Don't over twist your bullets. If you are up over 2.0 SG, that's too much. Wanna be about 1.6-1.8 SG
Berger Twist Rate Stability Calculator (need help)
- Thread starter Bill Norton
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Mikecr
Well-Known Member
Less stability.
The 9 in 9tw or 1:9 means 9" travel per bullet turn.
That is 9" of resistance to point forward travel that a bullet revolution is trying to compensate for. That resistance is air density, so resultant stability from a fixed turn rate is very heavily tied to air density.
Given this, twist rate requirements are always reported as based on sea level standard air density conditions. That's your beginning consideration.
So for example: If your hunting is at 1200ft above sea level, and warmer than sea level standard 59deg, then your air density is less,, less resistance to point forward bullet travel,, you can get away with further distance between bullet turns. A slower twist rate.
Bill, based on your data either 7 or 8tw are plenty. I would go 8 or even 9 myself.
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1-8 twist will work will the bullets you listed. I have a 6.5 Sherman, very similar 6.5 280. It's an 1-8. I have shot the 156s without issue at sea level.
Jay
Jay
nkyshooter
Well-Known Member
Greater, less, more, faster, slower, etc - it IS a bit confusing until you realize it is just 'backwards' ...
1 twist in 8 inches will stabilize bullets longer than a 1 twist in 9 inches will ... and 1 twist in 7 will stabilize bullets longer than a 1 twist in 8 can.
So 1 in 8 or "more" means 1 in 8 or "faster" ... the lower the number, the 'faster' the twist and therefore the longer the bullet it will stabilize.
As others have said though - we really only want to go to "fast enough" and not faster ("low enough and not lower") ... I have a 1 in 7 twist 223 that separates the jackets from the core as the barrel begins to heat up when shooting 50gr varmint bullets. But, I can shoot very heavy for 22 cal vld's quite accurately and subsequently leverage the higher BC's and get to 1000 yards easily with it.
For the Berger 140 vld, 1 twist in 8 is optimal ... you'll be able to shoot 140's in a 7 and probably in a 9 but will get best blend of stability at all altitudes and muzzle velocity with the 8 twist. If you think you might shoot the 156's, then get a "faster" 1 in 7 twist and know that you "may" sacrifice some muzzle velocity and "may" sacrifice some accuracy the lighter you go until you eventually find the point you are separating jackets from cores by spinning them so rapidly.
Cheers
1 twist in 8 inches will stabilize bullets longer than a 1 twist in 9 inches will ... and 1 twist in 7 will stabilize bullets longer than a 1 twist in 8 can.
So 1 in 8 or "more" means 1 in 8 or "faster" ... the lower the number, the 'faster' the twist and therefore the longer the bullet it will stabilize.
As others have said though - we really only want to go to "fast enough" and not faster ("low enough and not lower") ... I have a 1 in 7 twist 223 that separates the jackets from the core as the barrel begins to heat up when shooting 50gr varmint bullets. But, I can shoot very heavy for 22 cal vld's quite accurately and subsequently leverage the higher BC's and get to 1000 yards easily with it.
For the Berger 140 vld, 1 twist in 8 is optimal ... you'll be able to shoot 140's in a 7 and probably in a 9 but will get best blend of stability at all altitudes and muzzle velocity with the 8 twist. If you think you might shoot the 156's, then get a "faster" 1 in 7 twist and know that you "may" sacrifice some muzzle velocity and "may" sacrifice some accuracy the lighter you go until you eventually find the point you are separating jackets from cores by spinning them so rapidly.
Cheers
Hugnot
Well-Known Member
I have found the Miller Sg estimator to be on the conservative side. I have got extremely good accuracy with Miller Sg's under 1.5 but conditions were warm at 3000 ft. elevation. On the other hand my 8 twist .243 shooting the 87 VMax at 3250 has a Miller Sg of 2.89 and is very accurate having prairie dog hits at 400 - 85*, 3000 ft. elevation; no jacket separation but what would happen with a 7 twist at 3400 fps? 29.92 (H83) is sort of a standard but at higher elevations it would be less causing the Sg estimate to increase.
