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5.56 NATO - Barrel twist vs bullet weight?

Discussion in 'Rifle Country' started by shield20, Oct 14, 2007.

  1. shield20

    shield20 New Member

    I have a couple AR-180s which have the older 1/12 twist - meant for the 55 gr 5.56. How much of an effect will this have on the accuracy of the M855 rounds? I am questioning the accuracy/bullet stability of one of these rifles.

    Last edited: Oct 14, 2007
  2. shield20

    shield20 New Member

    Ahh - never mind - found it!

    "While the slow 1 in 12" twist is adequate to stabilize the 55 grain M193, it will not stabilize the 62 grain M855. As a result, the newer M855 ammo will group 1-2 feet at 100 yards, with bullets flying through the air sideways, instead of shooting to about 2" at 100 yards, like military ammo should."

    Sounds like what I was seeing.
  3. Harley Quinn

    Harley Quinn New Member

    :scrutiny:First there is this:

    Then we need to mention that the 62 grain is not going to be well stablized with a 1/12 some will mention. I was under the impression that is the way it is or was back then.

    The lighter bullets will shoot just fine I figure, but the proof is in the puddin as they say. Shoot it with both at 100 yds and you will see:uhoh:

    What year were the guns made and what was the ammo then that was being shot:scrutiny:
  4. S-K

    S-K New Member

    As borrowed from AR-15.com:

  5. RockyMtnTactical

    RockyMtnTactical New Member

  6. GunTech

    GunTech New Member

    The rate of twist will generally be determined by the length of the bullet, which is why round nose bullets require a slower twist than spitzers of the same weigh, while copper monolithics require a faster twist for the same weight. Ogive taper, boat tail length, meplat and a number of other factors determine the ideal twist. So rifles will stabilize one brand of bullet and not another of the same weight for this reason. Initial velocity and station pressure also factro in. Cold dense air requires a faster twist than warm air at high altitude.
  7. LeibstandarteAdH

    LeibstandarteAdH New Member

    Id kind of like to know what the rate of twist is on my StG-2003C in 5.56x45 for this very reason of projectile selection.
  8. LeibstandarteAdH

    LeibstandarteAdH New Member

    In that case, whats wrong with a lil overtwist? AKA why not just go with 1 in 7 for everything? If less twist is less stable, id much rather spin it a little faster then in likes then be keyholing things.
  9. taliv

    taliv Moderator

    army seems to concur, as all their M4s are twisted 1/7
  10. Jim Watson

    Jim Watson Active Member

    1x14 - if you want to understabilize the .224 bullet, use this twist. Useless except for tumbling effect it causes upon bullet impact.

    This would be news to Remington, Winchester, Harvey Donaldson, Jerry Gebby, and a whole host of others. All commercial and most wildcat .22 centerfires except early .22 Hornets did just fine with 14" twists until the military started messing with .22s and had to stabilize cheap boattails in Siberia, then keep adding weight and length trying to make the .223 into a full time infantry rifle. The 7" twist is required only for the tracer; a 9" is ample for M855/SS108 hardball.

    Liebwhatthehellever, if you want to know the twist rate of your barrel, measure it. A swivel handle cleaning rod, a tight patch on a jag screwed in tight, a pen mark or tape flag on the rod. Measure the distance you have to move the patch for the rod to make one turn.
  11. possum

    possum New Member

  12. Kurt_D

    Kurt_D New Member

    I'm not 100% positive but it's probably 1/7. The STG is basically a dressed up Romanian WASR, I've heard the Romanian 5.56 AKs (PAR-3, SAR-3, WASR-3) use 1/7.

    Even if it's not, it'll be at least 1/9 which will get you to 69 gr. Actually 77gr works in my Bushmaster 1/9 but that varies gun to gun.
  13. LeibstandarteAdH

    LeibstandarteAdH New Member


    So simple and potentially useful insight, but if 1 in 14 inches is as stable as you say it is, unless my silver bear 55 grain JHPBT's ever start exploding in mid air, i really could care less.
  14. JohnKSa

    JohnKSa Active Member

    I don't understand where all this super-fast twist rate information for .224 diameter bullets is coming from.

    Lilja says that 1:12 is sufficient for up to 60 grain bullets and that 1:9 will work up to 75 grains.

    Berger suggests that 1:12 is sufficient for up to 64 grain bullets and that 1:9 will work up to 75 grains.
  15. Kurt_D

    Kurt_D New Member

    I think the fast twist rates come from ARs and surplus ammo.

    1/12 will stablize 55 gr no problem and probably work with up to 60-62 gr regular bullets, not the best but workable. Problem is ss109 62gr bullets are longer than a regular 62 gr bullet and will keyhole in a 1/12. I have read win 64 gr powerpoints will also keyhole from a 1/12. 1/9 works best with these ss109 bullets and also allows you up to 75-77 gr normal bullets, once again not the best but doable.

    1/7 was for the tracer rounds to stablize. It's "mil-spec" that's why many opt for that twist. 1/8 will handle everything up to 80gr.

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