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Thread: Why not 1/8 twist?

  1. #111
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    We're in a discussion of twist rate; gyroscopic stability is what we've been talking about from the beginning, even by Litz's own language. When someone says "stability" in a discussion of twist rate, it's no fault to assume they're talking about gyroscopic.

    And if you read my posts, I never denied that the bullet could become more precise as it heads downrange as the proportion of gyroscopic stability decreases lesser than external factors like precession and such. I only said it can't become more gyroscopically stable. However, his explanation does not perfectly fit the observed effects more than someone describing earth's gravity and calling it magnetism.
    Last edited by DreadPirateMoyer; 02-18-14 at 14:28.

  2. #112
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    Quote Originally Posted by DreadPirateMoyer View Post
    We're in a discussion of twist rate; gyroscopic stability is what we've been talking about from the beginning, even by Litz's own language.

    And if you read my posts, I never denied that the bullet could become more precise as it heads downrange as the proportion of gyroscopic stability decreases lesser than external factors like precession and such. His explanation does not perfectly fit the observed effects more than someone describing earth's gravity and calling it magnetism.
    Hahahha. You just can't deal with being wrong. You said "As for the bullet being more stable as it slows down, I don't see how that's true." You didn't say anything about gyroscopic stability. The fact is the bullet becomes more stable as it heads downrange. You can play semantic tricks, dance around with pedantic arguments, insist you knew exactly what the deal was the whole time, etc... but the simple fact is you plainly stated "As for the bullet being more stable as it slows down, I don't see how that's true" and Litz offers an explanation that other than a single word is perfectly accurate.

  3. #113
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    Quote Originally Posted by Obscenejesster View Post
    I don't know how this thread turned into a discussion about a couple individual rounds and what twist rates they like better.

    As a general rule of thumb, 1/8 barrels will stabilize most rounds under 62gr better than a 1/7 all things being equal. It will also stabilize all rounds up to 77gr. Some rounds in the high 60's up to the mid 70's well stabilize better coming out of a 1/8 and others will do better coming out of a 1/7.


    Bottom line. Both twist rates will be battle accurate over a wide range of weights.



    I personally own more 1/7 because that is what's widely available in hammer forged chrome lined barrels. I own a couple 1/8's because that is what's widely available in SS match barrels.

    Sent from my SGH-T999 using Tapatalk
    It is the bullets length, not the weight, that determines twist rate. A longer bullet needs a faster twist than a short one.
    Example: The RRLP bullets (no lead) that we use at work are only 62 grains but are the same length as a 77 grain Sierra MK and so they need at least a 1-8 twist to stabilize. Ive also used some of the old PRL powdered tungsten core bullets that were 87 grains yet were only the same length as a Sierra 69 grain bullet, so they only needed a 1-9 twist to stabilize.

    To make a bullet heavier you typically make it longer and that's why some people assume its weight. But when using powdered metal cores or solid copper you can get a really long bullet that is lightweight.

  4. #114
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    Quote Originally Posted by sniperfrog View Post
    Example: The RRLP bullets (no lead) that we use at work are only 62 grains but are the same length as a 77 grain Sierra MK and so they need at least a 1-8 twist to stabilize. Ive also used some of the old PRL powdered tungsten core bullets that were 87 grains yet were only the same length as a Sierra 69 grain bullet, so they only needed a 1-9 twist to stabilize.
    Those are 55gr aren't they? I load those suckers... and either way... they're long for their weight. And yeah... it's length that drives twist requirement.
    "You people have too much time on your hands." - scottryan

  5. #115
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    Quote Originally Posted by CC556 View Post
    Hahahha. You just can't deal with being wrong. You said "As for the bullet being more stable as it slows down, I don't see how that's true." You didn't say anything about gyroscopic stability. The fact is the bullet becomes more stable as it heads downrange. You can play semantic tricks, dance around with pedantic arguments, insist you knew exactly what the deal was the whole time, etc... but the simple fact is you plainly stated "As for the bullet being more stable as it slows down, I don't see how that's true" and Litz offers an explanation that other than a single word is perfectly accurate.
    Right, because we're in a thread discussing twist rates, which is a discussion of gyroscopic stability. I assumed we were still talking about that, especially when someone cites an authority who calls something gyroscopic stability that isn't. I was totally, crazily out of line for thinking that's what we were discussing, wasn't I? And please stop quoting me out of context in some weird attempt at an internet pissing contest. My next sentence said "...but as far as gyroscopic stability"; it's clear what I was talking about.

    Anyway, this discussion isn't about who is wrong or right. Stop taking it so personally. It's just a discussion about twist rates and bullets. Bullets lose gyroscopic stability as they leave the barrel. They also happen to gain dynamic stability, which I didn't know and never denied ("This may have tangible, counter-intuitive effects of greater precision at certain ranges than others (I'm not making a claim on that, I don't know)"), and which is awesome. Crazy what these things do.

    As for twist relation to bullet, indeed, it's the length that matters. It usually corresponds to weight as well since most bullets are made of lead, but it gets weird when you get into all-copper and non-standard bullets.
    Last edited by DreadPirateMoyer; 02-18-14 at 15:16.

