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Sub-sonic .308 barrel twist?

Discussion in 'NFA Firearms and Accessories' started by wally, Jul 4, 2019.

  1. wally

    wally Member

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    Not interested in debating the merits or stupidity of the idea. I'm asking because I ran into a couple of guys at the range working up sub-sonic .308 reloads in anticipation of getting their suppressors in another half year :)

    Their "rainbows" were mostly connecting at 300 yards on the steel plates so I'd say they had the loads worked out pretty good. What I'm asking about is what barrel twist would be required to insure adequate "initial stability" to insure no baffle strikes when they get their new cans?

    If a "standard" .308 twist barrel is fine they will be thrilled, if not, the wisdom and experience here could save them a lot of grief when their wait is finally up!

    I was hit with the "initial stability" issue with a .22lr suppressor and the Aquila 60gr "sub-sonic" ammo causing a baffle strike. Not catastrophic, but not a happy day at the range either.
     
  2. s76

    s76 Member

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    https://bergerbullets.com/twist-rate-calculator/

    A 1:10 is pretty much standard for 30 cal barrels. I have a 1:7 for my 300BLK (same bullet). The faster twist allows a heavier, longer bullet to be stabilized. A 1:10 will stabilize 150 grain size bullets but at 1000 fps it will only be a fun range toy.
     
    Last edited: Jul 4, 2019
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  3. mcb

    mcb Member

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    For heavy 200+ gr bullets I would want 1:7 twist especially with the increasingly popular all copper subsonic expanding projectiles. From my 1:7 twist 9-inch 300BO barrel launching 220gr Maker REX bullets at ~980 fps that bullet is starting to yaw at 100 yards, round holes at 50 yards.
     
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  4. JeffG

    JeffG Member

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    My subsonic 180 roundnose (950fps) are fine with 1-10 twist. BTW it is a great idea to work up your load before introducing your suppressor. Assess stability, accuracy, etc.
     
  5. mcb

    mcb Member

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    Remember the all copper bullets are longer for the same weight and thus require a faster twist. Should have included this image with my previous post.

    Ltp7gk1.jpg

    You can see the all copper Maker REX on the right even with a blunter Ogive is substantially longer than the spire pointed Berry's platted lead bullet on the right despite both being 220gr bullets.
     
    Last edited: Jul 5, 2019
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  6. wally

    wally Member

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    Thanks this is exactly the info I was looking for. They were chronographing, looking to be as fast as they could but still sub-sonic. The way they were hitting steel plates at 300 yards I'd say they'd worked up some good loads. I forget to tell them about the potential "suppressor boost" which some times gives a bit of extra velocity, so they may end up being just super-sonic once they get the cans if they are too close to it now. But they will be relieved about the barrel twist issue.
     
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  7. JeffG

    JeffG Member

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    If you have worked out sight settings for 300 yards, you are way ahead of me. I keep mine under 100 yards. The longer bullets carry a lot of energy, even at lower velocities. At 50 yards on steel, all I find is the banana peeled jacket. The lead is gone.
     
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