Barrel twist

[1RUGERMAN1]

What would be the best twist for a25-06 barrel shooting 100-117 grain bullets?

 

[Rick in Oregon]

I'd go with a ten-twist, but that's just me. Here's Pac-Nor's twist chart, it may help you out choosing what twist to choose:

http://www.pac-nor.com/barrels/

Good luck with the new build.

 

[Webster]

The Berger has a program for determining the proper twist.

http://www.bergerbullets.com/litz/TwistRuleAltWP.php

The proper twist depends on the bullet weight, bullet length and muzzle velocity. Sierra has a chart on their website but I cannot find it.

The Berger manual recommends a 10 twist for a 25-06 and their 115 gr Match Bullet

 

[1RUGERMAN1]

I've read all the twist charts already and was thinking about the 1:10. Shilen recommends 1:9 for over 100gr just wondering if it's worth it to go with 1:9 or the 10. I already built an 30-06 with a Shilen # 7 HV and a 1:12... It's recommended for up to 170gr but I'm getting a cloverleaf at 100yds with 185gr vld's. Thanks for your input.

 

[RGRobinett]

I've read all the twist charts already and was thinking about the 1:10. Shilen recommends 1:9 for over 100gr just wondering if it's worth it to go with 1:9 or the 10. I already built an 30-06 with a Shilen # 7 HV and a 1:12... It's recommended for up to 170gr but I'm getting a cloverleaf at 100yds with 185gr vld's. Thanks for your input.

It (Sg) is WAY more length dependent than weight related: the latter just happens to be convenient 'DATA'. For a .25 Caliber project, I'd opt for the 1:9" twist, which, especially with BT configurations, will provide sure-fire gyroscopic stability (Sg).

Once I went to 1:9" twist barrels, my competition quarter bores, in combination with longer BT bullets, "came to life". The bullets in question [100 to 110 Gr.] feature a 0.10" long, 9 deg. BT and are about 1.170" long.

The 1:10" twist is perfectly well suited to FB bullets of about the same length. A good rule-of-thumb: compared to a FB, a BT, of similar length, in order to achieve the same Sg, demands about a 1.0" faster twist rate - BUT, BTs, of equal weight, are invariably LONGER! Another pretty safe bet: in this length area, for this caliber (and 6mm as well), every additional .032" of length, will increase the twist requirement by about 0.75" . . . and then, you may have to consider the EXTREMELY LONG plastic tips, which, can, and DO require a twist-rate up to a full 2.0" faster, "than normal"!

The plastic tips really demonstrate that length is far more important than weight, as for given weight range, plastic tipped bullets are necessarily much LONGER! RG

 

[1RUGERMAN1]

Thanks rg

 

[timeout]

I have a 25-06 in 10 twist and it stabilizes the 115 Berger VLD's just fine, at least to 600. I can't shoot any farther than that here. There may be other bullets that would need a 9 twist. As long as you are building and plan to shoot the longer high BC bullets, may not hurt to go with 9.

 

[MrMajestic]

RG, your comments on "Plastic Tips" may be incorrect. Use the link below and change nothing but add the tip length into the equation and you will see that the SG increases. You are correct about the length but the plastic tip is not a one to one if I am interpreting the results and your statement correctly. Perhaps Litz will chime in and shed some light on this.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

 

[RGRobinett]

RG, your comments on "Plastic Tips" may be incorrect. Use the link below and change nothing but add the tip length into the equation and you will see that the SG increases. You are correct about the length but the plastic tip is not a one to one if I am interpreting the results and your statement correctly. Perhaps Litz will chime in and shed some light on this.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

I could be incorrect, but doubt it. ???

If your link is to the, 'Miller Formula', you'll need to go to something else: the original [Miller] calculator results do not correlate to either McDrag, or, Tioga Engineering Sg calculations, and leaves MUCH to be desired. Making a bullet longer ALWAYS increases the twist rate requirement, as it increases the overturning moment, while REDUCING the specific gravity, which also demands more twist: a "double whammy.