  6. #116
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    Quote Originally Posted by CC556 View Post
    Do you have a link to some data? I'm having a hard time believing that in an application where even tiny changes to the ogive shape or meplat diameter make a significant difference the presence or absence of a boat tail would make no practical difference.

    This touches on it a bit:

    http://bulletin.accurateshooter.com/...-and-accuracy/
    You're correct- changes in the shape of the front portion of the bullet do make a real difference. That's why the spitzer bullet was considered a game changer in the early days of smokeless powder

    The boat tail reduces drag and turbulence, which helps keep the bullet stable as it goes transonic. It doesn't only help at transonic speeds, it helps all the time and helps as the bullet hits that transonic range. As you can see in the pic below from the article, the turbulent area behind the bullet is made smaller by the boat tail. The end of the boat tail is basically the flat base, and with a proper tail angle the effective size of that base is reduced compared to a bullet with no boat tail and a flat base (and resulting area of turbulence) that is much larger.
    That is an awesome photo. I really dig shadowgraphs.

    Ok, I have to acknowledge there may be a measurable difference in drag between the BT and FB. Drag from the base of the bullet comes from a reduction in air pressure trying to pull the air back in. The difference between a BT and FB is very small because the difference in area is very small. However The greatest affect the BT has on the bullet's flight is at the transonic range where it reduces buffeting that occurs because there will both supersonic and subsonic airflows causing turbulence. The base of the bullet has less affect on drag than the nose and the average shooter isn't going to see a practical difference between the two at ranges where the bullet remains supersonic

    Quote Originally Posted by Obscenejesster View Post
    Again, I don't know what your fixation is with comparing a single bullet and using it as a basis for all twist rates.

    Going by what you just said, if we can assume all 69gr bullets are more stable and accurate being shot from a 1/7 barrel, then we can just all assume shorter bullets 62gr and below are more accurate and better stabilized being shot from a slower twist barrel like a 1/8.
    You are confusing yourself. What I said was
    Quote Originally Posted by MistWolf View Post
    Sierra uses doppler radar to measure the ballistic coefficient of bullets in flight. They have found the more stable the bullet, the less drag it has because it has less precession. The Sierra 69 gr Matchking (which is shorter than the 70 gr TSX), was found to have a better BC when fired from a 1:7 twist barrel than anything slower, including the 1:8
    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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  7. #117
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    Quote Originally Posted by sniperfrog View Post
    It is the bullets length, not the weight, that determines twist rate. A longer bullet needs a faster twist than a short one.
    Example: The RRLP bullets (no lead) that we use at work are only 62 grains but are the same length as a 77 grain Sierra MK and so they need at least a 1-8 twist to stabilize. Ive also used some of the old PRL powdered tungsten core bullets that were 87 grains yet were only the same length as a Sierra 69 grain bullet, so they only needed a 1-9 twist to stabilize.

    To make a bullet heavier you typically make it longer and that's why some people assume its weight. But when using powdered metal cores or solid copper you can get a really long bullet that is lightweight.
    I know it's a bullets length....That is the entire argument. I was being facetious with this particular post. Sorry I it wasn't so obvious.

  8. #118
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    Quote Originally Posted by MistWolf View Post
    You are confusing yourself. What I said was
    No, I'm really not. You're still fixated on one particular bullet at one particular load fired out of one particular barrel attached to one particular rifle. Lastly, only 10 rounds were fired from the 1/7 and 12 rounds were fired from the 1/8. We already know the difference was minimal so this also tells us that the results could have been entirely different if the round was loaded to 2600 FPS and fired out of a different barrel. Hell, if they would have taken 10 shots with the 1/7 and 12 shots with the 1/8, that may have reversed the results.


    If this thread was titled "Which twist and barrel is most accurate with 69gr SMK's" then you might have an argument.

    Again...Bottom line.....There's a reason why most SS AR barrels come with a 1/8 twist. These top barrel manufactures are more than capable of bringing them to the market with a 1/7 twist but they don't. Like I said before....I happen to own more 1/7 barrels and I don't own any 1/9 barrels. I have no bias in this argument.

  9. #119
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    Quote Originally Posted by markm View Post
    Isn't stability an absolute state. A bullet is either stabile or it isn't. No? There is no "more stabile" or "too stabile"...
    Ya know, sometimes you come across facts that cause you to reconsider your conclusions. If the definition of "over stable" means the bullet has so much gyroscopic stablity that it won't nose over to point in the path of the trajectory, then yes, it can be over stable. However, it does affect dynamic stability, which is, the bullet will not nose over to point along it's flight path.

    Everything has a tolerance range
    It is well known that bullet stability is critical for accuracy, but it is not well understood that there are different degrees of bullet stability. BC measurements give us some insights into varying degrees of bullet stability
    Still, a stable bullet is a stable bullet. A bullet at the edge of stability is still stable and once stability is lost, it cannot be recovered
    Last edited by MistWolf; 02-18-14 at 17:58.
    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

  10. #120
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    Quote Originally Posted by Obscenejesster View Post
    No, I'm really not...
    If you don't understand the use of an example, I cannot help you
    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

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