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  #1  
Old 11-21-2017, 08:32 PM
drewh drewh is offline
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Default twist rate stability calculator

Anyone used this to help select a twist rate, did it help with accuracy?

http://www.bergerbullets.com/twist-rate-calculator/

I have a 20 viper that I am using 24gr NTX lead free bullets in, and I got an 11 twist barrel. It shoots decent, not spectacular. Ran across that when I was looking into faster twist rates to try and make the lead free bullets come apart easier.

I used the calculator with length of .625 and ballistic coefficient of .170, and 0 and 4000 ft for elevation and 30 and 80 for temperatures. it seems like in order for the bullet to be fully stable all of the time for my target velocity of 3600 I would need to run an 8.5 twist. ( a stability factor of 1.5 is considered fully stable) As temp goes up and altitude goes up so does stability. I do shoot close to sea level, but rarely at 30 degrees so there is some margin for error in there.

For comparison purposes I set up the 32 vmax I used to run in my 11 twist 20-222 at 3800 and it gave a factor of 1.31 which was close but not fully stabilized - it was a very accurate rifle.

the 24 ntx from the viper in an 11 twist calculated at 1.02 which is listed as marginal. Same everything except for twist of 8.5 gives me 1.71 - supposedly stable.

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  #2  
Old 11-21-2017, 09:20 PM
Hog Patrol Hog Patrol is offline
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If those bullets are long for caliber, you'll PROBABLY need a faster twist than 11. I've found the twist calculator and their ballistic chart more accurate with G7.
You may have the same problem I have. I develop my loads at 400 msl and hunt them at 4000 to 6000 msl so I have to use a faster twist to get acceptable stability at the lower altitudes.

Another option is to figure RPM. 240,000 plus or minus usually gives good stability.

http://bulletin.accurateshooter.com/...and-stability/

MV X 720/Twist Rate = RPM

Edit. I checked Brian Litz bullet library and .224 is the smallest he has listed. This book shows tested bullets and true BC.

Having said all that, manufacturer listed BCs are calculated using a forumal and generally not derived from real world testing like Litz has done.

Last edited by Hog Patrol; 11-21-2017 at 09:57 PM.
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  #3  
Old 11-23-2017, 02:25 PM
william t. oviatt william t. oviatt is offline
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Not talking about precision or bench rest-type shooting, but there is absolutely no reason not to use a 1/9 twist minimum in new chamberings, across the board for 17-6mm calibers.

Eventually, you will see most factory offered rifles to all be in 1/9 twist rates. The differences between this twist and marginal, just sufficient twist rates, is just not worth all the potential issues that are all washed away by going to a 1/9 TR.

it will be like the old 17REM issues of having a 1/10 twist. Or, the 244 Remington (that is EXACTLY the sane cartridge as the 6mm Remington, just a faster twist rate on factory barrels). People that know the limitations of these slower twist rifles, realize contemporary factory offered bullets will not stabilize and steer away or have these rifles rebarreled with faster twist rates.

Most folks looking for just enough twist for stabilization, are trying to save resistance to boost Velocity. That is just not worth the gain, and there are a ton of other issues in rifle and cartridge construction (reloading) that can have far greater impact to this one characteristic of chasing faster FPS.

To the average shooter, this is a non-issue in performance, for their rifle to be in a 1/9 twist rate, and it ensures their future value of the rifle in ability to shoot majority of bullets offered. Or, if they ever want to sell it.

JMH&EO,

Bill
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  #4  
Old 11-23-2017, 04:29 PM
South Pender South Pender is offline
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Bill, I'm not very knowledgeable about this, but isn't the "just stabilized" twist rate more for optimal accuracy than reducing resistance to increase velocity? At least that's the way I remember it. My recollection of the theory (like a 14 twist in the first 6 PPCs) is that by being close to the ragged edge of stability, you're likely to see the best accuracy.
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  #5  
Old 11-23-2017, 04:49 PM
Hotshot Hotshot is offline
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Being on the ragged edge of stability for accuracy does not make common sense to me. I know a 12 twist for 204R can be very accurate with some bullets but I have superb accuracy with 10 and 11 twist barrels using most any bullet.
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  #6  
Old 11-23-2017, 07:05 PM
South Pender South Pender is offline
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Quote:
Originally Posted by Hotshot View Post
Being on the ragged edge of stability for accuracy does not make common sense to me. I know a 12 twist for 204R can be very accurate with some bullets but I have superb accuracy with 10 and 11 twist barrels using most any bullet.
Let me put it a little differently. The theory has been that the slowest twist that will adequately stabilize a bullet is the best twist rate for optimal accuracy. I believe the thinking is that causing higher RPM via faster twist exacerbates the effects of any flaws or inconsistencies in the bullet, causing larger groups.
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  #7  
Old 11-23-2017, 07:27 PM
drewh drewh is offline
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I ended up ordering an 8.5 twist barrel. Once I get it I will try and test it agaInst the 11 to see if I can see a difference on the target.

Bill, I have pretty much come to that way of thinking. My primary purpose is trying to get the best performance out of the lead free bullets we are going to be stuck shooting here soon.

I need to reread the Litz articles to see if he addresses stability vs accuracy,
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  #8  
Old 11-23-2017, 10:04 PM
william t. oviatt william t. oviatt is offline
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All of the above has truth in the applications as some put their rifles to...I am just saying that an accurate rifle in the field that shoots little bug holes...might need some improvement to win at Bench Rest Shooting....but to me it is a moot issue when so many other characteristics need to be followed to get that BR performance.
A good Gun Smith, quality barrel and action and a little know-how in the reloading room and 99% of the shooters will be very happy with their results.

Pulling one "truth" from one shelf and adding to other combinations of "truths" from other shelves, will not give most shooters any advantage. I'm not talking about someone that studies each component of their rifle build to the point they could write their own testament...Those guys are in search of "Nirvana" in performance. I'm talking about building a rifle that performs every time the shooter picks it up and works without thinking whether the last shot was 3/4s of a Badger hair to the left or right! There are just too many other variables that will take precedent over any issues from a 1/9 twist rate.

It's just not that big of an issue..... Unless one chooses too slow of a twist rate!!!

Respectfully,

Bill
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Last edited by william t. oviatt; 11-23-2017 at 10:07 PM.
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  #9  
Old 11-24-2017, 12:06 PM
rhouser rhouser is offline
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I use this calculator most of the time. http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi. JBM Ballistics has been working this stuff for a long time.

Hope this is helpful.

rch
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