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AR rifle twist 1-in-7, 1-in-9, overspin,... etc

Discussion in 'Rifle Country' started by wacki, Nov 2, 2008.

  1. wacki

    wacki Senior Member

    Sep 16, 2006
    Reminiscing the Rockies
    I saw the below text on a another forum. Why is overspinning bullets bad? How much extra accuracy will you get with a 1-in-7 over a 1-in-9 on the milspec bullets?

  2. briansmithwins

    briansmithwins Senior Member

    Aug 1, 2005
    My 1/7 AR has both 50gr and 55gr bullets delivering 1.5" groups at 100 yards.

    It may be possible to 'overspin' bullets but I've yet to see it happen in real life. BSW
  3. cracked butt

    cracked butt Senior Member

    Jan 3, 2003
    SE Wisconsin
    Its possible to cause bullets to 'blow up' if they are driven too fast with too tight of a twist rate- but this usually occurs near the end of the barrels' life span when the throat gets rough.

    FWIW, my AR currently wears a barrel with a 6.5:1 twist rate which was meant for 80-90gr bullets, but shoots 52 gr bullets very well.
  4. chrissmallwood

    chrissmallwood Member

    Jul 12, 2007
    Seymour, Indiana
    Im no expert so take this with a grain of salt but I was under the impression that the issue of bullets disintegrating due to too tight a twist only arose when using the thin jacketed 40-45grn varmint bullets.
  5. Jim Watson

    Jim Watson Senior Member

    Dec 24, 2002
    MY 6.5 twist .223 meant for 90 grain bullets, will blow up 75 grain A-Max in midair, if pushed fast enough to make it to 1000 yards supersonic. Fine 600 yard load, though.

    Military FMJs are tougher than varmint and target bullets, and stand the extra centrifugal force better.

    The quote in the OP "I’m not sure the reasons why a 1:7 twist rate in particular was selected;" is readily cleared up. The 7 twist is for the SS 110/M856 tracer bullet. A 9 twist is adequate for the ball ammo. It is marginal for 75-77 grain bullets, though, so if you want the heavies, 75-82 grains, get an 8 twist, or a 7 for the extra heavies of 90 grains from Sierra, Berger, and JLK.

    The first recorded use of fast twist .223 barrels was the early effort by the USMC to make the M16 into a target rifle, with a 10 twist to add a little stability and accuracy to cheap 55 gr M193s.

    Everybody is twist concious these days, and the matter is considerably overanalyzed for most purposes.
  6. RockyMtnTactical

    RockyMtnTactical Senior Member

    Oct 21, 2006
    I think 1/8 is the ideal twist for anyone not shooting tracers (M856). I wish that were the standard.

    1/7 was a compromise between the M855 round that needed a 1/8 or 1/9 twist and the M856 that needed a 1/6 twist.
  7. Larry E

    Larry E Member

    Mar 15, 2005
    Billings, MT
    Aside from causing some bullets to come undone, a too fast twist isn't going to degrade practical accuracy for the average rifle, cartridge, or shooter. The only place it's really important is in competitive benchrest - maybe. For field use, a twist that's 'too fast' is better than one that's 'too slow' IMHO.

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