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300 Blackout Carbine

Discussion in 'Articles & How-Tos' started by Beeg, Oct 10, 2018 at 9:10 PM.

  1. Beeg

    Beeg New Member

    Sep 24, 2018
    I recently purchased a 300 Blackout carbine. Right out of the box I started having trouble with subsonic rounds not reliably ejecting the casing and not picking up the following round. I have learned that the 300 BLK pistol barrel "gas port" is around 7" fro the breach and the carbine is around 11". This difference doesn't allow the subsonic to create enough back pressure to fully recharge the bolt. I'm told that a suppressor might create enough back pressure to shoot subsonics? Does anyone out there agree with this?
    Next question: I can buy a carbine barrel with a pistol gas port. Does this created a problem with shooting supersonic rounds??? The twist is 1:7 on a carbine barrel and 1:8 on the barrel drilled for the pistol gas port. Is that a problem? I appreciate any knowledge on this!

  2. Younggun

    Younggun Doer of Deeds TGT Supporter Admin

    Jul 31, 2011
    hill co.
    Yes, a carbine length gas system can be finicky with subs.

    Yes, a suppressor can help with cycling issues. Depending on the gas port size you may still have problems but it sounds like you don’t need much, just a hair more to get it cycling.

    A pistol length system would probably fix the issue. The cartridge was designed around a short barrel and suppressor.

    Yes, a 1/8 twist will be just fine.

    There are also a few things you can do to help your rifle function with subs.

    First, get heavy subs. The heavier the bullet is, the more gas pressure it will take to push it while remaining subsonic. What weight were the Subs you were using?

    You can order an extra buffer spring (cheap) and clip a few coils off. Go a little at a time and only trim what you have to. Get it too short and it won’t drive the bolt carrier forward with enough force. It could also cause the buffer to start battering against the buffer tube which is hard on everything. Especially when you switch to supers.

    All that said, I don’t see much point in shooting subs if they aren’t suppressed. You loose a lot of punch for hunting, the ballistics are kinda crap, and bullets that will expand become fairly expensive.

    Sent from my iPhone using Tapatalk
  3. Gummi Bear

    Gummi Bear Member

    Mar 24, 2015
    My rifle is carbine length, and it cycles everything. My buy built an identical rifle (just different furniture) and his is flawless as well.

    We drilled our barrel to .125” (YHM low profile gas block)

    I have a rifle length stock, and use a carbine spring and buffer (I cut and fit a dowel into the buffer tube, so the length is the same internally)

    I primarily shoot supersonic, and hunt with it all the time.


    Since this picture (4-1/2 years ago), I’ve added a suppressor. The last year or so, I’ve been having some fun stretching it out. I shot a pig at 300, and a coyote the next morning at 310, one shot each.

    I went to the woods because I wished to live deliberately...

    Henry David Thoreau

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