Quantcast
  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Why a 1/9 twist barrel?

Discussion in 'Rifle Country' started by Redfisher60, Sep 13, 2017.

  1. Redfisher60

    Redfisher60 Member

    Joined:
    Jul 31, 2017
    Messages:
    65
    In the AR world, is a 1/9 twist less expensive? I see from 1/7 to 1/9 twist in complete AR's.
    Why would they choose 1/8 or 1/9 over 1/7?
     
  2. Hokie_PhD

    Hokie_PhD Member

    Joined:
    Jan 29, 2017
    Messages:
    684
    Location:
    SW Virginia
    I haven't seen any noticeable difference in prices based on twist rate. Most cost difference seems to be based on quality, name brand, length and material.

    The most common twist rates right now seem to be 1:7 in 16" barrels. These give decent performance for 62gr and 55 gr bullets.

    If I'm not mistaken 1:8 is the next most common then 1:9. Likewise after 16" the longer you get the barrel the less common they become. Ironically, short lengths for SBRs and pistol length ARs seem to be growing in Popularity.

    Don't forget to add in gas length. Right now carbine is the most popular, then pistol, mid and rifle.

    In my case I'm building a gun I plan to use for 3 gun. So I'm going with an 18" barrel with rifle gas and 1:8 twist.
    The idea is for a soft shooting gun, that is good to about 300 yards and will handle a wide range of ammo.

    If you search there's a great chart on twist rate vs bullet weight that someone posted.

    My suggestion to you is to figure out what you want your AR to do then find the right barrel for it. Shop around and you'll find a good deal as there are lots out there right now.
     
    1stmarine likes this.
  3. jmr40

    jmr40 Member

    Joined:
    May 26, 2007
    Messages:
    10,401
    Location:
    Georgia
    There is probably less difference between a 7, 8 or 9 twist than most of us think. The original M-16's had either 12 or 14 twists. Seems like they started with 14 and quickly changed to 12 but I'm not certain. I see conflicting reasoning, but some people argue that this slow twist created a slightly unstable bullet that would tumble more easily after impact resulting in a more deadly bullet. I won't say there is any truth to that, but is an argument I've heard.

    This instability lead to poor performance at longer ranges. Going to a faster twist and heavier bullets gives better accuracy at longer ranges. The 7 twist is probably ideal for the heaviest bullets, but even the 8 or 9 twist do pretty well and are seen as a compromise considering the original 14 twist. Some bolt guns in 223 still have 12 twist barrels. At least in theory a 8 or 9 twist should be a better all around barrel twist for the guy who wants to shoot bullets from lightest up to heaviest.

    My AR's all have 7 twists and I get good accuracy with bullets as light as 50 gr. Never tried anything lighter. I have one bolt gun with an 8 twist and it does fine up to 77 gr, which is as heavy as I've tried.

    I've concluded this is one of those things we worry about more than necessary.
     
    easy and RussellC like this.
  4. TheVision

    TheVision Member

    Joined:
    Jan 26, 2011
    Messages:
    107
    Location:
    Connecticut
    The Armalite rifles were originally outfitted with 1:14" barrels, because that is what was commonly available in .22 caliber guns at the time. The first in-country weapons generated positive reports (of the destructive capabilities of the round), but as you mentioned, instability concerns resulted in the 1:12" twist becoming standard.

    The move to a much faster twist was made during NATO's Russian helmet penetration requirement and the (eventual) standardization on the SS109 projectile. 1:9" would have been ideal for this bullet weight, but it would not stabilize the longer, heavier tracer projectile (at all service temperatures), so 1:7" was arrived at.
     
    Orcon likes this.
  5. FL-NC

    FL-NC Member

    Joined:
    Feb 10, 2016
    Messages:
    1,784
    Location:
    Fl panhandle
    It seems to me 1:9 is most common in the civilian barrels. I have read claims that they are optimum for most projectiles under 70 grains. I have also read that 1:8 is the best for heavier projectiles like 77 grain MK 262. In practice, I fire heavy bullets from barrels ranging from 10" to 18" in 1:7, 1:8 and 1:9 and get great stability and accuracy.
     