If you use JBM, skip right over the Miller calculator, and use the Drag/Twist calculator. While I have yet to use Brian's calculator (Berger Bullets), I would expect very close correlation to both the McDrag (Robert McCoy based) and the Tioga Eng. bullet design program, written by [the late] Bill Davis. McDrag and Tioga are usually within 0.10" of recommended twist rate . . . and nearly 20 years of using these two calculators has produced reliable direction. The McDrag version, at JBM has some up-dates from the original, including a specific gravity data entry box - most entries must be converted into CALIBERS, as opposed to actual lengths/diameters.

Back to the original topic - I can state that, even for a BR competition rifle, going to the 1:9" twist rate certainly will not hurt precision, and it produces sea-level > Sg 1.5. That is, bullets will be, "put to sleep" - a red-neck term which Brian may not like, but, with which, I doubt he'll argue! ;D RG

 

[Michael Courtney]

I could be incorrect, but doubt it. ???

If your link is to the, 'Miller Formula', you'll need to go to something else: the original [Miller] calculator results do not correlate to either McDrag, or, Tioga Engineering Sg calculations, and leaves MUCH to be desired.

I tend to regard experimental verification as more impressive than correlation with alternate theoretical approaches.

The Miller twist rule (and the newer twist formulas to account for plastic tips and open tipped match bullets) has proven accurate (within 5%) with every careful experimental test that has been published to date. We also have a significant body of unpublished data showing additional agreement with experiments. In addition, others have communicated privately that the Miller formulas agree with their results, and not a single party has reported disagreement with their results when all the needed information is measured to apply the formulas properly.

Science is not about different computational approaches agreeing with each other. It is about agreement with experiment. There may be some oddball cases where the Miller formulas are less accurate (> 5% error), but I suspect most of these will be things like bullets shot backward, bullets with large hollow points, composite bullets with sintered metal cores, armor piercing bullets, etc.

We are always willing to consider and evaluate experimental data relating to the twist rules. Please email me: Michael_Courtney@alum.mit.edu

 

[dkhunt14]

Seeing this is on the hunting forum I will add my 2 cents. I have noticed if you turn a bullet harder it seems to open up better. So twist has a little more to do than to stabilize the bullet.

 

[RGRobinett]

I could be incorrect, but doubt it. ???

If your link is to the, 'Miller Formula', you'll need to go to something else: the original [Miller] calculator results do not correlate to either McDrag, or, Tioga Engineering Sg calculations, and leaves MUCH to be desired.

I tend to regard experimental verification as more impressive than correlation with alternate theoretical approaches.

The Miller twist rule (and the newer twist formulas to account for plastic tips and open tipped match bullets) has proven accurate (within 5%) with every careful experimental test that has been published to date. We also have a significant body of unpublished data showing additional agreement with experiments. In addition, others have communicated privately that the Miller formulas agree with their results, and not a single party has reported disagreement with their results when all the needed information is measured to apply the formulas properly.

Science is not about different computational approaches agreeing with each other. It is about agreement with experiment. There may be some oddball cases where the Miller formulas are less accurate (> 5% error), but I suspect most of these will be things like bullets shot backward, bullets with large hollow points, composite bullets with sintered metal cores, armor piercing bullets, etc.

