Thanks easy to use…
I wonder why it doesn't require barrel length or have a minimum length ? I'm
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Bullet stability
- Thread starter scpaul
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I wonder why it doesn't require barrel length or have a minimum length ? I'm
scpaul
Well-Known Member
I bought several box's of 7mm burgers before I had to quit shooting. I think that the heaviest one was around 195gr hybrid. I can't be sure, they are outside in the loadroom.
MagnumManiac
Well-Known Member
@Mikecr
My point was regarding drag in relation to atmospheric conditions, which does change not only drag but also BC with differing air density…just didn't phrase it well.
Another point is that when a bullet is so long to be marginal with high velocity, there is always a point where the bullet is travelling either with the tip up or down, this changes drag far more than rotational speed. I can't remember the term used for it.
Cheers.
My point was regarding drag in relation to atmospheric conditions, which does change not only drag but also BC with differing air density…just didn't phrase it well.
Another point is that when a bullet is so long to be marginal with high velocity, there is always a point where the bullet is travelling either with the tip up or down, this changes drag far more than rotational speed. I can't remember the term used for it.
Cheers.
scpaul
Well-Known Member
Screwing up or down, haha
Hugnot
Well-Known Member
The .284 Berger Hybrid is one long pointy bullet needing a fast twist rate.
Using the accepted Miller Sg calculation:
A 9 or 9.5 twist will not stabilize the 195 Berger Hibird at sea level at mild temperatures- it's marvelous G7 BC (comparative to standard) will be severely compromised due to bullet wobbling - like tip describing a circle as it plows thru air instead of slightly pointing up. Chances are that 1.645 long bullet profiles might occur on paper. MV of 2850 looks good for most big 7m rounds. Cavitation most likely thru water at much lower speeds.
From Berger:
"A Bullet's Ballistic Coefficient is comprised of 3 basic components: Weight, Diameter, and Form Factor. The weight and diameter of the bullet combine to determine the bullets sectional density; that's the amount of mass packed behind the frontal area of the bullet. Then form factor is what describes how streamlined the projectile is. A bullet with a long pointy nose and a boat tail will have less drag, and a lower form factor, than a blunt, flat based bullet. In particular, form factor is a number that relates the drag of a bullet to the drag of a standard such as G1 or G7. G7 form factors for long range bullets are from 0.95 (low drag) down to 0.88 (very low drag) for most typical long range bullets. The lowest drag projectiles made on lathes with very aggressive shapes, like our solid bullets, can have G7 form factors as low as 0.80, but that's pretty rare."
Calculating the 195 .284 HiBird FF.
The Berger twist calculator gets into BC degradation with inadequate twist rates - but - using a barometer will provide actual barometric pressures instead of an altitude table for Sg determination. Bullet revs over expected TOF degrade very little like over 1000 yards with a TOF of 1 sec.
Miller sez 1.5 Sg for adequate stability. I have found the Miller Sg to be conservative, obtaining good accuracy with Sg values between 1.4 - 1.5 but minor BC degradation might occur.
Looks like an 8 twist would be best for the long pointy .284 Berger Hibird. Adequate Sg values of 1.5 plus with a 9 twist may be obtained at high altitude/low pressure & high temperatures like 27-inch Hg (possibly 5000 ft plus) & 95 F but Sg's will be inadequate for most applications.
I'm real happy with my creepy .280 Rem shooting stubby 139 H SST's killing deers and such up to 400 yards. I also use the 139 SST against rodents and have done well up to 600. My .280 R has a 9 twist barrel & I would not waste money on something that would not shoot good like the 195 Hibird in my 9 twist .280 Rem.
Using the accepted Miller Sg calculation:
A 9 or 9.5 twist will not stabilize the 195 Berger Hibird at sea level at mild temperatures- it's marvelous G7 BC (comparative to standard) will be severely compromised due to bullet wobbling - like tip describing a circle as it plows thru air instead of slightly pointing up. Chances are that 1.645 long bullet profiles might occur on paper. MV of 2850 looks good for most big 7m rounds. Cavitation most likely thru water at much lower speeds.
From Berger:
"A Bullet's Ballistic Coefficient is comprised of 3 basic components: Weight, Diameter, and Form Factor. The weight and diameter of the bullet combine to determine the bullets sectional density; that's the amount of mass packed behind the frontal area of the bullet. Then form factor is what describes how streamlined the projectile is. A bullet with a long pointy nose and a boat tail will have less drag, and a lower form factor, than a blunt, flat based bullet. In particular, form factor is a number that relates the drag of a bullet to the drag of a standard such as G1 or G7. G7 form factors for long range bullets are from 0.95 (low drag) down to 0.88 (very low drag) for most typical long range bullets. The lowest drag projectiles made on lathes with very aggressive shapes, like our solid bullets, can have G7 form factors as low as 0.80, but that's pretty rare."
