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Thread: Which gas port size for 14.5 cut to 10.3 for both unsuppressed and suppressed use?

  1. #1
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    Which gas port size for 14.5 cut to 10.3 for both unsuppressed and suppressed use?

    I'm going to the F2/F4 SBR build through Rainier Arms.

    For this particular rifle, I want to cut down the existing barrel (Stag M4 14.5" with perm attached phantom) to 10.3".

    RA can do the cut and thread work, and enlarge the gas port.

    I need an idea of what size the gas port needs to be . . . ammo will be Speer 55 GD, AE .223, PMC Bronze, and occasionally some M193 or M855. I do not use Tula, Brown Bear, Wolf, etc.

    I have read that the MK18 spec calls for .070" and a H2. My best guess, having looked at this on different boards for months, is to go with 0.73", so it'll run .223 pressure ammo.

    Suppressors I have are a AAC 556-SD and a 762 SDN-6. I can use whatever buffer and spring I want. I use Colt gold extractor springs and carbine (black) inserts.

    Been looking through a lot of threads for months but not seeing an answer to my particular situation, hence I'm asking.

    Thanks, guys.

  2. #2
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    There are two ways to go about it. The easiest way is to use a slightly oversized port combined with an adjustable gas block.

    The second is to comb through this thread to get an idea what port size to use and what type of performance to expect
    https://www.m4carbine.net/showthread...essed-SBR-Ever
    INSIDE PLAN OF BOX
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    Yeah, read through that thread already.
    Do not want to use a AGB, although if I did, it would be a Govnah. I may do that down the road for this gun.
    I have a switchblock on non-NFA carbine and like it, but am not going to use a SB for this particular gun. I may buy a 10.5 SB barrel for my next SBR build, but I have yet to figure out all the rail issues that would incur.
    Inferring from MicroMoa's info that .003" is what they find to be the difference between 5.56 and .223, hence my inclination to enlarge the port from the ~.064" is it now, to .073" (.003" over the .070" M18 port size).
    Funny how the DD MK18 is overgassed at .083" (or now, at .078"), but Colt's 10.3" is .093" (for which SAW recommends a H3), if I'm reading correctly.

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    Heck with it. I think I just sold myself on a Govnah.

  5. #5
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    My 10.5 has a gas port of .082" and it's just too large. It works well enough unsuppressed with a rifle RE assembly, but it works even better with the SLR AGB. I gave the Govnah a serious look but in the end, the lighter weight and ease of fine tuning of the SLR won out
    Last edited by MistWolf; 08-19-15 at 07:31.
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  6. #6
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    OP,

    The gas port on your Stag barrel is likely fairly large anyway so I doubt that you would need to have it opened up much (if at all) in order to function .223 pressure ammo. For a 10.3" barrel I would honestly start with no more than a .070" port and just see if it runs suppressed. If you're going to be suppressed the majority of the time, then I would suggest starting closer to .060" and working up, if necessary.

  7. #7
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    Quote Originally Posted by sff70 View Post
    Yeah, read through that thread already.
    Do not want to use a AGB, although if I did, it would be a Govnah. I may do that down the road for this gun.
    I have a switchblock on non-NFA carbine and like it, but am not going to use a SB for this particular gun. I may buy a 10.5 SB barrel for my next SBR build, but I have yet to figure out all the rail issues that would incur.
    Inferring from MicroMoa's info that .003" is what they find to be the difference between 5.56 and .223, hence my inclination to enlarge the port from the ~.064" is it now, to .073" (.003" over the .070" M18 port size).
    Funny how the DD MK18 is overgassed at .083" (or now, at .078"), but Colt's 10.3" is .093" (for which SAW recommends a H3), if I'm reading correctly.
    Has this been confirmed by measurement?

  8. #8
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    Not sure where you are reading that, but a Crane spec 10.3" barrel with a .071 gas port will work just fine with military ammunition.

    Quote Originally Posted by sff70 View Post
    Yeah, read through that thread already.
    Do not want to use a AGB, although if I did, it would be a Govnah. I may do that down the road for this gun.
    I have a switchblock on non-NFA carbine and like it, but am not going to use a SB for this particular gun. I may buy a 10.5 SB barrel for my next SBR build, but I have yet to figure out all the rail issues that would incur.
    Inferring from MicroMoa's info that .003" is what they find to be the difference between 5.56 and .223, hence my inclination to enlarge the port from the ~.064" is it now, to .073" (.003" over the .070" M18 port size).
    Funny how the DD MK18 is overgassed at .083" (or now, at .078"), but Colt's 10.3" is .093" (for which SAW recommends a H3), if I'm reading correctly.



    Owner/Instructor at Semper Paratus Arms

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  9. #9
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    You are trying to go cheap which usually ends up bad. Get a known quality Colt or BCM barrel with a carbine gas port of .063 and then have it opened up and cut down.


    Quote Originally Posted by sff70 View Post
    I'm going to the F2/F4 SBR build through Rainier Arms.

    For this particular rifle, I want to cut down the existing barrel (Stag M4 14.5" with perm attached phantom) to 10.3".

    RA can do the cut and thread work, and enlarge the gas port.

    I need an idea of what size the gas port needs to be . . . ammo will be Speer 55 GD, AE .223, PMC Bronze, and occasionally some M193 or M855. I do not use Tula, Brown Bear, Wolf, etc.

    I have read that the MK18 spec calls for .070" and a H2. My best guess, having looked at this on different boards for months, is to go with 0.73", so it'll run .223 pressure ammo.

    Suppressors I have are a AAC 556-SD and a 762 SDN-6. I can use whatever buffer and spring I want. I use Colt gold extractor springs and carbine (black) inserts.

    Been looking through a lot of threads for months but not seeing an answer to my particular situation, hence I'm asking.

    Thanks, guys.



    Owner/Instructor at Semper Paratus Arms

    Facebook: https://www.facebook.com/SemperParatusArms/

    Semper Paratus Arms AR15 Armorer Course http://www.semperparatusarms.com/cou...-registration/

    M4C Misc. Training and Course Announcements- http://www.m4carbine.net/forumdisplay.php?f=141

    Master Armorer/R&D at SIONICS Weapon Systems- http://sionicsweaponsystems.com

  10. #10
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    Im doing something similar.

    What do you mean "sbr build through rainier arms", do you mean you are buying the parts through them and having them gunsmith/assemble it?

    I have a chf bcm 11.5 and for this next upper want it about 10.7 (handguard+ can, was originally planning on 10.3), I bought parts through them, and got
    their FN chf, rainier mountain, which is a 14.5 I was told had a .068 port so it might be just about right.

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