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Thread: Why not 1/8 twist?

  1. #61
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    When shooting High Power matches I always had the best luck at 600 yards with a 1:8 twist barrel. I shot 80g projectiles at 600 yards and 68g projectiles at 200 and 300 yards with a great deal of success, with the goal always being to shoot well under 1 MOA. With a quality 55g bullet I have been able to shoot MOA out to 300 yards at reduced distance matches. Other competitors who shot 90g-100g projectiles preferred a 1:7 twist over 1:8 twist for better accuracy at long distance.

    When selecting a barrel for a 16" carbine I fired a lot of rounds through both the 1:7 twist barrel and 1:9 twist barrel. Unless the 50g-62g projectiles were high quality, I was able to shoot them more accurately with the 1:9 twist barrel. My 1:8 twist barrel shot 55g and 62g ammunition really well, but it was a 20" barrel so I am not sure how a 1:8 carbine length barrel will shoot. For shooting bulk 55g and 62g ammunition with reasonable accuracy, I currently shoot with a 1:9 twist barrel. A 16" 1:7 barrel is good enough for defensive accuracy, so I can understand why some people prefer it. I also suspect that a 1:7 barrel may be better than a slower twist rate if the barrel length is considerably less than 16".

    A lot of the accuracy testing I did was before I owned a computer and my barrel/projectile choice was based on what kind of groups I could shoot at 200, 300 and 600 yards. I can't tell you how software will predict how a barrel/projectile combination will perform, but I do have 30 years worth of shooting data recorded for each barrel/projectile combination I have used in competition and my opinion is based on my data.

    I would prefer a 1:8 twist 16" barrel for all round use and when they become readily available I will start shooting with them. If I had a 1:7 barrel on my rifle, I would not run out and buy a 1:8 barrel if the 1:7 barrel was still serviceable. If you can shoot 4 MOA out to 300 yards with a 1:7 barrel and M193 ball, that should be good enough.
    Last edited by T2C; 02-16-14 at 17:53.
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  2. #62
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    The one thing I have learned over 45+ years of reloading (handguns and rifle). There is a bullet weight/shape/brand,that works best in what ever different twist rate barrel you are loading for and all can be picky. Then you get to find the right powder and load to make that bullet weight work out of that twist rate best. Throw in different barrel lengths and add even more to the equation. For the most part,just about all 5.56/.223 will shoot out of any MSR and it's barrel twist. It's finding and reloading the right bullet weight and powder load (to the grain), that's the real challenge of the hunt and keepin those tight groups on the target. 1-7, 1-8, 1-9, 1-10, 1-12. Just need to take the time and make it happen.

  3. #63
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    Looking through my Sierra manual today, I could find nothing about a 1:7" barrel better stabilizing a 69 or 77 gr. It merely gives a range I adequate twists. Perhaps we have different editions? It does speak of choosing different bullets for different ranges, but we already know that. It's pretty common knowledge that flat-based bullets shoot best at shorter ranges while BTHP (I know it's not as tacticool to say BTHP---OTM is now the cool guy phrase for the same old product) do a better job at retaining velocity, and thus defeating wind while carrying more energy.

    Now let's forget about naval weapons and get back to AR barrels. I'd like to some research showing adequacy/inadequacy of match 1:8" barrels vs 1:7" now that we've established the large majority of chrome lined combat barrels are 1:7" and the majority of stainless match barrels, with the exception of Noveske, are 1:8".





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  4. #64
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    The real affect the boat tail has on long range aerodynamics is when the bullet velocities are in the transonic range. When the bullet is supersonic, or when it's subsonic, there is no difference between a flat base and an boat tail base. But there is when the bullet is traveling in the transonic range.

    When the bullet is traveling at supersonic velocities all airflow is supersonic. At subsonic velocities, all airflow is subsonic. But at transonic velocities, part of the air flow is subsonic and part is supersonic which creates turbulence which will hit the base of the bullet, buffeting it around. The boat tail reduces the affect of the turbulence. The boat tail only helps as the bullet transitions from supersonic to subsonic velocities.

    Another place the boat tail helps is barrel life. Tests by the Army shows that there is significant increase in barrel life when shooting nothing but boat tails through a barrel compared to shooting nothing but flat based bullets.

    I believe calling a bullet OTM is a political expedience. There is a general misconception that the "Geneva Convention forbids the use of hollowpoints". As most everyone on this forum knows, that it's the Hague that deals with "humane" bullets for warfare and nothing in it's wording bans the use of hollowpoints. Sierra Match Kings, by construction, are hollowpoints but are not designed to offer expansion. In fact, it is felt by some military legal types that the terminal performance of the Sierra MKs meets the standard set by the Hague. But since it's a hollowpoint by construction and hollowpoints are traditionally used to facilitate violent expansion, it would be a political mess for the US government to declare it legal for warfare.

    While most match grade barrels we're familiar with on this site might be 1:8, I think of you look at true match barrels, you'll probably find most are 1:7. Yes, there is a difference between a match barrel and match grade barrel.

    In a couple of my past posts, I give edition & page for the information found in the Sierra manual. Not much help, but unfortunately my books are currently in storage a couple hours away
    Last edited by MistWolf; 02-18-14 at 09:34.
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
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  5. #65
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    Again, when we have some data, let's see it.

    This isn't about the purpose of Hague or the invention of the term OTM, or whether or not BT bullets have higher BC by design or happenstance.

    Anybody in this thread have accuracy issues with 1:8" bullets and 77s?