We are always willing to consider and evaluate experimental data relating to the twist rules. Please email me: Michael_Courtney@alum.mit.edu

I do not believe that either McDrag, or, Tiaoga are, "alternate theoretical approaches", but rather sound calculators which reliably dictate the twist-rate required for specific projectiles. I may have to stand corrected regarding the "corrected" Miller formula. ;D

I am guilty of having not used it since it first appeared in Precision Shooting Magazine, as it had no correlation to either of the proven calculators, mentioned above: 'Miller' would often predict Sg as low as .9xx (where by the old guy's logic, less than 1.0 is tumbling) for twist-rates and velocities which are known to produce Sg of greater than 1.4. Perhaps the errors have been corrected. I'll try it and see. RG

 

[Michael Courtney]

I do not believe that either McDrag, or, Tiaoga are, "alternate theoretical approaches", but rather sound calculators which reliably dictate twist-rate requirement for specific projectiles. I may have to stand corrected regarding the "corrected" Miller formula. ;D

I am guilty of having not used it since it first appeared in Precision Shooting Magazine, as it had no correlation to either of the proven calculators, mentioned above: 'Miller' would often predict Sg as low as .9xx (where by the old guy's logic, less than 1.0 is tumbling) for twist-rates and velocities which are known to produce Sg of greater then 1.4. Perhaps the errors have been corrected. I'll try it and see. RG

McDrag doesn't compute bullet stabilities or twist rates at all. The program that does that is McGyro. McDrag computes drag coefficients. Some colleagues and I just finished up a paper demonstrating experimentally that McCoy's assumption that drag coefficients are independent of air density is accurate to 1-2% for supersonic projectiles for which it is used under the expected range of conditions. The data that McCoy used to develop McDrag were collected at Aberdeen, all so close to sea level that the density independence had never really been experimentally tested over a range of Mach numbers.

Don Miller never claimed his original twist formula would be accurate for projectiles with non-uniform density. Neither did Bob McCoy make that claim for McGyro. There is a comment in the McGyro program:

560 PRINT "[NOTE: Do not enter the center of gravity"
570 PRINT "location. Program assumes the center"
580 PRINT "of gravity is at the volume centroid]"

This is equivalent to assuming constant density. McGyro would likely make predictions just as inaccurate for plastic tipped projectiles as Miller's original twist formula.

But now the Miller twist formula has been adapted to work with plastic tipped bullets and McGyro does not. McGyro also does not compute Sg for any other atmospheric conditions other than standard sea level conditions. It won't tell you that a 1 in 12" twist .223 Remington can stabilize 69 grain open tipped match bullets above 7000 feet or how slow you can shoot a 208 AMAX from a 1 in 10" twist at 5000 feet.

 

[RGRobinett]

I do not believe that either McDrag, or, Tiaoga are, "alternate theoretical approaches", but rather sound calculators which reliably dictate twist-rate requirement for specific projectiles. I may have to stand corrected regarding the "corrected" Miller formula. ;D

I am guilty of having not used it since it first appeared in Precision Shooting Magazine, as it had no correlation to either of the proven calculators, mentioned above: 'Miller' would often predict Sg as low as .9xx (where by the old guy's logic, less than 1.0 is tumbling) for twist-rates and velocities which are known to produce Sg of greater then 1.4. Perhaps the errors have been corrected. I'll try it and see. RG

McDrag doesn't compute bullet stabilities or twist rates at all. The program that does that is McGyro. McDrag computes drag coefficients. Some colleagues and I just finished up a paper demonstrating experimentally that McCoy's assumption that drag coefficients are independent of air density is accurate to 1-2% for supersonic projectiles for which it is used under the expected range of conditions. The data that McCoy used to develop McDrag were collected at Aberdeen, all so close to sea level that the density independence had never really been experimentally tested over a range of Mach numbers.

Don Miller never claimed his original twist formula would be accurate for projectiles with non-uniform density. Neither did Bob McCoy make that claim for McGyro. There is a comment in the McGyro program:

560 PRINT "[NOTE: Do not enter the center of gravity"
570 PRINT "location. Program assumes the center"
580 PRINT "of gravity is at the volume centroid]"

This is equivalent to assuming constant density. McGyro would likely make predictions just as inaccurate for plastic tipped projectiles as Miller's original twist formula.

