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rifling twist for 123 gr. 6.5 mm
- Thread starter logeorge
- Start date
Bergers twist rate stability calculator
https://bergerbullets.com/twist-rate-calculator/
bullet length chart
https://www.jbmballistics.com/ballistics/lengths/lengths.shtml
based on the information you gave, using the above information both bullets, estimating 1800fps they come in at right around a 1.05-1.06, marginally stable, below 1 is unstable above 1.5 is stable. Its super close to being unstable...
the 100g sierra is 3.54 and 120g sierras 2.24, both quite stable.
https://bergerbullets.com/twist-rate-calculator/
bullet length chart
https://www.jbmballistics.com/ballistics/lengths/lengths.shtml
based on the information you gave, using the above information both bullets, estimating 1800fps they come in at right around a 1.05-1.06, marginally stable, below 1 is unstable above 1.5 is stable. Its super close to being unstable...
the 100g sierra is 3.54 and 120g sierras 2.24, both quite stable.
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more velocity will mean more stability, but not much. I ran the numbers at 2500fps, and it only bumped it to a 1.17, still barely stable. If your reloading for it estimate 50-100 fps below max listed velocity in the manual. based on the stability numbers i would not wast the money. check the bullet length list for 6.5mm (.264) and try to find some bullets with shorter OAL's you are interested in. a rough estimate would be a bullet of 1.115 or shorter. that will keep them comfortable stable. you might get away with a bit longer with more velocity, but i try to stay out of the marginal stability range when i can.I emailed Hornady and they said I need a faster twist. I suppose the only way to be sure is to try some. I don't have a chronograph, so I don't know what velocity I'm getting.
the sierra pro hunter(litz) and Spitzer Pro hunter are both short enough in 120g.
the Speer Spitzer SP is also in the right range.
the swift a frames semi spitzer 120g is aslo stable
the barnes match burner 120g is at 1.26, just over the middle of marginalyl stable was the only match bullet in the weight range you were lookign at that looked close.
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I don't really have to have a match grade bullet for this rifle. I saw some videos on the 6.5 Grendel with 123 gr. AMAX and ELD and wondered if they might work in this one. If I were building a rifle in 6.5 mm with a case this large, I'd go with a 1-7 or 1-8 twist for the 140 gr. and up. I guess I'll stick to the Sierras l've been using. I appreciate your input. There's a long, strange story behind this rifle. If you want to hear it, I will send you a PM. Really, this cartridge with the 1-10 twist amounts to a .257 Roberts with a .007" larger bullet. Still a nice deer round, though.
A little better news for you
logeorge,
I think you will find both bullets shoot just fine for you. I'm not sure how Shadow9mm got his numbers. The lengths of those two 0.264" diameter bullets are 1.236 (A-max) and 1.247 (ELD Match). At sea level in 59°F and 0% RH (ICAO standard conditions) and 1800 fps, the stability factors I get from the Berger calculator are 1.39 and 1.35, respectively. But those are going to be low numbers. The reason is your bullets both have plastic tips. Berger doesn't make bullets with plastic tips, so their calculator doesn't try to account for them. The JBM calculator does.
The reason the tip makes a difference is the stability formula used by Berger and JBM, both, is Don Miller's modified Greenhill formula, and it assumes a typical profile with uniformly distributed density. The plastic tip has less density than the rest of the bullet, so it contributes less to angular momentum in any motion made by the tip. Of particular interest is stability near the muzzle while the bullet is recovering from initial yaw, which causes it to trace circles with nutation lobes that quickly settle into what is called a coning motion¹. In that, the tip is circling. Lowering the mass of the tip makes it easier for precession to turn the bullet and thus settle it into the yaw of repose for the rest of the flight. You'd see the same thing if the bullet were shorter or were spinning faster, thus it corresponds to an increase in the stability factor.
The way the JBM calculator compensates is by subtracting the plastic tip length from the bullet length. I think this goes a little too far because even though the tip mass is small, there is still drag on it in proportion to its profile area times the square of the sine of the yaw angle so I use about 75% of the tip length, to create a safety margin. Your tips are 0.14" long, so I used 0.105" (75% of 0.147) in the JBM version of the calculator and for 59°F, 0% RH, at 1800 fps I get stability factors of 1.644 and 1.600 for the A-max and ELD, respectively, based on that plastic tip compensating length.