The calculations for the 87 VMax, are shown on the top bar. No great mystery - just simple math. The 87 VMax data/values are in column "H". Numbers found in column H, rows 76 to 83 were plugged into the equation in the top bar. The resultant Sg appears in column H, row 85. This stuff would be lots easier using FORTRAN or C. Just numbers used to come up with a estimated Sg. This is a real good tool for buying bullets & new barrels. Will my new 8 twist, 6mm-06 blow up 75 VMax's? I really like the 87 VMax out of a 10 twist @ 3450 +-, 6mm AI. If in doubt go for a faster twist.
Additional info:
The calculations for the 87 VMax, are shown on the top bar. No great mystery - just simple math. The 87 VMax data/values are in column "H". Numbers found in column H, rows 76 to 83 were plugged into the equation in the top bar. The resultant Sg appears in column H, row 85. This stuff would be lots easier using FORTRAN or C. Just numbers used to come up with a estimated Sg. This is a real good tool for buying bullets & new barrels. Will my new 8 twist, 6mm-06 blow up 75 VMax's? I really like the 87 VMax out of a 10 twist @ 3450 +-, 6mm AI. If in doubt go for a faster twist.
Additional info:
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Mikecr
Well-Known Member
Blowups would happen
Handy math for jacketed bullet velocity limits to mitigate blowups:
1.38889*((0.308-BulletDia)*238.095+290)*Twist
1.38889*((0.308-0.243)*238.095+290)*7 = 2970fps limit
They'd blow up right & left at 3400fps
Your 8tw 87 Vmax limit would be 3394fps
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Bill, for the Berger and Hornady Bullets you've mentioned, a 1:8 twist will work very well. You can feel confident ordering that twist rate.
Hugnot
Well-Known Member
Taking a look at this:
The 87 VMax @ 3250
The formula gives a blow up velocity of 3392 and change for a 87 VMax with a 8 twist. Bullet construction and jacket thickness varies, does the subject method consider this aspect? In any event, my 3250 fps velocities are 142 fps under the formula derived blow up limit. When shot thru my old 8 twist 6mm AI barrel at over 3400 (real close to 3392) they did not blow up. Hornady's notes their .224 SX bullets have velocity limits of 3,500 fps but do not specify twist rate.
Looking at a 53 .224, VMax shot from a 7.7 twist comes up with a blow up velocity of 3313 & change. This is a thin jacket bullet. They don't blow up at 3600 or so thru my .22-.250, 7.7 twist. An extremely smooth well lapped, 5R 7.7 twist McGowen barrel.
My old 10 twist .22-250 got about 4,000 fps with the 40 grain .224 VMax; the .224 blow up limit for a 10 twist is 4302 fps. Well within formula limits. Terrific performance on small rodents. The 10 twist barrel could launch a 68 grain Hornady HPBT and could hit silhouette chickens at 880 on a nice warm day despite having a Sg of 1.06. The nice smart gunsmith suggested going with a 10 twist.
In a recent article about the 6mm-06 in the October 2020, in Handloader magazine, bottom left page 36, gives a velocity of 3664 for the 87 VMax, 6mm-06, 8 twist, 6mm-06, with 52.5 grains of RL17. Group size at 100 was shown at .58 inches, 3 shot. I will soon attempt to duplicate this with my 8 twist 6mm-06. 3664 fps is well over the 3392 limit. Will I see gray smoke? Can't wait to report back.
I don't understand the how & why of constants (1.389, 238.095 & 290) used in the formula. Are there any references to this?
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Hugnot
Well-Known Member
The Miller stability estimator comes up with a Sg of 1.51 for the Berger .264, 140 VLD bullet, 1.421" long at 2800 fps, 8 twist, 50*. This should not be a surprise to anyone because they are sort of a standard 6.5 bullet and are used in many 6.5 rifles which have 8 twist barrels with great results. In any event, for a 6.5 a 8 twist is a sure bet. My 9 twist .260 does well with the 140 Si MK at 2000 ft. elevation on a warm day. I wish it had a 8 twist.
