas pressure drops faster the further one gets away from the chamber. True the greater volume of an extended gas tube should reduce the force applied to the bolt carrier. But, the closer the gas port is to the muzzle, the lower will be the pressure of the gas that is tapped. Simply look at the thickness of the chamber compared to other regions of a rifle barrel. The chamber is thicker because that is where the greater pressure is. The further from the chamber the lower is the pressure.
    Edit to add. the closer the gas port is to the chamber, the longer the gas will have also to apply pressure to the gas tube to ultimately accelerate the bolt carrier faster.
    Last edited by barnetmill; 05-15-2018 at 09:13 AM.
    One who hammers his gun into a plow plows for those who do not....Unknown
    ...at the end of the day its not about anything else but YOU AND YOURS..... Gabe Suarez
    ....WANT not NEED is what America is all about. ..... Gabe Suarez
    Its not about how fast you can load, but about how well you can shoot ..... Someone being saved by a speed load is not something that has happened with any regularity. Gabe Suarez

  3. #3
    Join Date
    Jan 2012
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    Volume of the gas tube has very little to do with anything; it's not even worth considering.

    Gas pressure at the barrel port (this is directly related to gas system length) and port size have to be balanced to feed the correct amount of gas to the BCG. The adjustable gas block has the same affect as changing port size to balance the system.

    If you had to shorten the buffer spring to get the rifle to cycle, you either have:
    a) an under-gassed system, fix it by opening the gas block or opening up the gas port if the gas block is all the way open. Also make sure you're using decent ammo and not underpowered cheap stuff.
    or
    b) the wrong combination of buffer system parts.

    Either way, you should not have to shorten the spring.

  4. #4
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    Quote Originally Posted by Yondering View Post
    Volume of the gas tube has very little to do with anything; it's not even worth considering.

    Gas pressure at the barrel port (this is directly related to gas system length) and port size have to be balanced to feed the correct amount of gas to the BCG. The adjustable gas block has the same affect as changing port size to balance the system.

    If you had to shorten the buffer spring to get the rifle to cycle, you either have:
    a) an under-gassed system, fix it by opening the gas block or opening up the gas port if the gas block is all the way open. Also make sure you're using decent ammo and not underpowered cheap stuff.
    or
    b) the wrong combination of buffer system parts.

    Either way, you should not have to shorten the spring.
    I have not seen the calculations by a qualified engineer on "Volume of the gas tube has very little to do with anything; it's not even worth considering. "
    So I just do not know. One has to calculate volumes and some things on gas dynamics that are way beyond my abilities to comprehend. What I have learned in school and scientific investigation is to know what you know and most importantly to know the limits of what you know.

    Spring shortening is something I would like to learn more about. I am guessing that under adverse conditions such as fouling a cut spring might result in malfunctioning of the AR.
    One who hammers his gun into a plow plows for those who do not....Unknown
    ...at the end of the day its not about anything else but YOU AND YOURS..... Gabe Suarez
    ....WANT not NEED is what America is all about. ..... Gabe Suarez
    Its not about how fast you can load, but about how well you can shoot ..... Someone being saved by a speed load is not something that has happened with any regularity. Gabe Suarez

  5. #5
    Join Date
    Aug 2008
    Location
    Richmond, VA
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    Was the barrel 16" with the carbine length gas system and you changed out the barrel to a 18" barrel with a rifle length gas system?

    If it's a rifle length gas system gas port it may be just a tad too small for a barrel shorter than rifle length (18" vs. 20"). The shorter barrel results in gas pressure dropping off quicker since the bullet leaves the barrel 2" prior (and some micro seconds). I don't know this for a fact, it's just what I've read. I do know that a FAL I used to have had gas issues due to a shortened barrel. A previous owner had the barrel shortened and a compensator welded to the barrel to get it back to legal minimum length. Even when the rifle was clean the gas regulator had to be within a couple clicks of wide open. Within a couple hundred rounds you'd have to open the regulator up a couple clicks to maintain functioning and then after a couple more magazines you were out of adjustment range and you had a short stroking rifle in your hands. Opening up the gas port cured that.

    I also have an 18" AR15 with a rifle length gas system that would not reliably run my pet 55 grain FMJBT reloads with IMR4198 powder. Short stroking again. IMR4198 is about the bottom of the window for AR15/.223 powders due to it's fast burn speed. However, it works in 10 or 12 other AR15's from 8.5" to 24". I opened up the gas port just a tad on this rifle, too. Functioning since has been 100%.

    As others stated, cutting the buffer spring shouldn't be a way to get it to function. Just not a good idea. Make sure you've got a carbine spring, not a rifle spring. Make sure you've got a carbine buffer and not a heavy buffer. Make sure it's well lubed. Make sure you've got decently powdered ammo. Make sure everything from the chamber back is clean and well lubed.

  6. #6
    Join Date
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    Quote Originally Posted by M1A's r Best View Post
    Was the barrel 16" with the carbine length gas system and you changed out the barrel to a 18" barrel with a rifle length gas system?

    If it's a rifle length gas system gas port it may be just a tad too small for a barrel shorter than rifle length (18" vs. 20"). The shorter barrel results in gas pressure dropping off quicker since the bullet leaves the barrel 2" prior (and some micro seconds). I don't know this for a fact, it's just what I've read. I do know that a FAL I used to have had gas issues due to a shortened barrel. A previous owner had the barrel shortened and a compensator welded to the barrel to get it back to legal minimum length. Even when the rifle was clean the gas regulator had to be within a couple clicks of wide open. Within a couple hundred rounds you'd have to open the regulator up a couple clicks to maintain functioning and then after a couple more magazines you were out of adjustment range and you had a short stroking rifle in your hands. Opening up the gas port cured that.

    I also have an 18" AR15 with a rifle length gas system that would not reliably run my pet 55 grain FMJBT reloads with IMR4198 powder. Short stroking again. IMR4198 is about the bottom of the window for AR15/.223 powders due to it's fast burn speed. However, it works in 10 or 12 other AR15's from 8.5" to 24". I opened up the gas port just a tad on this rifle, too. Functioning since has been 100%.

    As others stated, cutting the buffer spring shouldn't be a way to get it to function. Just not a good idea. Make sure you've got a carbine spring, not a rifle spring. Make sure you've got a carbine buffer and not a heavy buffer. Make sure it's well lubed. Make sure you've got decently powdered ammo. Make sure everything from the chamber back is clean and well lubed.
    I have been told that people shortening the barrel of the SKS so the muzzle is not so far from gas port can result in malfunction. Opening the gas port was one way to restore reliable functioning.
    One who hammers his gun into a plow plows for those who do not....Unknown
    ...at the end of the day its not about anything else but YOU AND YOURS..... Gabe Suarez
    ....WANT not NEED is what America is all about. ..... Gabe Suarez
    Its not about how fast you can load, but about how well you can shoot ..... Someone being saved by a speed load is not something that has happened with any regularity. Gabe Suarez

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