Twist Ratios Unveiled

Discussion in 'AR-15 Discussion' started by ar15noob, Feb 29, 2012.

  1. ar15noob

    ar15noob New Member

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    Can someone quickly explain the various barrel twist rates and how they apply to the ammo and type of shooting I will be doing. Need insight on mostly range shooting and home defense. Thanks.
     
    Last edited by a moderator: Feb 29, 2012
  2. Jpyle

    Jpyle New Member

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    Twist rate refers to length of barrel in inches needed to complete one full rotation. Usually expressed as a ratio, i.e. 1:7, 1:9, 1:12. All barrels have twist rates, generally speaking the lighter the bullet weight, the slower the twist rate and inversely the heavier the bullet the faster the twist. For example in an AR a twist rate of 1:12 per the original Mil Spec barrel would not be able to adequately stabilize heavier weights above 62 gr but is well suited for lighter varmint rounds. A 1:7 conversely could overspin a lighter bullet causing it to come apart in flight.

    Below is a link to a fairly comprehensive write-up and discussion on the subject...

    http://www.firearmstalk.com/forums/f20/understanding-barrel-twist-bullet-weights-44557/
     
    Last edited: Feb 29, 2012

  3. canebrake

    canebrake New Member

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  4. Flat4sti

    Flat4sti New Member

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    You know that there is a stickied thread above this that covers this subject.

    I was at the range with my 1:7 BCM, I have been shooting XM855 62gr and it is fine. My buddy had a bunch of 45 gr that i gave that a try at 25 yards and it seemed fine. accuracy wise and it held together fine. Even at 100 yards it seemed fine. I had heard that the bullet may spin itself apart, but out of the 15 or so that i shot i did not have one that did this.
     
  5. Quentin

    Quentin New Member

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    Y'know there's been a shift toward these misleading thread titles that seem to be an explanation when actually it's a question.

    ar15noob, is this common on the other discussion forums you frequent or do you guys take classes on this or did I miss the sticky here? Why would you phrase it Twist Ratios Unveiled instead of something more appropriate like Please explain Twist Ratios?

    I'm pretty sure this boils down to a way to intice people into opening your thread and that tactic is not needed here.
     
  6. purehavoc

    purehavoc New Member

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    I think this is what he may be asking
    its real simple to understand , 1:7 one complete revolution of the bullet every 7" of barrel , 1:9 1 rev every 9" 1:12 one rev every 12"
     
  7. cfraga1978

    cfraga1978 New Member

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    Havoc you better hurry and post something else you are at 666 posts.

    Maybe we should break twist rate down a bit more - 1:7= fast spin (heavy bullets) 1:12= slower spin (lighter bullets). If you shoot light weight bullets 40-50 gr with 1:7 the bullet can rip itself apart and if you shoot a heavier bullet 70-90 gr it may not stabilize and tumble in flight. That is as simple as I can put it.
     
  8. purehavoc

    purehavoc New Member

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    Got it thanks ;)
     
  9. AgentTikki

    AgentTikki New Member

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    just to be clear, its not really the weight of the projectile, so much as the length....
     
  10. ar15noob

    ar15noob New Member

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    Quentin.....nothing nefarious meant here. Truly a noob with questions...as you have seen and responded to my other ones this past week or so. Admittedly the title should have been worded better and with a question mark. I appreciate all the responses and look forward to my purchase.
     
  11. ar15noob

    ar15noob New Member

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    So is it safe to think that I would need to know what type of ammo I wanted to shoot first to know what type of barrel twist I should be using or is that over thinking it too much?
     
  12. cfraga1978

    cfraga1978 New Member

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    Not so much.. you need to know what type of shooting you are going to be doing and at what ranges. You have a lot of overlap between them. I shoot a 1:9 and it likes 40 up to 70 grain bullets and a 1:7 may like 55 up to 90 grain. I have heard a 1:8 is a good all around twist.
     
  13. purehavoc

    purehavoc New Member

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    It will vary from barrel to barrel even with the same twist . I have 2, 1:7 twist barrels one shoots 55 gr FMJ fabulous the other doesnt group well unless Im shooting 62 gr. both barrels are from the same manufacturer so you truley wont know what shoots best until you try it .
     
  14. FCross7

    FCross7 New Member

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    True, but within a given caliber, higher weight typically translates to a longer bullet. The only variances are when switching between FMJ and HP.

    -Fred
     
  15. Quentin

    Quentin New Member

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    Fair enough and I shouldn't single out just you, AR. It's just a trend I've seen here and elsewhere. Seems like the title of the thread should try to summarize the content and whether it's a question or a thesis.
     
  16. ar15noob

    ar15noob New Member

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    Thanks a million cfraga this helps a bunch defining grains and twists.