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***WARNING*** This Read requires a Brain (Brian)

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2011 at 19:59
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Scope Swapper

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This post is a great read even if not a .338LM or .338 Shooter.

Alfred George Greenhill

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:

Twist = \frac{C D^2}{L} \times \sqrt{\frac{SG}{10.9}}

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).

Brian Litz is a Sir Isaac Newton x338 to me...
 
 
 


Edited by 338LAPUASLAP - October/13/2011 at 20:12
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2011 at 20:11
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Scope Swapper

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I was trying to be ironic not rude sorry about those who are scratching their heads wondering if they should read this post...
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2011 at 20:15
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For those readers that do not know Brian he is probably the best Ballistician of the 21st century.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2011 at 20:27
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X = 180 Y = 90 (X+Pyro)+(Y-Pyro) = ?

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While a good read, the greenhill formula is a bit dated. You may want to try the miller stability formula too. They actually have a calculator for it on JBM's site. I've found it to predict needed twist rate with what I'd expect from my real world results.
 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2011 at 20:35
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I know, I use some other stuff I was actually just trying to get people that don't know hooked...
 
Funny part of the whole thing is Brian Litz asking Johnny from the Streets why???


Edited by 338LAPUASLAP - October/13/2011 at 20:36
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2011 at 21:01
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X = 180 Y = 90 (X+Pyro)+(Y-Pyro) = ?

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I'm guessing because what they are doing doesn't make sense to him. My bullets that Brandon shot out of his .338 RUM are much longer than Berger's 300 gr, and he shot them from a 1-10" twist with no trouble. I think he's making a point, or wondering why these guys see the need to go tighter. But that's just my best guess. I really have no idea why he'd ask such a question. I'm guessing if anyone would know the answer, he should.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2011 at 21:09
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Exactly, that is why.
 
Have you had a chance to read his books? 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2011 at 21:14
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X = 180 Y = 90 (X+Pyro)+(Y-Pyro) = ?

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I have not. I probably should. Just too many things to spend money on right now, and I guess I haven't made that a priority. It's on the list though.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/14/2011 at 07:14
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Originally posted by 338LAPUASLAP 338LAPUASLAP wrote:

Exactly, that is why.
 
Have you had a chance to read his books? 
 
I have read the second edition a few times. It takes me a little longer to take in what I read, rather than real world of "just doing it". I have also noticed you can have the perfect surface bearing and twist rate but if the agive and base is angled to much or not enough the bullet still will not fly like it should. Also you can cheat the twist rate "sometimes", but it depends on the load and rifle and is not a normal thing.
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