Twist rate calculator

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  • Last Post 09 August 2017
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David R. posted this 09 August 2017

http://castpics.net/dpl/index.php/reloaders-reference/twist-rate-calculation

I just stumbled on to this today. I've been fascinated with it and have been playing around to see what I can learn about my own bullets and barrels. Are other folks here using this? Does it seem to be accurate? Seems like a wonderful resource. I'm sure that there must be many people in this group who already know a lot about this subject, but it's new to me and I'm excited to learn about it. 

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Scearcy posted this 09 August 2017

I have spent the last 6 months shooting 2,500 cast bullets through a 243. My rifle has a 1x10 twist. My best bullets are .930 to .940 in length. This calculator as well as another similar agree that I should really be using about a 9 1/4" twist rate. On cold, or even cool and cloudy, days my rifle does well at 100 yards and even ok at 200 yards. On a sunny, hot, humid day not too long ago I was unable to even stay on paper at 200 yards and my 100 yard targets showed significant instability. 

Unfortunately I have to say the calculations are apparently accurate for me even with a blunt cast bullet. Some have opined that the math does not apply to cast bullets. It seems to apply to my cast bullets.

Jim

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frnkeore posted this 09 August 2017

Ballistics has come a long way, since the Greenhill formula. There is more to it than bullet length. The CG of a bullet effects it's stability, too.

 

The first link is to the Miller Formula, it's much better than the GH formula. Miller doesn't take CG into account.

 

The second one, Geoffrey-Kolbe, is the one I use and does take CG and subsonic flight, into account.

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi    

 

http://www.geoffrey-kolbe.com/barrel_twist.htm  

 

In my own shooting, I've found the formula that I use, to be very accurate.   Anomalys do occure and to get the bullet to the target, point on, faster twists are better than slower. I have not found any anmalys in any of my bullet to twist applications but, I've not messed with any long 22 cal (or smaller) bullets, belond my 22RF experiments, in 14 twist.  

 

Frank

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OU812 posted this 09 August 2017

My longer .224 caliber NOE 80 SP bullet shoots very well @100 yards using slower 1/12 twist barrel. Maybe @200 yards it will be unstable? I need to shoot @200 to find out.

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