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1:9 or 1:7 Twist in AR15

Discussion in 'Rifle Country' started by Heck, Oct 11, 2008.

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  1. Heck

    Heck Member

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    Wanting to build an AR to shoot longer distance and have heard the 75-77gr bullet is the way to go. My question is that if I go with the 1:7 to handle heavier bullets will it still shoot the 55-62gr rounds ok. How would the 1:9 handler the heavier stuff?
     
  2. peck1234

    peck1234 Member

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    From what I hear, 1:7 stay 65gr and above, 1:9 45-65gr...

    If your shooting 55gr bullets with a 1:7 I think the bullet spins so fast that it literally rips in apart though the barrel, which then becomes unstable in flight....
     
  3. Ridgerunner665

    Ridgerunner665 Member

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    I'd get a 1 in 8" barrel...

    The 1 in 7" will shoot lighter bullets fine...but if you shoot them at very high velocity, it may spin them apart.
     
  4. mckormicnavistar

    mckormicnavistar Member

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    1 in 7 or

    Service rifles that are being used for matches are1 in 7. What I have been told by highpower shooters 1in 9 should work ok with 75. I tried some 77
    with my 1 in 9 Not Good.:cuss:
     
  5. R.W.Dale

    R.W.Dale Member

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    about the only bullets a 1/7 will rule out will be the lighter weight thinly jacketed varmint bullets mostly 50 grn down. So given the parameters you lay out I would go with the fastest twist possible
     
  6. MMcfpd

    MMcfpd Member

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    55 grain works fine in a 1:7 and a 1:9, in my experience, is good with 68 grain.
     
  7. Claude Clay

    Claude Clay Member

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    1/7 69 gr & up at 500 + yards is very good. 55 gr at 100 yards is under 2moa. adequate for 3-gun shoots but sure aint no target round. ss109 or 64 gr at 100 yards in sub 1 moa but pricey to plink with.
     
  8. RP88

    RP88 Member

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    guess I should ask so I don't maybe mess something up on my first AR...

    what happens if you shoot heavier loads that do not cope well with a 1:9 twist? Is there any harm that it will cause if I shove 75gr or more through a 1:9?
     
  9. RP88

    RP88 Member

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    edit: computer problem, sorry
     
    Last edited: Oct 12, 2008
  10. RP88

    RP88 Member

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    Edit: computer problem, sorry.
     
    Last edited: Oct 12, 2008
  11. RP88

    RP88 Member

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    edit: computer problem, sorry.
     
    Last edited: Oct 12, 2008
  12. marsche

    marsche Member

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    I shoot a Bushmaster 1/9 in high power. I shoot the Nosler 77 grain Custom Competition at 200 and 300 yards and the 75 AMAX at 600. They both fly great for me and the 75 grain AMAX is made specifically for a 1/9 twist.
     
  13. marsche

    marsche Member

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    I shoot a Bushmaster 1/9 in high power. I shoot the Nosler 77 grain Custom Competition at 200 and 300 yards and the 75 AMAX at 600. They both fly great for me and the 75 grain AMAX is made specifically for a 1/9 twist.
     
  14. marsche

    marsche Member

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    I shoot a 1/9 Bushmaster in hi-power and it shoots great with both a 77 grain Nosler Custom Competition and a 75 grain Hornady AMAX. I usually shoot the 77 Nosler at 200 and 300 and shoot the 75 AMAX at 600. The AMAX loads way too long to load in a magazine.
     
  15. marsche

    marsche Member

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    I shoot a 1/9 Bushmaster in hi-power and it shoots great with both a 77 grain Nosler Custom Competition and a 75 grain Hornady AMAX. I usually shoot the 77 Nosler at 200 and 300 and shoot the 75 AMAX at 600. The AMAX loads way too long to load in a magazine. I heard the 77 SMK does not fly too good in a 1/9 but the 77 Nosler works great - for me. :)
     
  16. DinoCrayon

    DinoCrayon Member

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    "Another common misconception is that bullet weight determines the optimal rifling twist. This is incorrect in that it is actually bullet (projectile) length that should be used to determine the twist rate. Generally speaking, however, the heavier bullets are also longer so while technically incorrect it is common to say that a 1:7 twist is more desirable for the heavier 75 and 77 grain projectiles. Therefore, choosing a barrel twist really comes down to first choosing your projectile weight, and more correctly, length. If you work for a department that mandates or issues a certain ammunition then this should be your guide when choosing a rifling twist rate. A good rule of thumb is that 1:9 will stabilize bullets in the 45 to 62 grain range, and 1:7 will stabilize bullets in the 55 to 77 grain range. Like all things this is not a given, and any barrel should be tested with the intended ammunition to ensure the desired results are achieved.
    More information about rifling twist and ammunition selection can be found here: http://www.ammo-oracle.com/body.htm "
     
  17. hags

    hags Member

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    Right on the money.

    Also right on.

    I build, shoot and sell alot of ARs. Both "tactical" and varmint/match grade. I recently had a conversation with with one of my barrel suppliers regarding his decision to change a match barrel twist rate.
    It was changed from 1:7 to 1"8 due to many shooters complaining that the 1:7" barrels were causing the bullets to disintegrate in flight.
    Now, most people using these match barrels are top notch competitors and serious shooters. I've had people use up to 80 gr. bullets with the 1/7" twist barrels I supply with my builds with excellent results.
     
