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Thread: Twist rate and velocity for stability.

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    Default Twist rate and velocity for stability.

    Curious if anyone has any articles or experience relating velocity to bullet stability. I'm having my 243 bored out to Ackley Improved right now, and I'd like to put 100+ grain bullets in it if possible. The Ackley should give me about 150 fps more velocity, but the barrel is a 1:10" twist which generally wont stabilize bullets over 100 grains. Does bumping the velocity give it additional stabilization or not? Ultimately I'll go just try it out and see, but I dont want to order a bunch of bullets if it wont stabilize them.
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    Quote Originally Posted by highestview View Post
    Curious if anyone has any articles or experience relating velocity to bullet stability. I'm having my 243 bored out to Ackley Improved right now, and I'd like to put 100+ grain bullets in it if possible. The Ackley should give me about 150 fps more velocity, but the barrel is a 1:10" twist which generally wont stabilize bullets over 100 grains. Does bumping the velocity give it additional stabilization or not? Ultimately I'll go just try it out and see, but I dont want to order a bunch of bullets if it wont stabilize them.
    I'll tread carefully as you can get banned talking about twist rates and stabilization....

    The 100+ grain bullets designed for LR will likely be difficult to stabilize in a 1:10 .243 AI. I'd give them a go if that's the bullet you want to try as I've seen a bit of variation among 1:10 twist barrels from the same manufacturer, but generally speaking it's not enough for bullets like the 105 A-Max, 105 Scenar, etc..... Velocity does increase stabilization, but in your case it will not be enough to make the needed rotational velocity you'll need IMO.
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    A couple of thoughts, yes more velocity equates to higher rpm which may stabalize a bullet that is marginal, but the increase in rpm is nearly as significant as increasing the twist rate of the barrel. Even if you can get a 150 fps velocity boost, that's less than a 5% increase in velocity and hence less than 5% increase in bullet rpm. going to a 1:9 barrel willi increase your rpm by 10+%. If you've got a Remington 700 you might be right on the border of a fast enough twist, some of the them shoot 105's just fine, others don't quite stabalize them.

    My advice is see if the gun will stabalize 105's as a .243, if it won't then I'd say you're best off putting on a faster twist barrel if you want to shoot 105's. As far as the ackley, the .243 does not gain siginificant powder capacity when blowing it to an ackley chamber, and as the .243 is already a high pressure cartridge, the only way to gain 150 fps is to run loads that are 70kpsi or higher. The primary advantage of the ackley for high volume shooters is not having to trim brass as frequently. If you look at the details of the guys getting crazy high velocities with .243 AI's, most of that speed is coming from 26-29" barrels, not the increased powder capacity.

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    I think I will say (without the math) that 1 in 10" twist in .243" bore can stabilize a 120 grain spitzer. Bullet type is important and the VLD BTHP type bullets will be much longer than say a flat base soft point spitzer.

    I posted this a few years back so I'll redo it with some extra info.

    The formula actually looks like this. Twist=CD^2/L * sq rt SG/10.9


    C=A constant meant to represent velocity. Use 180 for velocity over 2800 fps.

    D= bullet diameter in inches.

    L=Length of bullet in inches.

    SG=Specific gravity (10.9 for lea core bullets)


    Barrel Twist Rates
    There have been some questions about this subject lately and I have responded to the best of my ability specifically for each of them.

    There is a post back on page three asking about the 450 Marlin and another on page one for the 458 WM twist.

    In those two posts are formulas called the Greenhill Formula. Well, I make up my formulas as I go along because I understand math and what we are calculation here so the formulas are not exactly alike.

    One is: Twist=150 X D/r (the length of the bullet in calibers)
    150 is a constant.
    D=diameter of the bullet.
    The other is: Twist=150 X D squared/L (length of the bullet in inches)

    These are the same thing and will give the same results with just simple check book balancing math.

    A .458" diameter bullet is 1.374" long (500 grains) the twist will be 22.9" with either formula. I just recommended using the constant value of 100 for better impact stability and penetration.

    That same bullet would be calculated at; 15.26" or 15.26" depending on which formula you choose.

    Good shootin'.

    Murphy


    I will add to this for those who care, a way to find the maximum bullet length that can be stabilized in a particular barrel.

    Maximum Bullet Length (inches)=D x D x 150/Twist Rate

    This way we can go backwards to find what bullet is optimum in our rifle.

    A bullet needs a certain RPM or more specifically a rotational speed of enough surface feet per minute to stabilize.

    To calculate this RPM= Velocity/Circumference of the bullet. OR MV* 12/Twist rate*60 = RPM

    A bullet fired at 3050 fps from a 1 in 7 twist barrel will rotate at 313,714 RPM.
    Last edited by Murphy; 04-02-2013 at 16:52.
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    To put some hard #'s into the mix, here's one example. My old Ruger #1 in 243 had a 26" barrel and 1:9 or 1:10 twist, but darned if I can remember which. It was distinctive for having a mile-long throat- so long in fact that I could seat the old Speer 105 grain boattail spitzer with the base flush to the base of the neck. Looked kinda funny, but with the bullet waaaay out there I'd bet the powder capacity was at least on par with the AI. Between that and the 26" barrel it was easy to hit 3100 with a variety of loads. And by a long shot that was the most accurate bullet in the rifle.