    Varminterror likes this.
  6. boom boom

    boom boom Member

    Joined:
    Sep 24, 2007
    Messages:
    1,238
    Location:
    GA
    If you want to play around with how barrel twist theoretically affects bullet performance, here is a link to an online barrel twist calculator for theoretically optimal performance twist for a particular bullet/velocity configuration. Also look for the Greenhill formula which was revised by Miller and others for further information. http://kwk.us/twist.html

    Reportedly, the switch from 1:14 to 1:12 was due to military testing it's ammunition under artic conditions in Alaska. The 1:7 twist adopted by the military, I have seen attributed to stabilizing a tracer round but at least in theory, is not optimal for varminting type light bullets which is not a concern of the military anyway. The 1:8 seems to be a civilian adaptation that stabilizes most heavy bullets available in .223 but allows a bit more latitude with cheaper lighter .223 bullets than military issue 1:7 twist rate. The 1:9 was the old civilian std. which as FL-NC points out works for most stabilizing most bullets under 70 grains (this can vary a bit by a particular individual barrels as all of these standards can) and works with cheaper lightly constructed varminting bullets.

    One of the frustrating or enjoyable aspects of firearms and load development is that the barrel is simply one more part of a system that has to function at a high degree to have an accurate firearm. There are a lot of factors that can cause one particular firearm (aside from the quality of the shooter) to shoot some loads better than others and sometimes to perform better than expected on some bullet configuration that it should by theory. Sometimes, facts are found out about bullets as a lot is still unknown about the totality of the trip from muzzle to target (see Hornady's recent use of radar snapshots during flight to adapt its bullet tips for greater accuracy at longer ranges). Would like to see them investigate with the same technique, the alleged .303 Mark VII tendency for the bullet to "settle down" and become more accurate at longer range than it appears when fired at shorter ranges. It is easier to make a rifle inaccurate and unreliable than ensure reliability and accuracy in a rifle which is generally why if you pay more, you should expect more accuracy and reliability.
     
  7. Jim Watson

    Jim Watson Member

    Joined:
    Dec 24, 2002
    Messages:
    22,793
    The M16 developmental history as I recall reading in American Rifleman and other gunzines back when they were moderately technical:
    The varmint rifle 14 twist was found not adequate to stabilize cheap 55 grain boattail FMJ in cold dense air. Nothing to do with lethality in the tropics. So they went to the 12 twist.
    The USMC rifle team built some 10 twist rifles with the first free float handguards to squeeze the most out of the 55 gr M193.
    After that, it was a continual process of longer/heavier bullets and the faster twists needed.
    The 9 twist and the 68-69 grain bullets seem to have come along hand in hand.
    And as said, a 9 twist is fine for the M855/SS109 but it takes a 7 twist for the very long tracer.
    Target shooters seem to have settled down with 7.5-8 twist and 80 grain bullets for 600 yards. Magazine length 77 grain for shorter ranges.
    I have a 6.5 twist barrel for 90 grain boattails but that seems to be a dead end.

    The .303 does not partake of the Legend of Sleepy Bullets, it "compensates." Barrel vibration frequency is such that bullets at the low end of the normal velocity range are launched at a higher angle than bullets at the high end. High angle low velocity and low angle high velocity trajectories converge at some extended distance, I think about 600 yards.
     
    Last edited: Sep 13, 2017
    boom boom and Klint Beastwood like this.
  8. Damon555

    Damon555 Member

    Joined:
    Jul 7, 2008
    Messages:
    877
    Location:
    The south
    An AR15 in .223/5.56 with a 9 twist is ideal for most of the bullets loaded in factory ammo....Personally I see no need to shoot the heavier bullets out of a short barreled rifle (or longer barreled AR15 for that matter) due to the poor velocity. The 9 twist 223/5.56 really shines with lighter bullets and even does well with not so light bullets like the 69 grain offerings for civilians. Unless you are trying to shoot long range and need every last little bit of BC to get you to your target there's no real need for a 7 twist barrel. Magazine length COL constraints really shove those heavies into that little case eating up a lot of space too....YMMV
     
    chicharrones and boom boom like this.
  9. Redfisher60

    Redfisher60 Member

    Joined:
    Jul 31, 2017
    Messages:
    65
    Thanks for the informative replies!
     