But now the Miller twist formula has been adapted to work with plastic tipped bullets and McGyro does not. McGyro also does not compute Sg for any other atmospheric conditions other than standard sea level conditions. It won't tell you that a 1 in 12" twist .223 Remington can stabilize 69 grain open tipped match bullets above 7000 feet or how slow you can shoot a 208 AMAX from a 1 in 10" twist at 5000 feet.

Again, I will plead guilty, for believing the intro on the JBM sight: Bullet Drag and Twist
Calculates the bullet CD, and CD components, BC required twist and stability for input twist. Inputs required are the bullet measurements including nose length, total length, boattail length, meplat diameter, base diameter, caliber, weight, atmospheric conditions and drag function. This algorithm is based on the McDrag work done by Robert McCoy. See bibliography

I will restate, that my opinion is based upon a lack of correlation: the Bill Davis bullet design program addresses all of these issues quite nicely, and DOES work well with the tipped bullets, as well as hollow-point match bullets, etc., as it accommodates a specific gravity entry to compensate for the mass:length.

At some point, in the not so distant past, the Drag/Twist calculator, at JBM, was modified to consider specific gravity also, thus provides very accurate twist-rate calculation. In my experience, for every type of bullet, from HP, to plastic-tipped, the McCoy based program, in conjunction with the Tioga program, has produced reliable info. In the past, the McCoy program proved its worth via expiriments which I conducted with bullets of a constant weight, but varying lengths, through various barrel twists.

The results, including right up against Sg 1.0, proved spot-on with the expected - that is, 'wild' yaw and pitch (about 1 caliber - a poor man's chronograph ), and roughly 4-4.5" groups (100 Yd.) at just above the predicted 1.0x. And "BUG-HOLES" with anything in excess of 1.4. Beyond backyard 'testing' (above), I credit these two programs with helping me come up with bullets and twist-rates which, over the past 17+ years, have won a pretty fair pile of fake wood, for a good many competitors at registered bench-rest tournaments.

The Davis program does account for varying atmospheric conditions. The only attribute I do not like with that program: it only provides a twist which will produce Sg 1.5, at the selected muzzle velocity and atmosphere. I suppose this is because, at 1.5 and standard conditions, at sea level, the bullet will be stable most everywhere. Taking the exact data to JBM will provide a very close correlation, but will accommodate entering a specific twist, and calculate the Sg that twist. Both programs have several more data points, thus provide a finer degree of hair-splitting. ;D

I'm not arguing just to argue - I simply stated my OPINION, based upon experience, hoping to help steer people to a calculator which provides more precise and reliable results than the original Miller Formula. In the end, shooting for Sg 1.5, at sea-level and standard condition is a pretty good target. If the Miller Formula has been up-dated, to correlate with the McCoy and Davis programs, we can all be happy. 8) RG

 

[expiper]

Randy,,,,one of these days you will get it figgerd out,,,,after makin MILLIONS of match winning and record breaking bullets,,,and testing and developing bullet profiles and types,,,,some of us know who you are and respect your MANY years of experience with/without modern day compurter print outs,,,,thanks for the knowledge you share with us who realize the worth of it,,,Roger

 

[Michael Courtney]

Again, I will plead guilty, for believing the intro on the JBM sight: Bullet Drag and Twist
Calculates the bullet CD, and CD components, BC required twist and stability for input twist. Inputs required are the bullet measurements including nose length, total length, boattail length, meplat diameter, base diameter, caliber, weight, atmospheric conditions and drag function. This algorithm is based on the McDrag work done by Robert McCoy. See bibliography

McGyro is based on McDrag, but it includes considerable additional code.

The Bill Davis bullet design program addresses all of these issues quite nicely, and DOES work well with the tipped bullets, as well as hollow-point match bullets, etc., as it accommodates a specific gravity entry to compensate for the mass:length.

Is this program still available? How? It doesn't matter how accurate it is if it is no longer available.

