Pennsylvania Firearm Owners Association
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  1. #11
    Join Date
    Feb 2010
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    Levittown, Pennsylvania
    (Bucks County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    I think it has a carbine buffer in it. Where did you get your lighter buffer & spring?
    I'm not trying to be a wise guy. The question made me think of a hacksaw and a cut-off Dremel.
    There are two kinds of guns. Those I have acquired, and those I hope to.

  2. #12
    Join Date
    Jan 2007
    Location
    State College, Pennsylvania
    (Centre County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    OP, is your buffer spring also a carbine length spring? Also, take your buffer out and actually weigh it.

    You said with a lightweight bolt in it it cycled fine with subs but still wouldn't lock open when empty. You could take some weight off the buffer. Either take your buffer apart and remove a weight or buy a lighter buffer to test that. It not locking when empty could be that the bolt is going back far enough pick up the next round but not far enough back to let the magazine follower to get into position to actuate the bolt release.

    Buffers of different weights can be bought from just about anyone who sells AR parts.
    Ron USAF Ret E-8 FFL01/SOT3 NRA Benefactor Member

  3. #13
    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

    Quote Originally Posted by sbmania View Post
    I think it has a carbine buffer in it. Where did you get your lighter buffer & spring?
    First thing would be to be like xringshooter said, weight your buffer. that'll determine where you're starting and what modifications are needed. As others said, It sounds like your bolt isn't cycling fully back which is caused by the under gassing of subsonic rounds. Your choices are reducing the buffer weight, reducing the spring (reduced rate / yellow spring weight is about %20 less than standard), or increasing the gas pressure (but not really an option since you don't have an adjustable gas block).

    Here's an article about buffers and springs and a reference chart with weights.




    I got a JP Enterprises silent captured spring because I wanted to see if it really made that much of a difference in sound but honestly the jury is still out I'm not sure it's worth the extra cost.

    Hope that helps,
    Mitch

  4. #14
    Join Date
    Jan 2014
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    gilbertsville, Pennsylvania
    (Montgomery County)
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    Default Re: AR pistol converted to 300 Blackout - Won't cycle subsonic

    Not to Captain Obvious, but I chased an "under gas" issue for an afternoon before realizing there was a carbine length gas system on this 10.5" 300blk! It was from a few years back when we were still experimenting. So I re-tapered the barrel, threaded and plugged original gas port, and drilled the new gas port to .087" to be able to install a pistol length system. As imagined it cycles everything fine now! ....Have fun, Gerry

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