  10. Klint Beastwood

    Klint Beastwood Member

    Joined:
    Jul 12, 2017
    Messages:
    378
    This correct. for any questions on stability numbers, I would suggest investigating miller stability formulas and what they mean. Things that influence stability are bullet profile length and velocity. Which goes hand and hand with weight.
    As far as usefulness of stability, for those that shoot under 500y, no you won't see any real difference. However yes, it will have more of an effect at longer ranges especially as a bullet hits transonic.
     
    boom boom likes this.
  11. MTMilitiaman

    MTMilitiaman Member

    Joined:
    Apr 28, 2005
    Messages:
    2,656
    Location:
    Missoula, Montana
    If you are getting a .223 varmint rifle, a slower twist might serve your needs. But in a 5.56mm fighting carbine, I insist on a 1:7 twist because the longest and heaviest bullets have both the longest range and the best terminal effect.

    I consider the barrel to be the heart of a rifle. I am pretty picky about my barrels. It's the first thing I look at. If a rifle has a 1:9 twist, it's a varmint rifle, not a fighting rifle, period.
     
  12. Jim Watson

    Jim Watson Member

    Joined:
    Dec 24, 2002
    Messages:
    22,793
    The Army's meddling has screwed up the public perception of twist requirements and the concept of a varmint rifle. A 14 twist for 52 grain boattails and 55 grain flatbase spitzers was fine for several decades. Now people think they are limited to 40 grains in their fine old .222s etc.
     
    melbr00ks likes this.
  13. larryh1108

    larryh1108 Member

    Joined:
    Aug 29, 2008
    Messages:
    2,443
    Location:
    NC
    chicharrones likes this.
  14. mjsdwash

    mjsdwash Member

    Joined:
    Jan 14, 2011
    Messages:
    657
    from what I read, the ss109 bullet was found to be optimal for accuracy out of a 1/9, but a confused army doctrine on what an infantry rifle was for made them change it to use tracers, and that was once the only reason 1/7 existed.... however with 1/7 they started making heavy bullets to take advantage, but since 95% of the ammo bought in the US outside gov contracts is 55 grain, and 1/9 works well with it and its ideal for 62gr, the most common(1/7 is well known to blow up 55gr bullets at full speed) its a good all around number.
     
  15. kwg020

    kwg020 Member

    Joined:
    Dec 26, 2012
    Messages:
    305
    Location:
    Iowa
    It takes energy to create spin and it takes energy to create velocity. If you (me) are shooting under 69 grain bullets, I prefer more velocity. I think 1/7 is over kill and 1/12 is way too slow. For a 55 or 62 grain bullet, I see the 1/9 a real good compromise. I like a 1/8 for the occasional 70 plus grain bullet but for me, the 1/9 is a great option 99% of the time.
    kwg
     
  16. hdwhit
    • Contributing Member

    hdwhit Member

    Joined:
    Nov 16, 2015
    Messages:
    3,005
    Location:
    Frisco, TX
    While a faster twist rate is needed to adequately stabilize heavier (i.e. longer) bullets, the faster the twist, the more mechanical stress is placed on the bullet.

    As an example, say we have a 45 grain bullet fired at 3400 fps from a 1/7 barrel. At the muzzle, the bullet is spinning at approximately 350,000 rpm:

    3400 feet 60 seconds 12 inch 1 revolution
    -------------- * ----------------- * ---------- * ---------------- = 349,714 revolutions per minute
    1 second 1 minute 1 foot 7 inches

    At 1/8 it drops to 306,000 rpm and at 1/9 it drops to 272,000 (22% slower). This is why if I fire a cartridge I have loaded with 45 grain bullets at 3400 fps out of a 1/7 barrel, I end up with a puff of silver-grey vapor about 50 feet from the muzzle and nothing hitting the target because the bullet simply vaporizes on its way to the target.
     
    Last edited: Sep 13, 2017
    Geno likes this.
  17. Varminterror

    Varminterror Member

    Joined:
    Jul 17, 2016
    Messages:
    1,999
    What you don't see discussed enough in this type of thread is barrel length. What can be done with a 24" rifle is very different than a 10.5" pistol. I have ran 77's in a 1:9" 26" bolt gun which stabilize just fine, whereas I struggle with the same bullet in a 1:9" 16" carbine. Jump over to a 1:7" 10.5" pistol and I'm back in business.

