Twist rate Caculations.
- Thread starter PhulesAu
- Start date
riverwolf1
Silver $$ Contributor
Try going to Lija Barrels and I believe he has a chart that gives Caliber, Twist rate and Bullet weights that work in each particular twist rate. I copied it a while ago and keep it in my shop for reference.
Somebody else might know of one but for my needs I just use the suggested twists Berger publishes for their products. If you use bullets of other brands, simply measure their length & find something close to that (same dia. of course!) among Berger's products.
Most any 308 168 grains'll need minimum 1:13 but 1:12's a safer proposition. 1:10 ought to shoot just about anything you can find to stuff into a case... though I'd avoid anything lighter / shorter than 150's to avoid risks of losing 'em from over-stabilization / too rapid rotation.
Most any 308 168 grains'll need minimum 1:13 but 1:12's a safer proposition. 1:10 ought to shoot just about anything you can find to stuff into a case... though I'd avoid anything lighter / shorter than 150's to avoid risks of losing 'em from over-stabilization / too rapid rotation.
XTR
F-TR obssessed shooting junkie
JBM stability calculator
"Calculates the Miller stability factor. This formula was derived by Don Miller and published in Precision Shooting. This formula is much better than the antiquated Greenhill's formula. Stability value should be in the range of 1.3 to 2.0 to ensure bullet stability. Don Miller and Dave Brennan (editor of Precision Shooting) have also been kind enough to let me host his stability paper on my exterior ballistics bibliograpy page. The paper is titled A New Rule for Estimating Rifling Twist An Aid to Choosing Bullets and Rifles. "
Barrel length does not have any effect on stability except to impart velocity, which in turn increases the bullet RPM, but that is just a function of MV.
"Calculates the Miller stability factor. This formula was derived by Don Miller and published in Precision Shooting. This formula is much better than the antiquated Greenhill's formula. Stability value should be in the range of 1.3 to 2.0 to ensure bullet stability. Don Miller and Dave Brennan (editor of Precision Shooting) have also been kind enough to let me host his stability paper on my exterior ballistics bibliograpy page. The paper is titled A New Rule for Estimating Rifling Twist An Aid to Choosing Bullets and Rifles. "
Barrel length does not have any effect on stability except to impart velocity, which in turn increases the bullet RPM, but that is just a function of MV.
Weight isn't the factor that primarily determines the required twist rate - it's length in relation to calibre. The 168gn Sierra MK (and the others that are largely copies of it from Hornady, Speer and Nosler) is a short bullet for its weight and in relation to its 0.308" cal, so actually needs a relatively slow twist rate to be stabilised, working well in a 1 in13-inch barrel for instance. The 168gn Berger Hybrid is a longer design and needs a faster spin rate to be adequately stabilised, and its optimal twist is 1 in 12-inches, 1-13 being marginal.
Your 1 in 10-Inch rate will stabilise almost every 308 match bullet available in conventional gilding metal jacket + lead core form. Berger recommends it for the 230gn Hybrid for instance, although some F/TR shooting users on this forum reckon 1 in 9.5" or 1 in 9 is better.
Google Millers Twist Rule and you'll be able to download a simple little Microsoft Excel spreadsheet. You do need the bullet length to use it though. All Berger lengths are available on the 'Berger Bullets Quick reference Sheet' available as a PDF on the company's website, others you'd have to measure or get from Bryan Litz's book: 'Applied Ballistics for Long Range Shooting' which covers this subject in depth and shows Sg (coefficient of stability) values for all the bullets it analyzes in the extensive data section.
Taking the 168gn .30 Sierra MK as an example, it is 0.308" dia. and 1.215" length. You stick these in the Miller spreadsheet at whatever MV you're getting, let's say 2,600 fps and key in the rifling twist rate which we'll say is 13 inches in this example. Here are the results:
Don Miller's Twist Rule
Caliber 0.308 Inches
Bullet Weight 168 Grains
Bullet Length 1.215 Inches
Barrel Twist 13 Inches/turn
muzzle velocity 2600 fps
Temperature 59 degrees Fahrenheit (59 is standard)
Pressure 29.92 inches of mercury (29.92 is standard)
Sg = 1.45
Sg shouldn't be less than 1.4. If Sg is greater than about 2.0, you may
gain some accuracy by going to a slower twist barrel.