    Dunno what that would mean for even longer bullets, but in your shoes I'd round up boxes of everything long, then sit down for a shoot-and-see session.

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    If we consider this....the C constant is 150 for velocity up to 2800 fps and 180 for velocity above 2800 fps.

    Looking at this: Maximum Bullet Length (inches)=D x D x 150/Twist Rate

    .243 * .243 * 150/10 (twist for a 243 Winchester) = A maximum bullet length of .885735"

    .243 * .243 * 180/10.......................................= A maximum bullet length of 1.065882" Yes velocity matters.

    Just thought I'd try to answer your original question.
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    Quote Originally Posted by BrownBear View Post
    My old Ruger #1 in 243 had a 26" barrel and 1:9 or 1:10 twist, but darned if I can remember which.
    For what ever it's worth, my daughters Ruger .243 Win. M77 Hawkeye is a 1:9 twist.

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    Berger Bullets web site lists length, weight and recommended twist rates for their bullets. They list 1:8 twist for their 105gr VLD. VLD's are a bit longer than other manufactures offerings due to the longer nose design.

    Murphy has the math nailed. Run his formulas with the Berger 105gr bullet length, (1.235") and your forecast velocity.

    http://www.bergerbullets.com/products/hunting-bullets/

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    I guess the Remington 700's have 1:9.25" twist. That should be able to stabilize the sierra 100 grain game king, which is what I would really like to shoot.
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    Rem 700's will stabalize 105's. We've shot several 100's and 105's out of my son's 243 sps and it shoots them very well.
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    I would also like to say that AIing a 243 offers very litle in the way of additional powder capacity but just enough for the 4831 burn rate to work more efficiently. There is very little taper in the modern 243 (all the 308 family of cases) and was a cartridge developed after P.O. Ackley's time. The older style cases such as 250 Savage, 7x57, 30-06, etc. stand to gain the most from the Ackley improvement. I would not expect more than 100 fps from the 243 when AI'ed and then only with judicious loading. It's biggest improvement would be with the heavier bullets however.
    I've seen them all and loaded for every cartridge with the AI improvement. Some are just better than others.
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    Murphy is correct. But another way to get the same answer- bullet dia x bullet dia x 180 divided by bullet length = twist in inches. If your speed is under 3000 divide by 150 instead of 180. It's fun to work forward when you already know which bullets you want to shoot before selecting a barrel twist rate.

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    Quote Originally Posted by highestview View Post
    Curious if anyone has any articles or experience relating velocity to bullet stability. I'm having my 243 bored out to Ackley Improved right now, and I'd like to put 100+ grain bullets in it if possible. The Ackley should give me about 150 fps more velocity, but the barrel is a 1:10" twist which generally wont stabilize bullets over 100 grains. Does bumping the velocity give it additional stabilization or not? Ultimately I'll go just try it out and see, but I dont want to order a bunch of bullets if it wont stabilize them.
    As it's said, the 1-10 is a roll of the dice on the 105's. AI is a nice upgrade though, but most likely you'll have the NBT, NAB, or TSX to choose from. Not a bad way to roll, you just don't get the .5 BC with the Berger/Scenar/Amax 105s. I actually shoot all the above mentioned bullets out of my real 243ai, with I had twisted 8, in a 3gr PN barrel. No imaginary rifles here. 105's kill very well btw.

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    There are a couple different ways to look at stability. Target shooting (external stability) and hunting (terminal stability).

    Things that affect stability...
    Density Altitude
    Temp
    Velocity
    Cal of bullet
    Weight of bullet
    Length of bullet
    Specific gravity of bullet, i.e., monometal copper vs cup and lead core, etc.
    CG of bullet, i.e., plastic tip vs solid tip vs hollow point... boat tail vs flat base, etc
    Muzzle velocity
    and of course, twist rate

    I've been learning this stuff because I'm going to be shooting some really long 110 gr copper mono's out of a 6x284 and ordered a 7 twist barrel for the job.

    If you check out the Berger page that Marshal linked, you'll see that Berger has a twist calculator there.... upper right. It doesn't take in all the factors but it gives you a pretty good idea of what to expect. Berger recommends an SF of 1.5 for adequate stability and that a 1.5 is usually on the conservative side for hunting/terminal stability. 1.1 is normally good for good for accurate shooting and if you don't mind your bullet tumbling through flesh and bone... well, it's good for that too.

    http://www.bergerbullets.com/products/hunting-bullets/
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    Just for grins I ran some numbers through the calc...

    243
    100 gr
    1.1" in length
    9.25 twist
    3000 fps MV
    Sea Level (0' alt)
    -10 F

    That gives an SF of 1.32.... border line for worst case conditions, but optimistic velocity. Warmer temp and higher altitude = greater SF

    Raise the Temp to 40 and alt to 1000" and that increases the SF to 1.52.
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