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Twist rate for 155gr .308?
- Thread starter Curly T
- Start date
The Oehler Ballistic Explorer shows three 155gr Palmas, catalog numbers 2155 and 2155M (molycoated) and 2156. The 2155s list a bullet length of 1.130", the 2156 1.192".
The gyroscopic stability calculated for the 2155 and 1:14" twist rate is 1.6, fast enough to stabilize.
Curiously, the 2155M/1:14" comes up 1.1 and too slow. Guess that moly coat is more significant than I knew. A 1:12" twist is recommended for an SG of 1.5.
The longer 2156 and 1:14" twist SG come out at 1.0 and too slow. Recommended is a twist rate of 1:11" for an SG of 1.6.
A search engine will provide links to other calculators, your own measurements can check the bullet length.
The gyroscopic stability calculated for the 2155 and 1:14" twist rate is 1.6, fast enough to stabilize.
Curiously, the 2155M/1:14" comes up 1.1 and too slow. Guess that moly coat is more significant than I knew. A 1:12" twist is recommended for an SG of 1.5.
The longer 2156 and 1:14" twist SG come out at 1.0 and too slow. Recommended is a twist rate of 1:11" for an SG of 1.6.
A search engine will provide links to other calculators, your own measurements can check the bullet length.
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The JBM Stability Calculator comes up with different numbers.
The 2155 is listed with a length of 1.131 inches, and from a 1:14 twist is stable at 2750 fps with a factor of 1.440. The longer 2156 is not stable with a 1:14 twist even to 3250 fps in case someone were going to suggest rechambering to a magnum to use that bullet.
So if you are inclined to try, I would start with the 2155s. They are an older design, but there is nothing wrong with them.
Jimro
The 2155 is listed with a length of 1.131 inches, and from a 1:14 twist is stable at 2750 fps with a factor of 1.440. The longer 2156 is not stable with a 1:14 twist even to 3250 fps in case someone were going to suggest rechambering to a magnum to use that bullet.
So if you are inclined to try, I would start with the 2155s. They are an older design, but there is nothing wrong with them.
Jimro
Nosler guy
New member
It seems best to use a bullet that is best recommended near the limits for the twist you have. I used to think that a fast twist is best for everything but with a light bullet any run-out in that bullet can cause significant accuracy problems where a slower twist closer to the tolerance limit of the bullet weight can make the same bullets very accurate.Also Nosler custom comp and the accubond long range bullets are the only thing that have shot through the same hole where the computer screen registered a 0.0 group in the lab.
Inside of 550 yards (500 meters roughly) you'll be fine with any of the options listed.
Even 144~150gr FMJBT bullets should shoot very well from your slow twist barrel, which is why so many Fullbore shooters use slow twist barrels. The open tip match bullets should provide better terminal effects on varmints though.
Good luck hunting!
Jimro
Even 144~150gr FMJBT bullets should shoot very well from your slow twist barrel, which is why so many Fullbore shooters use slow twist barrels. The open tip match bullets should provide better terminal effects on varmints though.
Good luck hunting!
Jimro
Chuck Dye said:Curiously, the 2155M/1:14" comes up 1.1 and too slow. Guess that moly coat is more significant than I knew. A 1:12" twist is recommended for an SG of 1.5.
Chuck, the stability depends on bullet spin and shape and density. Moly should make no difference at all, so I am curious whose software you used to get those numbers?
Barrel 24" Twist 1:14 Taper .990"-.980" MV 2750fps Sierra 2156
Barrel 24" Twist 1:14 Taper .990"-.980" MV 2750fps Sierra 2156 .30 Caliber/7.62mm (.308) 155 gr. HPBT PALMA® MatchKing
RPM 141428.5
Mauser Action, Timney Trigger, and a Circassian Walnut Stock.
Comes up with a Stability of .970 which is less than optimal. That being said it works just fine for 600 Yards no Keyhole I am shooting 1/2 Moa.
Barrel 24" Twist 1:14 Taper .990"-.980" MV 2750fps Sierra 2156 .30 Caliber/7.62mm (.308) 155 gr. HPBT PALMA® MatchKing
RPM 141428.5
Mauser Action, Timney Trigger, and a Circassian Walnut Stock.
Comes up with a Stability of .970 which is less than optimal. That being said it works just fine for 600 Yards no Keyhole I am shooting 1/2 Moa.
Anything less than 1.0 is outright unstable, if the calculator is accurate. I see now that Chuck used the Oehler Ballistic Explorer. No idea why it thought the moly would be different. The only thing I can think is that it automatically reduced velocity, as moly bullets tend to run about 50 fps slower if you don't adjust the load. But if you do adjust the load to get the same MV, it should make no difference.
The 20" barrel the OP has should produce Jimro's 2750 fps with a near full load of Varget or anything else that passes about 2890 fps in a 24" standard velocity barrel, but the starting load will be about 2630 fps. The Miller estimator gives that a GS of 1.2, which is not unstable, but not good. A Sierra tech once told me they want to see 1.3–3.0 for hunting accuracy, and 1.4–1.7 for target accuracy.
Geoffrey Kolbe's more detailed twist estimator, based on the late Robert L. McCoy's McGyro program, thinks the 2155 will actually be a little more stable with a 14" twist all the way down to about 2050 fps, where it puts the GS factor at 1.3. It thinks 1.4 would be all the way up at about 3300 fps. This is based on entering the bullet's full set of dimensions from Bryan Litz's data. Note that I had to use a density of 10.0 to get the right weight estimate from those dimensions, as found using Kolbe's drag calculator.
Kolbe's calculator also finds the 2156 unsuitable. Note that this bullet has a lot of empty space in the long nose, so a density of 9.7 is necessary to get the weight right if you try it in Kolbe's calculator.
Litz's measured lengths are a little longer than the ones Chuck measured, and this accounts for part of the difference. He has the 2155 at 1.131" and the 2156 at 1.210".
The 20" barrel the OP has should produce Jimro's 2750 fps with a near full load of Varget or anything else that passes about 2890 fps in a 24" standard velocity barrel, but the starting load will be about 2630 fps. The Miller estimator gives that a GS of 1.2, which is not unstable, but not good. A Sierra tech once told me they want to see 1.3–3.0 for hunting accuracy, and 1.4–1.7 for target accuracy.
Geoffrey Kolbe's more detailed twist estimator, based on the late Robert L. McCoy's McGyro program, thinks the 2155 will actually be a little more stable with a 14" twist all the way down to about 2050 fps, where it puts the GS factor at 1.3. It thinks 1.4 would be all the way up at about 3300 fps. This is based on entering the bullet's full set of dimensions from Bryan Litz's data. Note that I had to use a density of 10.0 to get the right weight estimate from those dimensions, as found using Kolbe's drag calculator.
Kolbe's calculator also finds the 2156 unsuitable. Note that this bullet has a lot of empty space in the long nose, so a density of 9.7 is necessary to get the weight right if you try it in Kolbe's calculator.
Litz's measured lengths are a little longer than the ones Chuck measured, and this accounts for part of the difference. He has the 2155 at 1.131" and the 2156 at 1.210".