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  6. #66
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    I did a search online and found the Sierra data
    http://www.exteriorballistics.com/eb...ned/5th/25.cfm

    Looking first at the group for the 1x 7 twist rate, the average BC values for this group of 10 rounds is 0.297 when rounded to three significant figures, sufficient for trajectory computations. The standard deviation (SD) of the measurements, 0.0022, is less than 1.0 % of the average BC value for the group, and the extreme spread (ES), 0.0079, is less than 4.0% of the average BC value. These figures illustrate the criteria that we use (SD no more than about 1.0% of average value, and ES no more than about 5.0% of average value) to determine whether the measured data are “good.” If either of these criteria is seriously exceeded, we look for a reason or repeat the measurements.

    The 12 round group for the 1x8 twist rate also has an average BC value of 0.297. The standard deviation for the group is 0.0039, which is about 1.3% of the average BC value, and the ES, 0.0129, is about 4.3% of the average BC value. This group obviously is not quite as “tight” as the previous group, but we would not call this “bad” because the SD does not seriously violate our standard deviation criterion.
    For a clearer understanding, read the whole article
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  7. #67
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    Thanks for the link I'll give it a read


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  8. #68
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    Quote Originally Posted by tylerw02 View Post
    Again, when we have some data, let's see it.

    This isn't about the purpose of Hague or the invention of the term OTM, or whether or not BT bullets have higher BC by design or happenstance.

    Anybody in this thread have accuracy issues with 1:8" bullets and 77s?


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    This was out of a BCM SS410


  9. #69
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    I just gave it a quick read and it appears that 1:7 and 1:8" both have the same BC value, but a larger group which cannot be accounted for because of twist rate.

    Even the 1:9" has virtually the same BC at .295 vs .297.

    They also point out the velocity, as I had said earlier, is important for BC. Their test says this bullet was fired "about" 2800 FPS. Looks like only ten rounds were fired in the 1:7" and a single barrel was used. There were 12 rounds fired for the 1:8", obviously opening up the likelihood of a larger group.

    This begs to question of these results could be repeated with another set of barrels. We know each barrel is an individual and that the .3% difference in accuracy for 12 rounds vs 10 rounds leaves a lot of unanswered questions.

    We also know that twist rates also can have an effect on velocity and pressure to some degree.

    One 1:7" being slightly more accurate for 10 rounds than one 1:8" for 12 rounds both having the same average BC isn't convincing to the argument that 1:7" is superior or that the 1:8" is inferior.


    It would be interesting to see more research and more data on this subject. It would be very interesting to see the same test with a 55 gr and a 77 gr fired through AR barrels vs bolt actions. Hell, it would be interesting to see the same test performed on a set of bolt actions with ammo tuned to the specific barrel vs a generic load of "about 2800 FPS".

    All in all, this data is VERY limited and doesn't answer very many questions given the limited scope of testing.


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  10. #70
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    Quote Originally Posted by MistWolf View Post
    The real affect the boat tail has on long range aerodynamics is when the bullet velocities are in the transonic range. When the bullet is supersonic, or when it's subsonic, there is no difference between a flat base and an boat tail base. But there is when the bullet is traveling in the transonic range.

    When the bullet is traveling at supersonic velocities all airflow is supersonic. At subsonic velocities, all airflow is subsonic. But at transonic velocities, part of the air flow is subsonic and part is supersonic which creates turbulence which will hit the base of the bullet, buffeting it around. The boat tail reduces the affect of the turbulence. The boat tail only helps as the bullet transitions from supersonic to subsonic velocities.

    Another place the boat tail helps is barrel life. Tests by the Army shows that there is significant increase in barrel life when shooting nothing but boat tails through a barrel compared to shooting nothing but flat based bullets.

    I believe calling a bullet OTM is a political expedience. There is a general misconception that the "Geneva Convention forbids the use of hollowpoints". As most everyone on this forum knows, that it's the Hague that deals with "humane" bullets for warfare and nothing in it's wording bans the use of hollowpoints. Sierra Match Kings, by construction, are hollowpoints but are not designed to offer expansion. In fact, it is felt by some military legal types that the terminal performance of the Sierra MKs meets the standard set by the Hague. But since it's a hollowpoint by construction and hollowpoints are traditionally used to facilitate violent expansion, it would be a political mess for the US government to declare it legal for warfare.

    While most match grade barrels we're familiar with on this site might be 1:8, I think of you look at true match barrels, you'll probably find most are 1:7. Yes, there is a difference between a match barrel and match grade barrel.

    In a couple of my past posts, I give edition & page for the information found in the Sierra manual. Not much help, but unfortunately my books are currently in storage a couple hours away
    Boat tails reduce drag at any speed, not just when transonic. Think of even large things in everyday life like actual boats, lots of passenger cars, those semi-trucks with trailers sporting those cone-looking things on the back, etc... They all use a "boat tail" design to lower drag at velocities far less than transonic. Just as with the boat tail bullets, such a shape reduces drag at all velocities.

    Also, "real" match barrels have a twist rate suited to the particular bullet being fired. There's no advantage to over-spinning the bullet but there are real-world downsides. Look at something in .224" diameter like a 22PPC and you'll find the "real" match barrels will be 12 or 14 twist since that's all they need for the bullets they shoot. If you're single-loading an AR and shooting the 80-90gr bullets you might find in some match formats you'll probably use something like a 7 twist but don't think that all "real" match barrels are such a fast twist.

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