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Thread: Assembling AR15 Upper: Gas Tube Considerations? Overgassing/etc?

  1. #1

    Assembling AR15 Upper: Gas Tube Considerations? Overgassing/etc?

    I am piecing together a new AR15 upper and was planning to take it to a local gunsmith to have them assemble the components. I dont own a vice nor a receiver block and due to living conditions will have difficulties installing a vice. I may do one of those tricks of bolting a vise to a sheet of MDF and sit on the floor and use it that way.

    What I am wondering is, when I read about AR15 uppers, I hear people complain that some backyard gunsmiths are assembling things wrong and overgassing or undergassing the guns. I assume that means when you insert the gas tube into the gas block that there's some configurability as to how much you install the gas tube over the hole in the barrel, and if you have it fully over the hole, then it might be overgassed, or if you partially block too much of the hole it's undergassed.

    I'm not sure if it's something that happens when you assemble, or even earlier, like maybe how the manufacturer made the barrel and gas block?

    I also plan to shoot suppressed, so I dont know if that means I need to configure it to be slightly undergassed, since the suppressor pushes back more gas. And if so does it mean the gun won't shoot as reliably without a suppressor on?

    None of my uppers currently cause any issues either with or without a suppressor, but they were built by reputable gunsmiths. I did have an issue with one SBR not working well either suppressed or unsuppressed, I don't remember, and I changed the buffer tube to make it lighter or heavier (dont remember) and then it worked fine in either configuration.

    I think I wound up having one of each of H, H1, H2 and Maybe H3 if that's a thing (somewhere in my closet of spare parts), which I got precisely for this kind of testing.

    I'm just really curious how this all works and I don't trust googling it and finding some random youtube video because a lot of stuff out there in general is not reliable info.

    Please correct my understanding of what I described. I assume if the gun is overgassed then using a heavier buffer will be indicated to slow down the cyclic rate to normal, and if undergassed then using a lighter buffer is better.

    I have seen adjustable gas blocks but that seems like a gimick. Am I really going to insert some fine tool inside my rail MLOK holes to adjust a gas block if I take my suppressor off? Maybe but it seems like a hassle. And seems like a potential failure point. Also all 3 of my current uppers (11.5" to 14.5" have no issues going from suppressed to unsuppressed once I dialed in the buffer.

  2. #2
    First off if you don't have the proper tools then find someone who does. What components are you putting together? Generally if you use good parts in a "standard" configuration you should not have a gassing problem.

  3. #3
    HAS ELECTROLYTES LittleLebowski's Avatar
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    Adjustable gas blocks are not gimmicks and can be reliable, easy to adjust, add to your shooting experience (way less gas to the face), and lower the noise of your shooting. Just install, set for your load, and forget.

    @Hansohn Brothers
    #RESIST

  4. #4
    Making sure your AR build is properly gassed starts by choosing a barrel with the proper gas port size for the gas system length you’re using: carbine vs mid-length vs rifle, etc. Generally your gas tube can only install one way within your gas block because the pin securing them together has to pass through a hole in the gas block and the gas tube. Assuming you bought quality parts, the gas tube hole should line up fine with the gas block hole. If your gun ends up not being properly gassed, there is a company that sells gas tubes with holes of different sizes to mitigate issues.

    You may observe that certain steps in your cycle of operations occur differently between suppressed and unsuppressed shooting. The last AR I put together was intended to be shot primarily suppressed. The upper was a Geissele with a 0.068” gas port. That small gas port, combined with an H2 buffer, a Sprinco extra power spring, and added mass to the bolt carrier from the Law folder bolt extension realsulted in the bolt failing to lock back on empty magazines with .223 pressure ammunition when shot without the suppressor. When shot with the suppressor, it locks back just fine.
    My posts only represent my personal opinion and do not necessarily reflect the opinions or official policies of any employer, past or present. Obvious spelling errors are likely the result of an iPhone keyboard.

  5. #5
    Quote Originally Posted by Sanch View Post
    I am piecing together a new AR15 upper and was planning to take it to a local gunsmith to have them assemble the components. I dont own a vice nor a receiver block and due to living conditions will have difficulties installing a vice. I may do one of those tricks of bolting a vise to a sheet of MDF and sit on the floor and use it that way.
    Harbor Freight sells a platform to put a vice in a trailer hitch, do you have a garage where you could work on your gun out of sight?

    As far as the gun being gassed properly, that was mostly decided when the port was drilled into the barrel, doesn't really have anything to do with how you screw it together.

    IMO the toughest thing about putting an upper together is being confident the block is aligned with the port. I bought the dimple jig from SLR Rifleworks so I would not be guessing.

    ETA:
    Quote Originally Posted by Sanch View Post
    What I am wondering is, when I read about AR15 uppers, I hear people complain that some backyard gunsmiths are assembling things wrong and overgassing or undergassing the guns. I assume that means when you insert the gas tube into the gas block that there's some configurability as to how much you install the gas tube over the hole in the barrel, and if you have it fully over the hole, then it might be overgassed, or if you partially block too much of the hole it's undergassed.
    I just reread this and wanted to elaborate that like I said, it is not an assembly issue. The port is supposed to directly align with the gas block, no matter how big the holes are. I think part of the probably might be some folks (me?...) tend to assign more complex terminology to simple things. If a gun is "overgassed" or "undergassed" it just means the manufacturer drilled a hole too big or too little. Add in some other variables like the shoulder position (some have clearance for the thin stamping that hold the front of a conventional handguard that is absent with a FF rail, some don't) and there is really no good way to align the gas block, and probably 99/100 of them done by people like you and I probably get eyeballed (unless the barrel comes dimpled). The thing has to be really off to not function the action, so who knows how many people are asserting online that their rifle is undergassed because whoever put it together got the gas block crooked.
    Last edited by mmc45414; 05-05-2021 at 10:16 AM.