  18. Will Learn

    Will Learn Member

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    I wish I could find a upper with a chrome lined 1/8 twist. How come they don't make these?
     
  19. JWarren

    JWarren Member

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    I went with a 1:7 in my M4-gery.

    While I am not disagreeing with the idea of overstabilization of lighter bullets, I do perhaps have some decent anedoctal evidence that makes me somewhat skeptical.

    I've put about 200 rounds of 50 grain FMJ through mine from distances between 10 yards (got a cottonmouth) and 100 yards. These are not any significant distances but I would think that if a bullet was going to rip apart, it would do so at its highest velocities. That would be in the first 100 yards.

    I haven't had a single one rip apart, nor have I experienced any erratic accuracy.

    I never intended to shoot 50 grain in this rifle, but I caught a value pack at Wal-mart for a good price when I first got the rifle, and figured I'd play with those and reload the brass.


    -- John
     
  20. Zach S

    Zach S Member

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    IIRC CMMG offers 18" uppers with CLed 1:8 bbls.
     
  21. rob_s

    rob_s Member

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    Jesus. Do people actually shoot or do they just shoot off their mouths?

    The issue, just to be clear, is not weight but length. The longer the projectile the faster the twist you want. Yes, generally speaking longer projectiles weigh more, but if they have denser, or less dense, cores they could weigh more or less than similar length projectiles. In other words, you could have a 55 grain round that had a core of (this is hypothetical, don't get in an uproar) foam which would potentially make it as long as a 77 grain round and which would therefore benefit from a tighter twist.

    But, given that everything you're shooting is a lead core, and we're talking about readily available ammo, heavier=longer.

    So, what weights are good for what twists? My experience shooting both 1:9 and 1:7 barrels from various makers tells me that
    1:9 are good for 45-62 grain
    1:7 are good for 55-77 grain

    These are approximate. I did not have good luck shooting the 75-77 grain bullets from my 1:9 Bushmaster or Rock River Arms. Despite the fact that I was shooting top-quality ammo (Black Hills), crappy XM193 grouped tighter in these guns than the BH.

    Similarly, I shoot 55 grain Wolf, XM193, and Prvi out of 4 different Col 1:7 barrels all the time and am able to hit 8" stell out to at least 130 yards. I also have made hits on 6" steel out to 200 yards using an Aimpoint, and 11.5" 1:7 barrel, and XM193. Given that this equals <3MOA with a "pencil" barrel SBR, I'd say that 1:7 stabilizes 55 grain just fine and it does not "disintegrate" (where do people come up with this nonsense?). I also shoot 62 grain Wolf and Barnaul out of these same Colt 1:7 barrels, and get a slightly tighter grouping than with the 55. The BH 75 grain that I like and stock up on prints SILLY tight out of these barrels.

    Finally, you need to test YOUR bullets in YOUR barrel. There are guys that shoot 1:9 barrels with 77 grain all day long and get sub-MOA accuracy out of them. There are also guys that shoot 45 grain out of 1:7 barrels and they don't vaporize on leaving the barrel.

    My suggestion is to figure out your application and choose accordingly.

    If your focus is on a defensive-use firearm, I would plan to use the 75-77 grain ammo that works best for this application and stick to the 1:7. Test out various known performing defensive loads in the 75-77 grain range and figure out which one groups best.
     
  22. rob_s

    rob_s Member

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    Something else to consider is the type of barrel. Many people find that 75 grain ammo that won't group worth a damn in a 1:9 M4 barrel groups great in a 1:9 bull barrel.

    You have to find the mix/match that works for you. If you're after match accuracy then nobody cares about terminal performance and I'd buy the barrel I want (probably a 1:8 simply because it seems popular with that crowd) I want and tune/buy my ammo to that barrel. I'd also buy one with a .223 chamber if I was reloading or buying math ammo.
     
  23. lmccrock

    lmccrock Member

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    I bought an upper that was supposed to be 1:9 and turns out it was 1:12. When I tried to shoot 68gr Black Hills, the groups were unpredictable. One time the holes in the target were a perfect silhouette of a bullet (i.e., going sideways) so I know it was a twist/stability problem. (With 52gr BH, groups were under an inch @ 100yds)

    Conclusion? 1:8" is what I want :)

    JWarren, What velocity? An M4 barrel is 6-10" shorter than a typical varmint barrel, so those bullets from an M4 spin nowhere near as fast so they would not come apart from your barrel. More anecdotes From THR archive. ;)

    Also, stability is related to spin (RPM) which is calculated from velocity and spin rate. See this Accurate Shooter article.

    Lee
     
  24. hags

    hags Member

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    Dude, there's no need for the "Jesus". It is Sunday.

    Yes, as someone has already mentioned, it is length and not weight per se.

    No need to rehash.
     
  25. JWarren

    JWarren Member

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    Hiya... I haven't chrno'd it for the velocity, but my M4-gery has a 16" barrel (not 14.5.) Obviously, it won't have the velocity of a 20" or greater length.

    Looks like I am not alone in my observations based upon that link you provided.


    -- John
     
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