300 Blackout Build questions

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  • OnyxATX

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    Sep 24, 2013
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    Austin, TX
    Hey all,

    I've got some questions, or rather asking advice to make sure I'm on the correct path with my 300 blackout build. Basic AR platform, with a DI pistol length system on a 1 in 8" twist 7.5" barrel. (My 5.56 7.5 is an adjustable piston, so different game with this one.)

    I do plan to run a can on it, just waiting on the ATF to do their thing. (Saker 762)

    For testing it out, I plan to use a standard carbine buffer spring. I currently have a standard carbine buffers, but know that putting in an H2 would probably be a good idea to get the speed correct. (Locking back every time but failure to chamber rounds is too light a buffer from my understanding and pretty common with 300 blackout.)

    I have a few boxes of 220 grain subsonics now, and hear they will be more picky without a can on it. (As expected) 120-125 grain supersonics should run better without a can... but they aren't something that's available right now in my area.

    Is there anything specific I should watch out for when sighting in it? (and breaking it in as well.) I know the usual suspects on 5.56, but since the 300 blackout is a different round was curious if there were any more "unique" problems that came up in these builds.

    Thank you all!
     

    1slow01Z71

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    Jun 24, 2012
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    Kyle
    Where are you getting a 7.5" 300blk barrel? Thats kind of an odd length for the cartridge. Standard carbine buffer and spring should work fine. Supers run great with the can too. You can expect little gas face but its nothing terrible. Realoding for this cartridge is pretty easy and I'd highly recommend doing it. Its hard to get this most out of 300blk without reloading. Pretty much every 300blk I've loaded for likes something different depending on the bullet. It is a very fun cartridge to shoot and very versatile.
     

    TXARGUY

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    May 31, 2012
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    Wildcat Thicket, Texas
    Where are you getting a 7.5" 300blk barrel? Thats kind of an odd length for the cartridge. Standard carbine buffer and spring should work fine. Supers run great with the can too. You can expect little gas face but its nothing terrible. Realoding for this cartridge is pretty easy and I'd highly recommend doing it. Its hard to get this most out of 300blk without reloading. Pretty much every 300blk I've loaded for likes something different depending on the bullet. It is a very fun cartridge to shoot and very versatile.

    It is odd but I've been seeing these 7.5" 300BLK barrels popping up in ads everywhere.

    1.5" short of a full burn and not even a single full twist.

    I don't know who is making them but I'm going to find out because I would like to ask them some questions.

    I believe they came in response to the SIG brace as it seems they popped up very soon after it came on the market.

    A 'hey, check it out, people are buying these so they can build an SBR of sorts with no stamp on an impulse. Let's exploit that market segment somehow' type of thing.
     

    OnyxATX

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    Sep 24, 2013
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    Austin, TX
    Certainly reloading is going to be down the line. Not a reloader now, but getting into the 300 blackout game it would be nice to make my own.

    For the manufacturer of the barrel, I'll let you all know in a week or so. Would like to do some testing on it and everything before making a judgement and discussing that side of things.

    I was looking at 8.5-9" or so originally... so if the 7.5" has any weird issues it isn't going to kill me to swap barrels. However, from everything I'm reading about 7.5" it seems to work out just fine.
     

    Leper

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    Runner runner sells a 7.5 complete 300blk upper, so i imagine they ok to use.
     

    1slow01Z71

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    Jun 24, 2012
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    Kyle
    Only thing I worry about with shorter barrels is it requires larger gas port to function correctly. Add a can on the end with a big gas port and a stout super load and youre going to be pretty rough on your gun. That's not to say youll actually have any durability problems but an adjust able gas block may be needed to get recoil tamed down a bit. I think itll run just fine as long as who made the barrel knows what theyre doing. Im doing 6.5ish" build as soon as I can find someone to section my rail. I intend on running an adjustable gas block though for switching from subs to supers.
     

    Chief Gunner

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    Jun 10, 2014
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    Corpus Christi Tx.
    Just don't like the fact that unless you are getting at least one full revolution imparted on the bullet you are not that stable. And I would be Leary on putting a can on it.. I run a pistol gas system 10.5 barrel functions with supers and subs no problem. Also very accurate considering the length


    Sent from my iPhone using Tapatalk
     

    OnyxATX

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    Sep 24, 2013
    291
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    Austin, TX
    Got to test it out today, with supers (125 federal premiere match) it was on point. Cycled flawless, accuracy was good. The optic I had on top isn't for precision by any means (Meprolite M21) but no problems I could spot.

    I'll probably put some glass on top for further testing and reach out a bit further to make sure. From what I could tell everything was stable and nothing went weird. (80 rounds or so total for the day). Any shots that were off were my own doing. I do understand the concerns of going that short and not a full rotation... but so far so good.

    It was a bit goofy with 220 subs though, as I kind of expected. It wasn't getting enough dwell time to really cycle it perfectly. This is something I'm not too worried about though... I could fix it with the buffer spring / buffer. But, it seems timed dead on for supers, and with a can on there I'm confident it would cycle subs just fine. I'm concerned if I tune it to run subs without a can, it will be out of wack for supers.
     
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