Subsonic 300 blackout cycling

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  • jpo183

    Ultimate Member
    Mar 20, 2013
    4,116
    in Maryland
    You guys really are cheap.

    I guess its a time = money trade off for me. I'm going to load 155 scenars in my 300 BLK for supersonic and 208 Amaxs for sub.

    Hobby my dear watson, a hobby.
     

    BenL

    John Galt Speaking.
    100 yards.

    I believe BenL has gotten a few into the sub-moa as well. My loads thru that Rem 700 actually surprised me, I thought they'd be close to MOA, but they were under .8" at 100 yards, string of 5 rounds.

    With supersonics, and admittedly butchering the fundamentals, I've gotten close to MOA on 110gr Vmax's and H110.

    <img src="http://www.mdshooters.com/attachment.php?attachmentid=95011"/>

    With more magnification and not chunking my breathing, I know for a fact that load would be sub-moa.

    I have. I used 220 SMK with 1680 and got 1040fps; I forget the charge weight. They were sub MOA, but the same gun with 208gr Amax's was terrible (5" at 100 yards.) I picked up some 225gr Hornady's to try next.
     

    rsilvers

    Active Member
    Dec 24, 2010
    113
    The first factor is the gas system. A pistol length gas system is much easier to get subs to cycle without a can due to the smaller length the gases travel: i.e. much more of the gas is funneled into the gas tube before the bullet leaves the barrel. With this system subs can cycle with virtually any powder used, such as H110 and Lil gun and IMR 4227.

    Aside from a custom order, very few 16" or longer 300 AAC barrels are available with pistol length gas systems.

    With a carbine gas system, only one powder has been found to be reliable in cycling subs without a can, and that is AA1680. From what I have been told, this is the same powder Remington uses in its factory subsonic loads.

    I would't describe it that way. It is all about gas port size. The correct gas port size for a pistol length gas system does not allow H110 or Lil'Gun to cycle with subsonic.

    The correct subsonic powders, with any length gas system, remain the same - A1680 (not called AA1680), A5744, RE-7, or H4198.
     

    Winterborn

    Moved to Texas
    Aug 19, 2010
    2,569
    Arlington, TX
    I would't describe it that way. It is all about gas port size. The correct gas port size for a pistol length gas system does not allow H110 or Lil'Gun to cycle with subsonic.

    The correct subsonic powders, with any length gas system, remain the same - A1680 (not called AA1680), A5744, RE-7, or H4198.

    So a larger gas port on a carbine length will allow easier cycling with subsonic loads?

    Please elaborate.

    Sent from my EVO using Tapatalk 4
     

    Moorvogi

    Firearm Advocate
    Dec 28, 2014
    855
    i tested some 230gr this weekend also. shot fine but did NOT cycle the bolt. I have a pistol length gas block though.. and using a different powder though so i'll kick in some 1680 and see what happens.
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,818
    i tested some 230gr this weekend also. shot fine but did NOT cycle the bolt. I have a pistol length gas block though.. and using a different powder though so i'll kick in some 1680 and see what happens.

    Try the A-1680.

    What powder were you using?
     

    Glock4arms

    Member
    May 8, 2015
    66
    You will have much better cycling with a pistol length gas system and a 1:7 barrel ratio for those heavy subsonic rounds
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,818
    i used LIL GUN. I have LIL GUN, 4227, and A1680 all on hand to try.

    When I built my 300, I barreled it with a Rainier 10.5" Ultramatch. I had all kinds of cycling issues with it. I even had an SLR gas block along with a Tubb lite flat wire spring. No joy. Eventually, I had to have the gas port opened up(thanks Chad!) and all my problems disappeared.

    If the A-1680 doesn't do it for you, you may want to consider having that looked at.
     

    Moorvogi

    Firearm Advocate
    Dec 28, 2014
    855
    When I built my 300, I barreled it with a Rainier 10.5" Ultramatch. I had all kinds of cycling issues with it. I even had an SLR gas block along with a Tubb lite flat wire spring. No joy. Eventually, I had to have the gas port opened up(thanks Chad!) and all my problems disappeared.

    If the A-1680 doesn't do it for you, you may want to consider having that looked at.

    noob question time.. if the gas port is opened up, doesn't the extra just leak out or is it trapped in the block blowing more thru the tube? must be the latter but.. want to understand.

    it cycles supers just fine, and 230gr.. SOMEtimes.. maybe a hotter load. dont care about super/sub sonic really.. just the bullet weight itself and not having a pipe bomb in my hands HAHHA.

    I looked again.. i have a little bit of h110 also, enough for testing anyway.
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,818
    noob question time.. if the gas port is opened up, doesn't the extra just leak out or is it trapped in the block blowing more thru the tube? must be the latter but.. want to understand.

    it cycles supers just fine, and 230gr.. SOMEtimes.. maybe a hotter load. dont care about super/sub sonic really.. just the bullet weight itself and not having a pipe bomb in my hands HAHHA.

    I looked again.. i have a little bit of h110 also, enough for testing anyway.
    That's a question better answered by Chad or someone other than me. I do know that after a few rounds, the block becomes "sealed" to the barrel by carbon fouling that leaks out around the block. This is a normal occurrence with AR 15s.

    I believe the port of the GB is larger than the gas port in the barrel, so a somewhat larger barrel port will still be smaller than the intake port on the block.

    Sent from my SM-G965U using Tapatalk
     

    Pinecone

    Ultimate Member
    MDS Supporter
    Feb 4, 2013
    28,175
    You are limited by the smaller hole. Gas block or barrel. If one is larger, you can open up the other until the match, then you have to open them both up for more gas.

    Also alignment can be a factor. If the hole in the barrel is not properly located or the gas block canted, the holes will not be aligned. And you will effectively have a smaller hole.
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,818
    You are limited by the smaller hole. Gas block or barrel. If one is larger, you can open up the other until the match, then you have to open them both up for more gas.

    Also alignment can be a factor. If the hole in the barrel is not properly located or the gas block canted, the holes will not be aligned. And you will effectively have a smaller hole.

    It just so happens, Santa brung me a bore scope for Christmas. ;)
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,818
    What model did Santa get you?

    I found they are useful for other things too.

    I got the Lyman. $180 on Amazon prime(before Christmas) and it came Chad and DA approved. They run around $200 regularly.
     

    ToolAA

    Ultimate Member
    MDS Supporter
    Jun 17, 2016
    10,500
    God's Country
    I got the same one. I found myself using it to locate a bolt i accidentally dropped into a space over the firewall in my car and it was rolling around while driving. I didn’t want to take the covers off so the so once I located it I used a magnet to fish it out.
     

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