I'm not sure why Berger considers stability factors of 1.3 to 1.5 as marginal. Various ballistics authorities have had different opinions about what stability factor best balances the need for the bullet to settle out promptly without spinning it so fast that small mass asymmetries in the bullet introduce eccentric spin in flight (aka, wobble). Wobble deteriorates both BC and accuracy. These authorities have given estimated nominal values anywhere from 1.4 to 1.7, with the largest number of them settling on 1.5 as nominal. Years ago, a Sierra technician told me he advised 1.3 to 3.0 as fine for "hunting accuracy", while 1.4 to 1.7 was best for match shooting, probably because he'd read the same authorities. Nonetheless, the 10" twist 30 cal military gas guns fired the 168-grain Sierra MatchKing at a stability factor of about 2.4, and it won mountains of matches that way just fine, so this is all relative to what accuracy extremes you are trying to arrive at.
Where everyone can agree is that between 1.0 and 1.3 groups are opening up unacceptably, even for some hunting purposes.
Bottom line: the two bullets you mentioned should shoot well, especially in warmer weather or at higher altitudes. I haven't calculated how they will do in the transonic range. You need the drag function and the CG location and the axial and transverse moments of inertia for that, but I expect them to be OK.
¹ The circling bullet tip turns the bullet axis around its center of mass (CM), and thus with the circle as the base, the axis traces the shape of a cone whose tip is at that CM, with the base axis tracing a smaller cone on the other side of the CM.
logeorge,
I think you will find both bullets shoot just fine for you. I'm not sure how Shadow9mm got his numbers. The lengths of those two 0.264" diameter bullets are 1.236 (A-max) and 1.247 (ELD Match). At sea level in 59°F and 0% RH (ICAO standard conditions) and 1800 fps, the stability factors I get from the Berger calculator are 1.39 and 1.35, respectively. But those are going to be low numbers. The reason is your bullets both have plastic tips. Berger doesn't make bullets with plastic tips, so their calculator doesn't try to account for them. The JBM calculator does.
The reason the tip makes a difference is the stability formula used by Berger and JBM, both, is Don Miller's modified Greenhill formula, and it assumes a typical profile with uniformly distributed density. The plastic tip has less density than the rest of the bullet, so it contributes less to angular momentum in any motion made by the tip. Of particular interest is stability near the muzzle while the bullet is recovering from initial yaw, which causes it to trace circles with nutation lobes that quickly settle into what is called a coning motion¹. In that, the tip is circling. Lowering the mass of the tip makes it easier for precession to turn the bullet and thus settle it into the yaw of repose for the rest of the flight. You'd see the same thing if the bullet were shorter or were spinning faster, thus it corresponds to an increase in the stability factor.
The way the JBM calculator compensates is by subtracting the plastic tip length from the bullet length. I think this goes a little too far because even though the tip mass is small, there is still drag on it in proportion to its profile area times the square of the sine of the yaw angle so I use about 75% of the tip length, to create a safety margin. Your tips are 0.14" long, so I used 0.105" (75% of 0.147) in the JBM version of the calculator and for 59°F, 0% RH, at 1800 fps I get stability factors of 1.644 and 1.600 for the A-max and ELD, respectively, based on that plastic tip compensating length.
I'm not sure why Berger considers stability factors of 1.3 to 1.5 as marginal. Various ballistics authorities have had different opinions about what stability factor best balances the need for the bullet to settle out promptly without spinning it so fast that small mass asymmetries in the bullet introduce eccentric spin in flight (aka, wobble). Wobble deteriorates both BC and accuracy. These authorities have given estimated nominal values anywhere from 1.4 to 1.7, with the largest number of them settling on 1.5 as nominal. Years ago, a Sierra technician told me he advised 1.3 to 3.0 as fine for "hunting accuracy", while 1.4 to 1.7 was best for match shooting, probably because he'd read the same authorities. Nonetheless, the 10" twist 30 cal military gas guns fired the 168-grain Sierra MatchKing at a stability factor of about 2.4, and it won mountains of matches that way just fine, so this is all relative to what accuracy extremes you are trying to arrive at.
