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Thread: My 14.5 Mid-length doesn't like 55gr.

  1. #11
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    Seems that the short stroking issues are being caused by insufficent dwell time because you cut your barrel down without opening the gas port.

  2. #12
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    It's actually the Sabre Defense upper they sell that is 14.5" with 1/7 twist, and has the mid-length 9" handguards from the factory. It's made this way. They permanently weld an "Extended A2 flash hider" making OAL 16" which I had removed and welded on a vortex.

    http://www.sabredefence.com/commercial.php?focus=barrel

    I will look into the ammo issue, but my bolt ran fine in another AR.

  3. #13
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    I agree, as long as the extractor spring/assembly isn't worn out, it's definately a lack of gas pressure. The Hornady TAP rounds worked because they have quite a bit more natural recoil with a heavier bullet and a heavier charge - and could push the BCG further backwards on it's own without proper gas pressure.
    Are you certain your gas tube is clean?? It takes a lot of shooting to get one dirty enough (600-750 rds) to clog or stop... I would remove the gas tube, rinse it out well with Hoppe's #9 or another suitable powder/carbon solvent, and run a few clean gas-tube-sized pipe cleaners through it. Then I'd do the same thing with the FSB, and BCG's gas key. If the 55 gr. loads STILL doesn't function.. you may need a lighter bolt/buffer.
    The rifle's internals are pretty heavy, since it's designed to shoot 68 - 85 gr. bullets (optimally), since it's a 1/7" twist. 55gr. is even too light to get great accuracy from most 1/9" twist barrels... they were designed to shoot 60 - 70 gr. bullets optimally... The 55gr. was designed for the M16A1, which had a 1/12" twist.
    I hope you get it working soon!
    "A government big enough to give you what you want, is strong enough to take what you have." -T. Jefferson

  4. #14
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    Here's the simple test for weak ammo or gas leakage (i.e. short-stroking)

    Take 10 known good magazines, clear your gun. Close the bolt. Insert 1 of the 10 empty mags into the gun and pull the bolt all the way to the rear the bolt should lock back. Remove the mag and try the other 9 mags one at a time all 10 mags should lock the bolt to the rear while manually cycling the action.

    Next:
    Load 1 round (and only 1 round) into each mag that you tested.
    Load 1 mag into the gun and chamber and fire the round.
    It should lock the bolt to the rear everytime the gun runs empty on the 10 mags.

    If it doesn't it could be leaking gas at the gas key, gas tube or gas block, try another known good bolt carrier (using your bolt).

    If all this is all good and in good working order you may need to have the gas port enlarged.

    If yours is the 14.5" midlength with the 2pc YHM flip up front sight gas block, those are notorious for leaking as well. If I had that I would replace it with a steel low profile gas block and pin it.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

  5. #15
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    Who cut the barrel back for you?
    "Respect all, fear none"

  6. #16
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    Quote Originally Posted by k9dpd View Post
    Who cut the barrel back for you?
    https://www.m4carbine.net/showpost.p...6&postcount=12
    He said it's factory Sabre Defence.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

  7. #17
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    Isn't most 55 gr stuff (XM193) supposed to be some pretty hot loaded stuff?

    Some of our SWAT guys had Saber Defense M4's, actually they got them from the manufacture locally for a pretty sweet price but they had several issues with them and ultimately followed in my foot steps and bought a Colt.

  8. #18
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    Quote Originally Posted by gotm4 View Post
    Next:
    Load 1 round (and only 1 round) into each mag that you tested.
    Load 1 mag into the gun and chamber and fire the round.
    It should lock the bolt to the rear everytime the gun runs empty on the 10 mags.

    If it doesn't it could be leaking gas at the gas key, gas tube or gas block, try another known good bolt carrier (using your bolt).
    We may have something here. I know quite a few times the bolt didn't lock back on an empty mag ( the few times it did fire correctly ) and I did check the mags as you described and they were known reliable mags.

  9. #19
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    I have two 14.5" midlengths. A CMMG and a Sabre. Both have H buffers. I've never had any trouble at all after thousands of 55gr rounds.

  10. #20
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    Knowing these facts I am thinking that the gas port is wrong, the gas tube is burned out or excess leakage at the FSB. Nothing else makes sense and if the bolt and carrier assy. has worked in another rifle w/o problems and the mags have been checked then that's all it can really be.

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