Anything with an Sg above 1.0 is theoretically stable, but as it says above, it's strongly advised to be at 1.4 or above. Often though, you can get away with an Sg in the 1.2s and nearly always in the 1.3s unless your shooting is in exceptionally high atmospheric pressure and/or seriously cold conditions. (Dense air sees the bullet need a higher rate of spin.)
In this example, at standard ballistic conditions, (59-deg F and 29.92 inches mercury pressure) 2,600 fps in a 1 in 13-inch twist barrel gives this bullet 1.45 which is a near ideal stability level.
Your 1 in 10-Inch rate will stabilise almost every 308 match bullet available in conventional gilding metal jacket + lead core form. Berger recommends it for the 230gn Hybrid for instance, although some F/TR shooting users on this forum reckon 1 in 9.5" or 1 in 9 is better.
Google Millers Twist Rule and you'll be able to download a simple little Microsoft Excel spreadsheet. You do need the bullet length to use it though. All Berger lengths are available on the 'Berger Bullets Quick reference Sheet' available as a PDF on the company's website, others you'd have to measure or get from Bryan Litz's book: 'Applied Ballistics for Long Range Shooting' which covers this subject in depth and shows Sg (coefficient of stability) values for all the bullets it analyzes in the extensive data section.
Taking the 168gn .30 Sierra MK as an example, it is 0.308" dia. and 1.215" length. You stick these in the Miller spreadsheet at whatever MV you're getting, let's say 2,600 fps and key in the rifling twist rate which we'll say is 13 inches in this example. Here are the results:
Don Miller's Twist Rule
Caliber 0.308 Inches
Bullet Weight 168 Grains
Bullet Length 1.215 Inches
Barrel Twist 13 Inches/turn
muzzle velocity 2600 fps
Temperature 59 degrees Fahrenheit (59 is standard)
Pressure 29.92 inches of mercury (29.92 is standard)
Sg = 1.45
Sg shouldn't be less than 1.4. If Sg is greater than about 2.0, you may
gain some accuracy by going to a slower twist barrel.
Anything with an Sg above 1.0 is theoretically stable, but as it says above, it's strongly advised to be at 1.4 or above. Often though, you can get away with an Sg in the 1.2s and nearly always in the 1.3s unless your shooting is in exceptionally high atmospheric pressure and/or seriously cold conditions. (Dense air sees the bullet need a higher rate of spin.)
In this example, at standard ballistic conditions, (59-deg F and 29.92 inches mercury pressure) 2,600 fps in a 1 in 13-inch twist barrel gives this bullet 1.45 which is a near ideal stability level.
I have a 26" Rock Creek barrel with an 11.5 twist. I had bought a pile of 208 Amax before I became aware of the Sg factor and according to the calculator I'm in the 1.2's
Today I decided to test the stability on paper since the temps were so low and this is what results I had
.308
208 Amax (length 1.532")
2700 fps
28.63 station pressure
14 degrees F
46% RH
1.13 Sg
I fired 3 shots at 100 yards and they went into a bughole. I then fired 5 shots at 400 yards and had 2.5 vertical and 3.5 horizontal wind 5-10 mph (not correcting for wind)
I've been shooting this load for a few years and have always been in the 1/2 moa range.
Somebody please explain why this bullet / twist shoots so well given the Sg.
Today I decided to test the stability on paper since the temps were so low and this is what results I had
.308
208 Amax (length 1.532")
2700 fps
28.63 station pressure
14 degrees F
46% RH
1.13 Sg
I fired 3 shots at 100 yards and they went into a bughole. I then fired 5 shots at 400 yards and had 2.5 vertical and 3.5 horizontal wind 5-10 mph (not correcting for wind)
I've been shooting this load for a few years and have always been in the 1/2 moa range.
Somebody please explain why this bullet / twist shoots so well given the Sg.
Jay Christopherson
Not An Admin
If you are interested in an update to the Miller rule, that takes into account the type of bullet you are shooting:
http://forum.accurateshooter.com/index.php?topic=3834705.0
http://forum.accurateshooter.com/index.php?topic=3834705.0
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