  6. #6
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    Quote Originally Posted by Sanch View Post
    backyard gunsmiths are assembling things wrong and overgassing or undergassing the guns. I assume that means when you insert the gas tube into the gas block that there's some configurability as to how much you install the gas tube over the hole in the barrel, and if you have it fully over the hole, then it might be overgassed, or if you partially block too much of the hole it's undergassed.
    The deciding factor is not the way of inserting the gastube (wouldn't mess with that), but the gasport in the barrel.
    If its on the small size for the barrel length, it may be better suited for suppressed use (and hot 5.56 ammo).
    If its too big (overgassed), as many cheaper brands do, I would look at one of these:

    https://blackrivertactical.ecwid.com...ine-p103167251

    for easy switching between suppressed and unsuppressed mode in 300 BLK, I got one of these:

    https://bootleginc.com/product/full-...table-carrier/

  7. #7
    @Sanch

    https://primaryandsecondary.com/unde...15-gas-system/

    That article should answer all your questions about the AR gas system.

    I miss my friend Will Larson.
    Last edited by tango-papa; 05-05-2021 at 10:25 AM.

  8. #8
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    The New World
    Quote Originally Posted by Sanch View Post
    I am piecing together a new AR15 upper and was planning to take it to a local gunsmith to have them assemble the components. I dont own a vice nor a receiver block and due to living conditions will have difficulties installing a vice. I may do one of those tricks of bolting a vise to a sheet of MDF and sit on the floor and use it that way.

    What I am wondering is, when I read about AR15 uppers, I hear people complain that some backyard gunsmiths are assembling things wrong and overgassing or undergassing the guns. I assume that means when you insert the gas tube into the gas block that there's some configurability as to how much you install the gas tube over the hole in the barrel, and if you have it fully over the hole, then it might be overgassed, or if you partially block too much of the hole it's undergassed.

    I'm not sure if it's something that happens when you assemble, or even earlier, like maybe how the manufacturer made the barrel and gas block?

    I also plan to shoot suppressed, so I dont know if that means I need to configure it to be slightly undergassed, since the suppressor pushes back more gas. And if so does it mean the gun won't shoot as reliably without a suppressor on?

    None of my uppers currently cause any issues either with or without a suppressor, but they were built by reputable gunsmiths. I did have an issue with one SBR not working well either suppressed or unsuppressed, I don't remember, and I changed the buffer tube to make it lighter or heavier (dont remember) and then it worked fine in either configuration.

    I think I wound up having one of each of H, H1, H2 and Maybe H3 if that's a thing (somewhere in my closet of spare parts), which I got precisely for this kind of testing.

    I'm just really curious how this all works and I don't trust googling it and finding some random youtube video because a lot of stuff out there in general is not reliable info.

    Please correct my understanding of what I described. I assume if the gun is overgassed then using a heavier buffer will be indicated to slow down the cyclic rate to normal, and if undergassed then using a lighter buffer is better.

    I have seen adjustable gas blocks but that seems like a gimick. Am I really going to insert some fine tool inside my rail MLOK holes to adjust a gas block if I take my suppressor off? Maybe but it seems like a hassle. And seems like a potential failure point. Also all 3 of my current uppers (11.5" to 14.5" have no issues going from suppressed to unsuppressed once I dialed in the buffer.
    Do you know a reputable armorer? What is your location?
    Are you loyal to the constitution or the government?

  9. #9
    Quote Originally Posted by LittleLebowski View Post
    Adjustable gas blocks are not gimmicks and can be reliable, easy to adjust, add to your shooting experience (way less gas to the face), and lower the noise of your shooting. Just install, set for your load, and forget.

    @Hansohn Brothers
    What AGB would you recommend? I have not heard of any DI gas block that is reliable and also easy to adjust, outside of maybe the Novseke Switchblock, Seekins Select, or now discontinued MicroMOA Govnah. The SLRs seem to be decent if you're okay with set-and-forget, same with the Superlative, but both seem to have a enough failures to make me leery (though SLR is suppose to have excellent CS).

    I don't really see the point of shooting unsuppressed once you have a can, outside of niche situations or if you a bitch, so I would personally just go for something like the BRT inserts or gas tubes and tune to gun to run suppressed only. Use an LMT e-BCG and A5 system to give a larger operating window. I have a Govnah on my 14.5", and I guess its nice enough, but honestly for my upcoming 10.3", which will be the only rifle I'll want to have for mixed use (if only because the can will be removed to keep it easier to transport when combined with a folder), I'll probably skip out on using my last Govnah on, just don't see the juice being worth the squeeze when a decent port size combined with other measures should have it work good enough.
    Last edited by Default.mp3; 05-05-2021 at 12:20 PM.

  10. #10
    Quote Originally Posted by Default.mp3 View Post
    What AGB would you recommend? I have not heard of any DI gas block that is reliable and also easy to adjust, outside of maybe the Novseke Switchblock, Seekins Select, or now discontinued MicroMOA Govnah. The SLRs seem to be decent if you're okay with set-and-forget, same with the Superlative, but both seem to have a enough failures to make me leery (though SLR is suppose to have excellent CS).
    Rainier Arms recommended the superlative, when I was trying to diagnose some accuracy issues on a recent build. It worked fine enough for the short time I used it. The main reason for installing a SGB on a 16" SPR barrel was to try and smooth out the cycle, via the AGB, as excess gas can upset the system. It did help to bring down the group size

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