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LR308-Specific Gas Tube?


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The subject of rifle-length gas tubes in 308 builds was recently discussed on another forum. A well-respected gunsmith pointed out that the distance from the bore centerline to the gas key centerline is 1/16" greater on an LR308 than on an AR15.  Because of this, the gas tube requires a larger bend (offset) than the standard AR15 gas tube has.  According to him, while a standard AR15 gas tube may work, it is not correct and the incorrect fit can lead to future issues. As this site is dedicated to these rifles, I'd like to hear the opinions of those who have a lot of experience with them.  I'm working on my first 308 and I'd like it to be setup correctly. I have no problem altering the bend on the gas tube to achieve the additional 1/16" offset.  From the info he provided (including the attached drawing), it seems like the additional offset is the right way to build it. The only reason I hesitate is that I've found dozens of posts here saying that an AR15 gas tube is fine to use.


Thanks in advance for any info you can provide.


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Over the length of a rifle-length gas tube, the tube itself can easily flex enough to overcome a 1/16" rise at the upper receiver.  What you really have to watch for is whether the gas tube will clear the gas key on the BCG without dramatic impact or clearance issues - if it does, you have nothing to worry about.


There are big enough issues finding out of your barrel vendor advertising the DPMS-based 308 barrel you want is using an Armalite-spec gas system, thus forcing you to purchase another gas tube after your new barrel arrives.  I haven't seen the opposite yet - advertised Armalite AR-10-patterned barrels using DPMS LR308 gas systems.  Seems to only be the advertised DPMS using Armalite pattern gas.  That 1/16" you heard about isn't going to cause you issues if the gas key/gas tube mate well.

Edited by 98Z5V
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    As said above , if your gas tube fits into the Carrier Key properly , there is no issue .  We have seen some issues with Gas Tube length , with the Rifle Length gas tubes on some Barrels , I think Fulton Armory's Criterion Barrels reads you need their Gas Tube , But I have two of their Rifle Length gas , Barrels & I have been using std AR 15 Rifle length Gas Tubes . 

   I will soon find out if my third Criterion Barrel will need anything different , because I have a AR 15 Gas Tube ready & waiting for the Barrel to come from Criterion Barrels .


  One other thing you'all may want to think about , if the Barrel has a .750" Gas Block fitment , wouldn't that have the same centerline of the Barrel  to the .750" outside diameter ?  How would that get the center line of the Gas Tube higher ?  Be it a 308 AR or an AR 15   <dontknow>


  Now maybe Armalite AR 10 May be different , I don't know about that , but there are people who may know , they will speak up .

Edited by survivalshop
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survivalshop, the centerline of the barrel to the gas tube is the same as an AR15.  The issue is that the bolt is larger and the distance from the centerline of the bolt or bore to the gas key is greater on the 308.  I'd get the AR10 gas tube, but then I'd be concerned it was too long.  I'm guessing. as others have suggested, that's not a major issue. If it was, some company would probably be making an LR308 gas tube by now. I may try to add a little bend to mine when assembling it and see how it feels.  I guess if the BCG slides on and off the gas tube smoothly, then it's all good.

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  I wouldn't bend anything till I checked to see how it fits . If the Gas tube was off 1/16" at the Carrier Key , you would notice it , because it would not go into the Key , there's not a lot of room for error . If they are saying its being bent when fitted into the Upper Receiver , you would feel that also & know there was a problem . 


  I will be taking off my Gas Block & Hand Guard ( have a new Hand Guard coming ) Friday on my 16" 308AR & will see how it fits in there . I'm curious now .

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   I had my 16" Hand Guard & Barrel off today & probably because of the diameter of the larger 308 Barrel , as has been said , the Barrel Nut is of course sized in proportion to it , my Gas Tube does rise up a little , but I think most of it is the bulge at that end of the gas Tube . I inspected that end of the Gas Tube & can't see any rubbing or marks that would indicate the Carrier Key is even touching it . I guess I could make a measuring device to see if the Gas Tube at the Key end is not uniform in the cannel , but I don't think its measurement would be something we should worry about .

  I have shot a lot of ammo from this 16" bbl. & if it was going to cause some kind of problem , I  believe I would have seen something by now . If someone wants to make & use a specific 308 Gas Tube , that's fine , I just think its a non issue.

Edited by survivalshop
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