 

[RGRobinett]

Again, I will plead guilty, for believing the intro on the JBM sight: Bullet Drag and Twist
Calculates the bullet CD, and CD components, BC required twist and stability for input twist. Inputs required are the bullet measurements including nose length, total length, boattail length, meplat diameter, base diameter, caliber, weight, atmospheric conditions and drag function. This algorithm is based on the McDrag work done by Robert McCoy. See bibliography

McGyro is based on McDrag, but it includes considerable additional code.

[size=14pt]Yes, and that's why it's so GOOD![/size] 8)

The Bill Davis bullet design program addresses all of these issues quite nicely, and DOES work well with the tipped bullets, as well as hollow-point match bullets, etc., as it accommodates a specific gravity entry to compensate for the mass:length.

Is this program still available? How? It doesn't matter how accurate it is if it is no longer available.

Dan Lilja used to have a very similar program (free down-load) on his web-site - I cannot recall whether he worked with Davis or, McCoy - it may still be available there. I do not believe that the Tioga [Bullet Design] program is anywhere to be found - it is/was (?) copy righted software: I've used it for about 20 years. After reading MODERN EXTERIOR BALLISTICS, by McCoy, I stumbled onto the JBM site, and 'discovered' the McCoy based calculator, which, as stated earlier, has correlated very well with the Bill Davis calculator. When/if, the numbers don't 'jive', I know I'm doing something wrong!

In looking over the up-dated Miller formula, I will stick to the Bullet Drag/Twist calculator @ JBM, and continue to recommend it. The Miller calculator does not appear to factor for specific gravity, nose radius, or, nose length, all of which affect the 'balance'. Again, perusing Dan Lilja's excellent site may prove useful. RG

 

[Michael Courtney]

As I wrote before, McGyro also does not compute Sg for any other atmospheric conditions other than standard sea level conditions. It won't tell you that a 1 in 12" twist .223 Remington can stabilize 69 grain open tipped match bullets above 7000 feet or how slow you can shoot a 208 AMAX from a 1 in 10" twist at 5000 feet. The code at the Lilja web site has the same shortcoming.

So, for shooters who know the ogive radius of their intended bullet and are only interested in shooting at sea level, I suppose these are both fine tools. But for those who want to know the Sg of their bullets within 5% for any encountered atmospheric conditions and dimensions that are easily determined with a caliper, the Miller formulas should serve them well.

Another benefit of the Miller formulas is that one need not require all the margin for error of Sg > 1.5. Due to the accuracy within 5%, Sg > 1.3 is plenty to ensure optimal performance.

 

[Michael Courtney]

I should mention that another reason why I'm not too keen on Bill Davis codes is that they have been shown to be inaccurate for BC calculations. For many years, Berger bullets used the Bill Davis theoretical predictions for BCs.

See: http://www.bergerbullets.com/12/

I know Eric Stecker only acknowledges BCs that were 3-5% too high when compared with Bryan Litz measurements, but once Litz completed measuring all of the VLDs, the Bill Davis code BC predictions were as much as 11% high in some cases. For example, the .257 115 grain VLD was reported to have a G1 BC = 0.523 when the Bill Davis codes were used. However, the Litz measurements yielded a G1 BC = 0.466 for this bullet.

This is one reason I favor experimental validation for codes which are essentially theoretical calculations. Now, some folks might not mind 11% errors in ballistic coefficients, but I do. I had several lots of bullets where the errors in the Davis theory were compounded by some worn dies and quality control problems at Berger back around 2005. The actual G1 BCs of the 115 grain VLDs I received was 0.419, so the errors in the Davis theory were compounded with manufacturing defects to produce a 25% error between the advertised and actual BC.

http://arxiv.org/ftp/arxiv/papers/0705/0705.0389.pdf

Berger worked with me to identify the problem and eventually replaced the defective bullets. When I tested the new 115 grain VLDs, my results were within the experimental uncertainties of the Litz measurements.