Pennsylvania Firearm Owners Association
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  1. #1
    Join Date
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    Phoenixville, Pennsylvania
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    Default AR pistol converted to 300 Blackout - Won't cycle subsonic

    Replaced the 8" barrel on my .223 AR pistol with an 8.5" .300 blackout barrel. Shoots and cycles supersonic ammo fine, but will not cycle with subsonic ammo. It ejects the spent casing but will not pick up the new cartridge, running with either a flash hider or a Sig suppressor. I thought the increased backpressure from the suppressor would help but no. Acts the same with different magazines. Any suggestions on where to look?

  2. #2
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    Oct 2012
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    Jim Thorpe, Pennsylvania
    (Carbon County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    Check the length of the gas tube-may need shorter. Check the alignment of the gas port to the gas block to the gas tube.

  3. #3
    Join Date
    Oct 2007
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    Phoenixville, Pennsylvania
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    Checked all of the above which seem good. I put in a friend's lightweight bolt in the pistol and subsonic ammo cycled correctly. Although the bolt still doesn't lock back. Before I spend the money on a new bolt, any other thoughts on what to do? Thanks!

  4. #4
    Join Date
    Aug 2008
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    DeepInTheWoods, Pennsylvania
    (Warren County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    Quote Originally Posted by sbmania View Post
    Checked all of the above which seem good. I put in a friend's lightweight bolt in the pistol and subsonic ammo cycled correctly. Although the bolt still doesn't lock back. Before I spend the money on a new bolt, any other thoughts on what to do? Thanks!
    NOT a AR15 Guru, me. But I had a similar issue.
    My Ar pistol was working and then it quit cycling reliably.
    Somebody had fitted an aluminum combination gas block and Picatinny rail to it. The gas ate away the aluminum and gas was escaping.
    Not enough was getting back to the Bolt to TCOB.

    Subsonic rounds are producing less gas , YES?

    Look to an adjustable gas block ?
    American by BIRTH, Infidel by CHOICE

  5. #5
    Join Date
    Jan 2016
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    Pittsburgh, Pennsylvania
    (Allegheny County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    How subsonic are your loads ? You may have to bump up your charges or use a different powder or bullet combo. Have you chrono'd them ?

  6. #6
    Join Date
    Jan 2013
    Location
    Richboro, Pennsylvania
    (Bucks County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    You might want to consider a reduced power buffer spring for subsonic. It doesn't sound like your bolt is fully retracting.

  7. #7
    Join Date
    Aug 2011
    Location
    Moscow, Pennsylvania
    (Lackawanna County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    Subsonic routinely doesn’t cycle in these platforms without a suppressor. Which is what the subs are designed for

  8. #8
    Join Date
    Feb 2014
    Location
    Landenberg, Pennsylvania
    (Chester County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    My 2 cents, your buffer spring or buffer weight is too high. Do you know what buffer you have? I just replaced my buffer and spring with a captured recoil spring and buffer to help cycle subs out of a 300blkout conversion.

  9. #9
    Join Date
    Oct 2007
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    Phoenixville, Pennsylvania
    (Chester County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    Quote Originally Posted by mdd423 View Post
    My 2 cents, your buffer spring or buffer weight is too high. Do you know what buffer you have? I just replaced my buffer and spring with a captured recoil spring and buffer to help cycle subs out of a 300blkout conversion.
    I think it has a carbine buffer in it. Where did you get your lighter buffer & spring?

  10. #10
    Join Date
    Mar 2014
    Location
    NEPA, Pennsylvania
    (Wyoming County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    Do you roll your own subsonics?

    Once I got over the coolness I completely quit shooting subsonics. It's just my unsolicited opinion, but aside from the fun factor they don't serve me any self-defense purpose.

    *Since I quit shooting them I'm rusty on my recollection, but I think I may have run into the same problem using one powder until I was able to find A1680.
    "It seems that the Constitution is more or less guidelines than actual rules"
    My feedback: http://forum.pafoa.org/showthread.php?t=305685

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