Another AR15 question

Discussion in 'Large-Bore/Small-Bore Rifle/Shotgun' started by lonewolf204, Jul 5, 2012.

  1. lonewolf204

    lonewolf204 Active Member

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    Where can I find a 20" barreled upper with a 1:10 or 1:11 twist that is not a bull barrel?
  2. LDBennett

    LDBennett Active Member

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    lonewolf204:

    Those are barrel twist you are very unlikely to see. They are way too slow for the common 223 bullets in use today. 1 in 9 is about the best you will be able to do. I think the twists you want would only be good for extremely light bullets, like less than 40 grains. Take a look at the Greenhill formula:

    In 1879, Greenhill developed a rule of thumb for calculating the optimal twist rate for lead-core bullets. This shortcut uses the bullet's length, needing no allowances for weight or nose shape.[3] Greenhill applied this theory to account for the steadiness of flight conferred upon an elongated projectile by rifling. The eponymous Greenhill Formula, still used today, is:

    where:
    C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
    D = bullet's diameter in inches
    L = bullet's length in inches
    SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)
    The original value of C was 150, which yields a twist rate in inches per turn, when given the diameter D and the length L of the bullet in inches. This works to velocities of about 840 m/s (2800 ft/s); above those velocities, a C of 180 should be used. For instance, with a velocity of 600 m/s (2000 ft/s), a diameter of 0.5 inches (13 mm) and a length of 1.5 inches (38 mm), the Greenhill formula would give a value of 25, which means 1 turn in 25 inches (640 mm).


    From Wikipedia...

    Note that is the length that is important for a given bullet diameter in determining the minimum twist rate for stability of the bullet. long bullets require a faster twist rate.

    LDBennett
  3. lonewolf204

    lonewolf204 Active Member

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    I'm having a tough time with accuracy with the 1:9 and heavier bullets and was wanting to try shooting some of the lighter ones in a different twist just to see the difference if any. Maybe I exspect to much from the AR 15. Thanks for the info.
  4. keokeboy

    keokeboy Member

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    went to the range today with my sig 556 and blistering heat, i tried the reman-62 gr from dick's sporting goods ,very good results compare to the green tip from wal-mart. shooting 100 yds is a challenge for this old timer. any one got any feed back on range results shooting,their 556 16 in bbl with the 1.7 twist?
  5. aa1911

    aa1911 Active Member

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    I have a 10.5" shorty that easily reaches out to 500 meters plus with surprising accuracy using military green tip and similar. It's an Olympic arms I built with a 1:8 but the 1:7 14.5" barrels also do good groups out at long range. 16 would be nearly identical to 14" I would think. So many bullet options now too compared to 10 years ago.

    In my hi-power service rifle days, someone figured out that 1:7.76 was the optimal twist for the AR-15's but after reading resources about military terminal performance testing, the twist makes no difference. Accuracy is another thing but I think people over emphasize twist rate especially in AR's. The goal is to make sure that the bullet stabilizes in flight for both maximum accurcay and performance downrange.

    55-62gn and 1:7 thru 1:9 will do excellent for the most part.

    that's just my .02
  6. ms6852

    ms6852 New Member

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    Try ar15 barrels.com.
  7. cycloneman

    cycloneman Active Member

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    Stop. Before you spend money on a bbl. Go to the store and buy a box of 55 gr vmax bullets. vmax bullets move the center of gravity rearward. I have a rifle that shoots like crap with fmj. But when i put vmaxes in it i can do amazing things with it. Night and day. Sometimes it is not just the wt of the bullet it is the makeup of the bullet.
  8. oneshot onekill

    oneshot onekill New Member

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    62gr. Is about as light as I would go. The 556 is notorious for spitting 55gr. bullets all over the place. You'll find that 75gr. will be even better. FWIW...
  9. cpttango30

    cpttango30 Guest

    I shoot 60 gr V-Max bullets in my ar with a 1:8" twist and it shoots amazing groups.

    The 1-12" twist stabilizes spitzer bullets from under 40 grains to 60 grain flat-bases. An exception would be the short Speer 70-gr semi-spitzer that was made to stabilize in 1-12". Designed for 55-gr M193 FMJ-BT bullets.

    The 1-10" and 1-9" twist stabilizes spitzer bullets from 40 grains to 70 grains. An exception is that most 1-9" rifles will stabilize the shorter 75-gr Hornady BTHP, but sticking with the popular 68-69 grain BTHP match bullets as the heaviest in a 1-9" for all conditions is a sure thing for stabilization and accuracy.

    The 1-8" and 1-7" twist stabilizes spitzer bullets from 40 to 80 grains. One issue with the longer versions of 75 and 80 grain bullets is that they are so long that they cannot be loaded to magazine length and must be single-loaded into the rifle. The illustrated 75-gr Hornady A-Max is one of that type. Standard military M855 is a 62-gr FMJ-BT and specialized military long range match is Mk262 77-gr BTHP match (Nosler or Sierra).

    You also have to remember that twist rate is more set on length than weight. It just so happens that the more weight in a given caliber the longer the bullet. A 62gr bullet that is the same length as a 77gr bullet will need the twist of the 77gr bullet to shoot.
    Last edited by a moderator: Jul 6, 2012
  10. Raven18940

    Raven18940 New Member

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    Non-bull 20" barrels are rare to start. Then that's just not a common twist. I've seen the original 1:12 and then 1:9, 1:8, and 1:7.
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