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Thread: M-4 / Carbine Gas Port Modification

  1. #31
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    Quote Originally Posted by lunchbox View Post
    Cool thread, cool project. Pics and range report would be nice, if your taking tall orders. Thanks for sharing.
    Lunchbox,
    I just finished two bbls, so it is too late for pics during the maching process, but I need to do another bbl for meself. I'll take some pics during the process and a corresponding range report.
    I shoot all military stuff - Mk262Mod1, 70gr TSX & M855 - so maybe I'll post a comparison of the three ammo types for accuracy purposes.

    My pleasure -
    Scott

  2. #32
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    Range Report & Testing Port Sizes

    Quick range report based on different port sizes. Those that have followed this thread are aware that I am using cut down 20" match barrels that have been shortened & rechambered to M-4 configuration/dimensions.
    I had been using the original .093" gas port from the 20" barrel.

    Most are aware that this provides for a VERY stout functioning cycle. When the rifle is shot off the bench or just plinked - never seems to be too much of an issue. When run hard; i.e. transition drills, double tap, sustained firing, that's when the problems arise. FTE, short strokinging etc - all hall marks of an over gassed M-4.
    My plan was to install a threaded insert (8-32 set screw) in the FSB between the gas tube and the barrel. By drilling a new port size in the set screw, I could (hopefully) effectively reduce the gas impulse.
    Getting everything set up and machined was pretty straight forward - I machined 3 sets of set screws, .082 (#45), .076 (#48) & .063 (#52).
    Basic set up was an M-4 with a 14.5" Krieger bbl - chambered in 223 Wylde. Min Head Space - 1.464", 1 in 7.7 twist, with a YHM 5C comp.

    Standard Carbine Buffer Spring and an H Buffer.

    Bolt Carrier was a standard semi-auto carrier with the short complete diameter at the rear.

    Extractor had the standard M-16 spring and blue buffer w/ an O ring installed. (didn't have any M-4 extractor springs / inserts)

    Ammo was initially limited to standard M-855 (LC-09 lot)

    I installed the .082 bushing and tested the rifle hard.
    Not much difference - hard recoil, occasional FTE, failure to lock on empty mag etc.

    Next up was the .076 port size - easy swap out of the port bushing.

    Definite difference in recoil / cycling impulse. Shot the gun hard, got it hot and dirty. I still got intermittant failures to lock the bolt after running full mags through the gun. The FTE disappeared altogether. The failures to lock the bolt open were usually having the front face of the carrier locked by the bolt latch - like the BCG didn't cycle all the way back.

    At this point - I swapped out the bolt carrier for a heavy M-16 carrier and made no other changes.

    In testing - I could feel the difference in the weight - heavier / slower cycling. It did however cure the bolt locking open issue. No problems at all. No FTE - everything ran great.
    So - swapped the light carrier back - problems came back.
    Hmmm.
    At this point, I swapped out the bushing again for a .063. Went back to the original set up with the semi auto carrier.
    I honestly thought that this wasn't enough gas - totally wrong.
    Rifle ran like a dream.
    I mean it was a dream - smooth, gentle recoil / cycling. Fed, locked open, extracted etc etc.

    I tested different mags, std GI 20's & 30's and HK 416 30's.
    No issues.

    I then started to test different ammo. Up to this point, I had shot just M855.
    During the next 45 mins, i shot the following:
    M855
    Mk262 Mod 0
    Mk262 Mod1
    70gr TSX Military load
    55gr SP hunting ammo
    69gr Sierra - Fed Gold Medal
    M856 Tracer
    73gr Berger LTB hand Load

    264 rds - no failures of any kind.

    When I went to clean the rifle - it was noticeable that there was a clear difference in the reduction of soot - small port = rifle runs cleaner.

    So, I have seen the light with small gas ports - total eye opening experience.

    I will be using .063 port bushings/inserts in all my future conversions.
    I also have a number of pictures of the FSB / inserts /set screws etc - I'll post these in a seperate entry.
    Last edited by opsoff1; 10-17-13 at 16:41.

  3. #33
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    Pictures Added

    Sorry on the focus with all of these - done on a Blackberry - (POS)
    Attachment 18423
    These are the 8-32 Set Screws that have been drilled with .063 & .076 "ports" The .082 was installed in the FSB at this point.

    Attachment 18424
    This is a pic of the tapped gas transfer hole or passage that is between the bbl & the gas tube - this is where the port bushings will be installed - kind of like a restrictor plate in NASCAR terms.

    Attachment 18425
    Installing the set screw / port bushing with a 5/32 hex wrench.

    Attachment 18426
    Installing the set screw / port bushing. Different view. The set screw is short enough that it doesn't contact the gas tube or the barrel - it sits in between each.

  4. #34
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    More Pics

    Attachment 18432
    Machined steps in the barrel under the FSB rings - standard mod for virtually all NM rifles - very desireable feature since it allows exact windage zeroing of the front sight to the mechanical center of the rear.

    Attachment 18433
    FSB drilled / tapped with 4 8-32 set screws - these ride in the steps on the bbl. By loosening one side and tightening the other side - the FSB can be centered to the rear sight, thus preserving equal windage adjustments L & R from center.

  5. #35
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    Thanks for all the info, and pics!!
    ^^ Read with southern accent !^^ and blame all grammatical errors on Alabama's public school system.
    Technique is nothing more than failed style. Cecil B DeMented
    "If you can't eat it or hump it, piss on it and walk away."-Dog
    Go where the food is.

  6. #36
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    Awesome work! This is rifle building
    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

  7. #37
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    Excellent work!

  8. #38
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    Appreciate the feedback. One thing I have found over the years is that the learning curve never flattens. There is always something that can be done to improve some aspect.

  9. #39
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    Good stuff! I have a small bundle of cast-offs from my pal who is a high master in NRA High-Power...... saving them for similar treatment some day.

    Maybe you mentioned it, but are you reusing the old barrel extensions or new ones?

  10. #40
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    Is there any erosion concerns restricting the gas like this with the drilled out screws?

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