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Thread: M-4 / Carbine Gas Port Modification

  1. #1
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    M-4 / Carbine Gas Port Modification

    Looking to query the masses as to the suitability or success of modifying a gas port dimension.
    To be specific, I am re-utilizing previously shot out 20" custom barrels. These barrels have outstanding bores, it is just that the throats are gone from NM shooting. These are cut down to 14.5", re-threaded at the breach, chambered, headspaced and when assembled, have a longer muzzle brake pinned on. Previous ones have shown phenomenal accuracy, however there lurks a common problem - gas port size.
    The larger gas port in a 20" bbl is typically .093 in a Krieger.
    I am mulling over an option to tap the hole up in the front sight assembly where the gas travels between the barrel port and the gas tube, install a threaded bushing and re-ream a port to a smaller dimension than the .093 in the barrel or the .120 passage in the front sight between the barrel and the gas tube.
    This would be in essence, a "restrictor plate" in NASCAR parlance.
    Intent is to start at .062 then work up to where I have reliable functioning and avoid the FTE, o ring / extractor / buffer etc etc BS.

    Thoughts?
    Anyone ever tried this?

  2. #2
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    There's been a thread (my forum search-fu fails me at the moment) for a product that does exactly that. Simply a small cylinder with a hole through it, to restrict gas flow.

    Front sight assembly off, insert restrictor, assembly on and test.

  3. #3
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    That's us.

    Black River Tactical CustomTune Gas Port

    The port can be installed into a FSB.


    Also, when rechambering and chopping rifle barrels, I'd suggest a mid length gas system for 14-18" barrel lengths.

    CAR length gas is not the best choice for barrel lengths over 12.5"

    Cutting 3.375" off the breech end makes a 16.625" middy barrel.

    This would make a nice barrel, if the overall profile allows it.

    You always have the option of cutting and recrowning the muzzle if it needs to be shorter.

    Quote Originally Posted by patrick sweeney View Post
    There's been a thread (my forum search-fu fails me at the moment) for a product that does exactly that. Simply a small cylinder with a hole through it, to restrict gas flow.

    Front sight assembly off, insert restrictor, assembly on and test.
    Black River Tactical
    BRT OPTIMUM HFCL Barrels - Hammer Forged Chrome Lined 11.5", 12.5", 14.5"
    BRT OPTIMUM Barrels - 16" MPR, 14.5" MPC, 12.5" MRC, 11.5" CQB, 9" PDW
    BRT EZTUNE Preset Gas Tubes - CAR and MID
    BRT Covert Comps 7.62, 5.56, 6X, 9mm
    BRT MarkBlue Gas Tubes - BRT EXT, EXC and PDW Lengths
    BRT MicroPin Gas Blocks - .750" & .625"
    BRT MicroTUNE Adjustable Gas Blocks
    BRT CustomTUNE Gas Ports

  4. #4
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    Thanks for the input - necessity is the mother of invention!

  5. #5
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    One other option is to go with a 16" or 18" intermediate gas length.

    The INT gas system is 1.5" shorter than RIFLE and 1.875" longer than MID.

    The 16" INT should allow you to keep the .093" gas port as-is.
    Black River Tactical
    BRT OPTIMUM HFCL Barrels - Hammer Forged Chrome Lined 11.5", 12.5", 14.5"
    BRT OPTIMUM Barrels - 16" MPR, 14.5" MPC, 12.5" MRC, 11.5" CQB, 9" PDW
    BRT EZTUNE Preset Gas Tubes - CAR and MID
    BRT Covert Comps 7.62, 5.56, 6X, 9mm
    BRT MarkBlue Gas Tubes - BRT EXT, EXC and PDW Lengths
    BRT MicroPin Gas Blocks - .750" & .625"
    BRT MicroTUNE Adjustable Gas Blocks
    BRT CustomTUNE Gas Ports

  6. #6
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    Money Pit of Fail

    Quote Originally Posted by opsoff1 View Post
    ...To be specific, I am re-utilizing previously shot out 20" custom barrels...

    ...These are cut down to 14.5", re-threaded at the breach, chambered, headspaced and when assembled, have a longer muzzle brake pinned on...
    I need to make sure I'm tracking you...

    You are starting with a used 20" inch barrel that incorporates a rifle length gas system that you are going to reconfigure to 14.5 inches?

    If this is the case, when you cut a rifle length barrel to 14.5 inches the original gas port is gone. Why not just machine the barrel to 16 inches and avoid the need to pin and weld a muzzle device and ream a proper size gas port to begin with?

    Why would it be necessary to re-thread the breach and re-chamber it?

    Huge waste of time and money. Just because you can doesn't mean you should... Just sayin'

  7. #7
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    The chamber end is cut off due to throat erosion.
    "Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -Benjamin Franklin

  8. #8
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    Quote Originally Posted by Quiet-Matt View Post
    The chamber end is cut off due to throat erosion.
    Not possible without a new gas port. The chance of getting the barrel extension timed to an existing gas port is ZERO.
    Last edited by MarkG; 09-29-13 at 15:56. Reason: Syntax

  9. #9
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    Ask the OP, but as I'm reading his post he is basically using the old barrel as a blank. I assume that he would have to plug the old gas port and pretty much start from scratch, especially with the point of barrel extension timing that mk18 brings up. I don't see the cost benefit in this, hopefully he can enlighten us.
    "Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -Benjamin Franklin

  10. #10
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    Well, to clear up some of the confusion - yes, I am cutting down 20" barrels from the chamber end. The breech end is re-threaded, rechambered, a new barrel extension is installed and it is timed exactly to the exisiting gas port. It is actually very easy - so I am not sure what the fuss is about.
    I have been doing this for years and have done dozens of M-4 bbls - every one has hammered and there are no issues. Each extension and corresponding bolt is individually headspaced.
    Attached is a pic of one of them. If you look closely - the index pin in the bbl extension has not been installed yet - but I can assure you - the port, and the pin are in EXACT alignment - I use a fixture that is set up on my milling machine's table.
    Open to questions.

    Edited to add - a new Krieger 20" bbl is up in the $400 range - so when faced with using them as a tomato stake or reutizing them as tack driving M4 bbls - I choose the later - start to finish, I can have one completed in under 4 hours, with the majority of that time spent chambering. And I do it because I can...
    Last edited by opsoff1; 09-30-13 at 12:13.

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