    Twist rate x velocity = spin rate, so lowering velocity can mean the twist needs to increase to keep the same stabilizing spin rate.

    A lot of guys, myself included, will point out a great number of bullets will shoot well at certain ranges even in slow twist barrels. I can single load 75 A Max's in a 1:9" 16" carbine and shoot nearly if not as small of groups at 100yrds as anything it ever shoots. However, the wheels come off of that bullet around 350-400yrds, and I can even see keyholing by 650. Whereas the 69smk will stay true. It's not misleading to say a barrel shoots the 70+ grn bullets well, if you only ask them to fly well for a couple hundred yards, but that is not the same thing as saying they are properly stabilized.
     
    grampajack and boom boom like this.
  18. ColoradoMinuteMan

    ColoradoMinuteMan Member

    Joined:
    May 8, 2015
    Messages:
    223
  19. MistWolf

    MistWolf Member

    Joined:
    Aug 17, 2005
    Messages:
    2,405
    Not in the 5.56/223
     
    RussellC likes this.
  20. FL-NC

    FL-NC Member

    Joined:
    Feb 10, 2016
    Messages:
    1,784
    Location:
    Fl panhandle
    I've never seen a 55 grain "blow up" in a 1:7. I fire them regularly through my multiple 1:7 carbines and have had nothing but consistent performance from a number of manufacturers. In addition, one of the advanced shooting schools I attended at Ft Bragg exclusively uses 55 grain M193 through standard issue 14.5" M4 carbines, and they literally fire millions of rounds of this stuff, with engagements out to 300 meters. I believe they use that round in order to minimize wear and tear on steel targets. Never saw or heard of this happening there.
     
  21. FL-NC

    FL-NC Member

    Joined:
    Feb 10, 2016
    Messages:
    1,784
    Location:
    Fl panhandle
  22. grampajack
    • Contributing Member

    grampajack AR Junkie Extraordinaire

    Joined:
    Mar 31, 2016
    Messages:
    1,429
    The 1:7 is milspec, so it's prolific. 1:9 dominated the civilian market for a long time because it makes more sense, generally speaking, for what most people shoot. 1:8 is more versatile than the other two options. With bullet weights ranging from 30-80 grains and barrel lengths ranging from 5-30 inches, it's definitely necessary to have options. If you're only shooting 5.56 out of 16'' barrels then it probably doesn't matter that much. It starts becoming a problem when you move into extremes.
     
    Hokie_PhD likes this.
  23. RussellC

    RussellC Member

    Joined:
    May 26, 2013
    Messages:
    2,218
    Yup, I also shoot 52 and 53 grain SMK with great accuracy with a 1/7 twist.

    Russellc
     
  24. mjsdwash

    mjsdwash Member

    Joined:
    Jan 14, 2011
    Messages:
    657
    may do fine out of a 14', may do fine with heavy jackets, but I have personally seen 55's explode into dust at the 75 yard mark with enough consistantcy to demonstrate for other people, and its not exactly an unheard of thing. You can find THR posts about it. I was chronoing 55 grain bullets at about 3300 FPS at the time and was keeping them in 2' at 50, and off paper, at 100. Looking out between the two you could see puffs of smoke about 4 feet up, 75 or so yards out when the rifle fired. Its was a neat thing to see. Im sure military bullets would be fine even at the speed because the specify jacket material and dimensions. Commercial bullets (especially the cheap ones I saw this happen with) are looser. I agree the military specifies to avoid them in the A2 because of accuracy/wear.
     
  25. mjsdwash

    mjsdwash Member

    Joined:
    Jan 14, 2011
    Messages:
    657
    https://www.thehighroad.org/index.php?threads/are-my-bullets-disintegrating-in-flight.821778/ this is what I found on the subject without looking, and while 52gr is not 55gr, its fairly close, and he's also producing velocity well below the max for a 55. I had numerous 55's explode out of a 1 in 7 using commercial bullets. 1 in 9 did fine. I think military bullets would be fine, but military 55gr bullets are not exactly the standard.
     
    RussellC likes this.

Share This Page