Where everyone can agree is that between 1.0 and 1.3 groups are opening up unacceptably, even for some hunting purposes.
Bottom line: the two bullets you mentioned should shoot well, especially in warmer weather or at higher altitudes. I haven't calculated how they will do in the transonic range. You need the drag function and the CG location and the axial and transverse moments of inertia for that, but I expect them to be OK.
¹ The circling bullet tip turns the bullet axis around its center of mass (CM), and thus with the circle as the base, the axis traces the shape of a cone whose tip is at that CM, with the base axis tracing a smaller cone on the other side of the CM.
Unclenick: You are way better informed than I am on this subject. I will give them a try when I get a chance. I was a toolmaker, but never got around to finishing my engineering studies. Too much overtime work. I'm originally from Ohio, too. Worked at Ford's Lima Engine Plant for 40 years.
great info, thanks!logeorge,
I think you will find both bullets shoot just fine for you. I'm not sure how Shadow9mm got his numbers. The lengths of those two 0.264" diameter bullets are 1.236 (A-max) and 1.247 (ELD Match). At sea level in 59°F and 0% RH (ICAO standard conditions) and 1800 fps, the stability factors I get from the Berger calculator are 1.39 and 1.35, respectively. But those are going to be low numbers. The reason is your bullets both have plastic tips. Berger doesn't make bullets with plastic tips, so their calculator doesn't try to account for them. The JBM calculator does.
The reason the tip makes a difference is the stability formula used by Berger and JBM, both, is Don Miller's modified Greenhill formula, and it assumes a typical profile with uniformly distributed density. The plastic tip has less density than the rest of the bullet, so it contributes less to angular momentum in any motion made by the tip. Of particular interest is stability near the muzzle while the bullet is recovering from initial yaw, which causes it to trace circles with nutation lobes that quickly settle into what is called a coning motion¹. In that, the tip is circling. Lowering the mass of the tip makes it easier for precession to turn the bullet and thus settle it into the yaw of repose for the rest of the flight. You'd see the same thing if the bullet were shorter or were spinning faster, thus it corresponds to an increase in the stability factor.
The way the JBM calculator compensates is by subtracting the plastic tip length from the bullet length. I think this goes a little too far because even though the tip mass is small, there is still drag on it in proportion to its profile area times the square of the sine of the yaw angle, so I use about 75% of the tip length to create a safety margin. Your tips are 0.14" long, so I used 0.105" (75% of 0.147) in the JBM version of the calculator and for 59°F, 0% RH, at 1800 fps I get stability factors of 1.644 and 1.600 for the A-max and ELD, respectively, based on that plastic tip compensating length.
I'm not sure why Berger considers stability factors of 1.3 to 1.5 as marginal. Various ballistics authorities have had different opinions about what stability factor best balances the need for the bullet to settle out promptly without spinning it so fast that small mass asymmetries in the bullet introduce eccentric spin in flight (aka, wobble). Wobble deteriorates both BC and accuracy. These authorities have given estimated nominal values anywhere from 1.4 to 1.7, with the largest number of them settling on 1.5 as nominal. Years ago, a Sierra technician told me he advised 1.3 to 3.0 as fine for "hunting accuracy", while 1.4 to 1.7 was best for match shooting, probably because he'd read the same authorities. Nonetheless, the 10" twist 30 cal military gas guns fired the 168-grain Sierra MatchKing at a stability factor of about 2.4, and it won mountains of matches that way just fine, so this is all relative to what accuracy extremes you are trying to arrive at.
Where everyone can agree is that between 1.0 and 1.3 groups are opening up unacceptably, even for some hunting purposes.
Bottom line: the two bullets you mentioned should shoot well, especially in warmer weather or at higher altitudes. I haven't calculated how they will do in the transonic range. You need the drag function and the CG location and the axial and transverse moments of inertia for that, but I expect them to be OK.
¹ The circling bullet tip turns the bullet axis around its center of mass (CM), and thus with the circle as the base, the axis traces the shape of a cone whose tip is at that CM, with the base axis tracing a smaller cone on the other side of the CM.