The Berger 140 VLD, at 35* and 3000 fps the Sg is 1.50 - stable & flying good. At 20* Sg 1.46, cold day, but still stable & flying good. Real life experience shows this bullet works well, the numbers give additional confidence.
The Berger 140 VLD, at 35* and 3000 fps the Sg is 1.50 - stable & flying good. At 20* Sg 1.46, cold day, but still stable & flying good. Real life experience shows this bullet works well, the numbers give additional confidence.
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Mikecr
Well-Known Member
It is anecdotal and no more than what I came up with decades ago to define failures reported with heavy for cal Berger VLDs. Sierras survived a bit more, but not a lot. There are of course condition variances which would defy the math. Light for cal bullets have less bearing to heat and pull apart, and good aftermarket barrels challenge jacket integrity less -for a while.
The 'limit' is not hard but conservative & reasonable as to maximum reliable velocity. It takes an extreme overbore to pass it, which will produce great heat, and bore damage,, both seriously challenging high BC bullets. Best example I know of is with 22cal 90gr VLDs needing 6.5-7tw. With these, there is a very thin margin between stable and blowup, from overbores like a 22-243imp (supporting 1kyd with that bullet).
But I would be surprised if the 87vmax holds up to 3600fps+. If it does, I think that by ~500rnds you'll lose some.
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I believe you are referring to SG of 1.9. Mikecr gives a good explanation of what can happen if a bullet is "over spun". This "over-stabilization" CAN adversely affect precision, but terminal performance may also suffer. As others have pointed out, if a bullet spins too fast it may come apart in flight. If it just barely stays together in flight and impacts an animal, it may come apart on contact, leaving a surface wound with little penetration. So in the end, keeping the SG in that 1.4-1.9 range assures proper terminal performance. Again, there are many factors at work in all of this but they are good general guidelines.
Hugnot
Well-Known Member
On the subject of stability and bullet failures.
RPM = MV * (12/twist) * 60, or RPM = MV * (720/twist)
An 87 VMax @ 3600 plus fps out of a 8 twist would have 324,000 RPM's & be spinning like crazy. Any major jacket defect would cause failure and contact with any object would result in a chaotic impact. Not good for deep controlled expansion on big animals like elks with common angling shots. Any variance of the bullets center of mass would be magnified by huge spin rates and accuracy would suffer. Shorter bullets don't need fast twist rates.
High performance has liabilities like short barrel life, heat, short brass life, and magnification of bullet defects. The advantages are flat trajectory and reduced wind deflection. Weatherby rifles and cartridges have been in use for a long time despite their drawbacks.
The Sg values are derived using the formula shown on my post that included the excel spread sheet. Sg data includes bullet diameter, weight, length, twist, velocity, temperature, & pressure. The major items in the equation are velocity, caliber, twist & length - these receive exponential treatment in the equation. The environmental stuff that varies with conditions is secondary.
Making a decision on terminal performance based on Sg values has problems. A 12 twist (common for .375's) .375, 300 grain, 2500 fps, 1.382 inch long bullet at 80*, used to kill big beasts, would have a Sg value of 3.11, as per the Miller Sg estimate equation. I have a .375- .338 and I use the 270 grain Speer SPBT bullet at about 2650, this has an Sg of 3.33; I like to shoot steel with this combo because I like to hear bullet impact sounds. Bullets having relatively high and equal sectional densities may have markedly different lengths thus the shorter bullet would have a higher Sg value but equal sectional density and velocity. The Miller equation does not include bullet shapes.
Some time ago I read about Berger bullets failing in flight. I have my own idea's why this happened, which is, the bullet jackets were work hardened before the ogive was formed. Upon forming the ogive, longitudinal stress fractures occurred along the ogive when the previously work hardened brass was subjected to additional stresses and the failure occurred at these cracks. I have seen these longitudinal cracks running along some bullet ogives. The longitudinal crack stuff has been observed to occur, at right angles, to the force applied, in many materials upon exceeding plastic limits. The failing Berger bullets were not subjected to huge velocities or excessive twist rates. The Miller Sg equation does not include jacket data or quality. Failure might have occurred with normal Sg values, like under Sg 2.0.
