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Thread: Worth Opening a Gas Port?

  1. #1
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    Worth Opening a Gas Port?

    My new 300BLK SBR features a Faxon 8.5" CL 4150 1/8 with a gas port measured at 0.081", per Faxon. Gas Block is a pinned Geissele SGB done professionally by Rainier Arms LLC.

    I'm running a Toolcraft C158 BCG, Tubb's LW Flatwire spring (lower pressure version), and inter-switched with an H/H2 buffer. The rifle cycles 123-125gr supers fine but 200-220gr subs will fire and eject but won't cycle. I tried fresh pmags as well. I didn't try a carbine buffer yet. The rifle would seldom lock back on the last subsonic round (only when 1 round is loaded in the mag)

    I do have an AAC SDN6 still pending so I will fire subs suppressed BUT since I do like a variety of shooting any 300BLK rounds whether super/subs without a can reliably. Is it worth opening the gas port to .100 or .104? Would having a port this large cause any further issues when it is suppressed? I'm thinking excessive gas blowback.


    Specs:

    UPPER:
    *Rainier Arms Milspec Forged Upper
    *Faxon 8.5” 300BLK 5R Chrome-lined Barrel 4150 1/8 Twist (Rainier Arms Medium Contour) (.081" Gas Port)
    *AAC BLACKOUT 51T 7.62 Muzzle Brake
    *AAC 7.62 SDN-6 Suppressor (F4 Pending)
    *Geissele Super Gas Block (Pinned)
    *Toolcraft 1B1B6 C158 Phosphate BCG
    *Geissele Super Modular Rail MK4 7” DDC
    *Daniel Defense Fixed Sights Front/Rear
    *Geissele Airborne Charging Handle DDC
    *Geissele Super Precision MRO Lower 1/3 Mount DDC
    *Trijicon MRO (Post 89000 SER.)
    *SureFire M300C Scout Tan + UE07 Tape Switch
    *Cloud Defensive Light Control System Mount MK1 FDE

    LOWER:
    *Rainier Arms Forged Lower
    *LaRue Tactical MBT-2S Trigger
    *Geissele LPK
    *Geissele Maritime Bolt Catch
    *Magpul Enhanced Aluminum Trigger Guard
    *Griffin Armament Ambidextrous Safety Selector
    *Magpul MOE+ Grip
    *LWRCI Compact Stock
    *LWRCI H2 Buffer
    *BCM 7075-T6 Milspec Receiver Extension
    *Tubb’s Lightweight SS Flatwire Buffer Spring (300BLK/Low Pressure Spring)
    *BCM QD Endplate
    *CrossTac 1-2 Point Sling Coyote Brown



  2. #2
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    The barrel appears to be ported for supersonic ammo only.
    Black River Tactical
    BRT OPTIMUM Barrels - 16" EXT, 11.5" CAR, 9" PDW
    BRT Covert Comps 7.62 + 5.56 + 9mm
    BRT MarkBlue Gas Tubes - BRT EXT and PDW Lengths
    BRT EZTUNE Gas Tubes - CAR and MID
    BRT MicroPin Gas Blocks .750" & .625"
    BRT MicroTUNE Adjustable Gas Blocks
    BRT CustomTUNE Gas Ports

  3. #3
    Join Date
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    Many .300 Blackout barrels will not cycle properly with subs when not using a suppressor. This helps to address overgassing situations when the suppressor is added. Shooting subs without the suppressor serves no purpose and wastes ammo.



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  4. #4
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    This could also be a gas block issue. Personally I like adjustable gas blocks. The barrel might be too short to create enough pressure to send back that bolt at 220 gr subs. I was going to suggest Superlative arms but they haven’t made one for 300 blackout. I would say Adams arms but you will have to cut/modify that barrel but for Geissele as the gas tube isn’t compatible with any of Geissele’s barrel nuts.


    Sent from my iPhone using Tapatalk

  5. #5
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    Not sure why you'd want to shoot subs unsuppressed?

    I would not open it, I have some cans pending too. I have a 8.5" Aero precision barrel that cycled two kinds of subs with a an H2 buffer. I'm going ditch the barrel, or not bother with it suppressed as I think it have a gaping blow hole of a gas port. This thread makes me think I'll look at the faxon barrels.

  6. #6
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    If you want to use it all 4 ways - subs, supers, subs suppressed, supers suppressed you will need to open the port up. I would not do anything till you get your can and can do testing with it. As a reference the AAC 9" barrel is .100.

  7. #7
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    It sounds like I may have the same barrel as you have. I purchased it from Rainier had them install and pin a Centurian Arms Gas Block. When I received it, it wouldn't cycle a number of supersonic loads, which made me question the size of the gas port. I called Faxon, and at the time they said it was .082"... which is a little small compared to similar barrell's of the same length.
    I asked them about enlarging the gas port and was advised not to go over .100 . A friend had already gone through the process, so I followed his advise and went up to .086" since I could always go a step bigger, but couldn't go back smaller.
    The Pistol I assembled with this barrel now shoots everything supersonic, and without a suppressor will cycle most sub-sonic's...
    This approach seemed to make the gun very reliable as well... I'm using a standard Buffer Spring and H-Bufffer.

  8. #8
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    Strange if we have the same barrel. Mine cycles supers without issue. Since your port is now .086" are you experiencing any excessive gas when suppressed?

    This is the barrel I have

    https://www.rainierarms.com/faxon-fi...150-5r-barrel/

  9. #9
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    Sorry I have been a little busy... Yep, I have the same barrel. No I have not experienced any excessive gas since opening up the port. This may be in part for Two reasons, 1). I only went up .004 ( a very small increase from the original .082 ) ,and 2) the can has an internal opening of .378", which helps diminish back pressure. I have also developed my own Hand loads, which also helps prevent any over gassing.

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