I have no plans for a .224 using 88 or 90 grain bullets. I shoot 75 .224 ELDM's at 3300 fps out of a 7.7 twist with no problems, SG 1.9, the Hornady 4DOF Sg calculation claims a 7.7 is too slow and the label on the box recommends a 7 twist.
If I had to shoot something weighing over 150 pounds with my small caliber .224's & 6mm's I would use one of those solid copper alloy bullets and would not care about any Sg values and would select a pointy bullet having good ballistic stuff.
RPM = MV * (12/twist) * 60, or RPM = MV * (720/twist)
An 87 VMax @ 3600 plus fps out of a 8 twist would have 324,000 RPM's & be spinning like crazy. Any major jacket defect would cause failure and contact with any object would result in a chaotic impact. Not good for deep controlled expansion on big animals like elks with common angling shots. Any variance of the bullets center of mass would be magnified by huge spin rates and accuracy would suffer. Shorter bullets don't need fast twist rates.
High performance has liabilities like short barrel life, heat, short brass life, and magnification of bullet defects. The advantages are flat trajectory and reduced wind deflection. Weatherby rifles and cartridges have been in use for a long time despite their drawbacks.
The Sg values are derived using the formula shown on my post that included the excel spread sheet. Sg data includes bullet diameter, weight, length, twist, velocity, temperature, & pressure. The major items in the equation are velocity, caliber, twist & length - these receive exponential treatment in the equation. The environmental stuff that varies with conditions is secondary.
Making a decision on terminal performance based on Sg values has problems. A 12 twist (common for .375's) .375, 300 grain, 2500 fps, 1.382 inch long bullet at 80*, used to kill big beasts, would have a Sg value of 3.11, as per the Miller Sg estimate equation. I have a .375- .338 and I use the 270 grain Speer SPBT bullet at about 2650, this has an Sg of 3.33; I like to shoot steel with this combo because I like to hear bullet impact sounds. Bullets having relatively high and equal sectional densities may have markedly different lengths thus the shorter bullet would have a higher Sg value but equal sectional density and velocity. The Miller equation does not include bullet shapes.
Some time ago I read about Berger bullets failing in flight. I have my own idea's why this happened, which is, the bullet jackets were work hardened before the ogive was formed. Upon forming the ogive, longitudinal stress fractures occurred along the ogive when the previously work hardened brass was subjected to additional stresses and the failure occurred at these cracks. I have seen these longitudinal cracks running along some bullet ogives. The longitudinal crack stuff has been observed to occur, at right angles, to the force applied, in many materials upon exceeding plastic limits. The failing Berger bullets were not subjected to huge velocities or excessive twist rates. The Miller Sg equation does not include jacket data or quality. Failure might have occurred with normal Sg values, like under Sg 2.0.
I have no plans for a .224 using 88 or 90 grain bullets. I shoot 75 .224 ELDM's at 3300 fps out of a 7.7 twist with no problems, SG 1.9, the Hornady 4DOF Sg calculation claims a 7.7 is too slow and the label on the box recommends a 7 twist.
If I had to shoot something weighing over 150 pounds with my small caliber .224's & 6mm's I would use one of those solid copper alloy bullets and would not care about any Sg values and would select a pointy bullet having good ballistic stuff.
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You guys are so smart, so if you were to build a 257 Roberts with a 26 inch. Barrel. And plan too shoot more 87 & 90 grain than 120. M 70s. Are one in 10. Local gurues think it should be one in 9 or 8. Is that good or bad advice? Thanks...
foxtrotter
Member
1:10 is the standard twist for the 257 Roberts. My Roberts shoots all (87 to 120) well with that twist. I built a 25-06 with 1:12 specifically for 87 gr. bullets and it will not stabilize long 120 gr. bullet (Hornady 120 hollow point).