Calculating the 195 .284 HiBird FF.
The Berger twist calculator gets into BC degradation with inadequate twist rates - but - using a barometer will provide actual barometric pressures instead of an altitude table for Sg determination. Bullet revs over expected TOF degrade very little like over 1000 yards with a TOF of 1 sec.
Miller sez 1.5 Sg for adequate stability. I have found the Miller Sg to be conservative, obtaining good accuracy with Sg values between 1.4 - 1.5 but minor BC degradation might occur.
Looks like an 8 twist would be best for the long pointy .284 Berger Hibird. Adequate Sg values of 1.5 plus with a 9 twist may be obtained at high altitude/low pressure & high temperatures like 27-inch Hg (possibly 5000 ft plus) & 95 F but Sg's will be inadequate for most applications.
I'm real happy with my creepy .280 Rem shooting stubby 139 H SST's killing deers and such up to 400 yards. I also use the 139 SST against rodents and have done well up to 600. My .280 R has a 9 twist barrel & I would not waste money on something that would not shoot good like the 195 Hibird in my 9 twist .280 Rem.
akmtnhnt
Well-Known Member
Because barrel length doesn't have anything to do with stability.
Barrel length is a factor in muzzle velocity.
Correct, but MV does.
Regardless of the bullet stability calculator used, i.e., as provided in #11, factors affecting SG are already provided. Any changes in one of those attributes will affect the SG. Don't overcomplicate it unnecessarily.
I know bullet X manufacturer will claim their ballistic/SG apps will not work because of their bullet design. Most bullet manufacturers, i.e., Sierra, Berger, Hornady, etc., have their "ballisticians" that are subject matter experts in the field that can help you better understand if you want to go that route.
Cheers!
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Seabeeken
Well-Known Member
I ran this very scenario last night for a hammer bullet on Bergers stability calculator.The 51 gr AH in a 10 twist is marginally stable at 3000 fps and stable at 3300 fps
RockyMtnMT
Official LRH Sponsor
Before we started making bullets I shot marginally stable bullets regularly. I didn't know any better. If it shoots good then it will work good, right? When we started making bullets and impact testing, particularly low velocity, we got eye opening results. Basically the higher the sg the more reliably the bullets will properly expand on impact. When we saw stability factor directly related to the same bullets ability to function properly at the same impact velocity, it was a lightbulb moment. Memories of bullets that we used over the years that gave less than desirable effects on game, tumbling, that we chalked up to poor bullet performance. Likely not the bullets fault, but operator error not giving the bullet enough stability for proper terminal performance. It has been long accepted that dangerous game bullets should run a minimum sg of 2.0 for good straight line penetration. There is difference between enough stability to fly well (ballistic stability) and enough stability to kill well (terminal stability).
The Miller formula, Berger and JBM both use this, is the most common formula. We have come to the conclusion that it is close but flawed. It's the best tool we have though. It doesn't take into account material density and there wasn't bullets so long for caliber when the formula was developed. The more "normal" for length a bullet is the more forgiving it is for stability. The longer bullets get for caliber the more particular the the stability factor gets. We have adapted to this by calculating for hunting by only using standard atmosphere at sea level for a minimum sg of 1.5. People that are hunting at low elevation and cold temps should beware that minimum twist rates calculated by a ballistic calculator not a terminal stability calculator, could have issues on game.
In the end, it is not worth it to push minimum twist rates in order to get a heavier or higher bc bullet. You will get better performance on game with a lighter or higher stability bullet. You just can't beat physics.
The Miller formula, Berger and JBM both use this, is the most common formula. We have come to the conclusion that it is close but flawed. It's the best tool we have though. It doesn't take into account material density and there wasn't bullets so long for caliber when the formula was developed. The more "normal" for length a bullet is the more forgiving it is for stability. The longer bullets get for caliber the more particular the the stability factor gets. We have adapted to this by calculating for hunting by only using standard atmosphere at sea level for a minimum sg of 1.5. People that are hunting at low elevation and cold temps should beware that minimum twist rates calculated by a ballistic calculator not a terminal stability calculator, could have issues on game.
In the end, it is not worth it to push minimum twist rates in order to get a heavier or higher bc bullet. You will get better performance on game with a lighter or higher stability bullet. You just can't beat physics.
Hugnot
Well-Known Member
The Miller stability or Sg calculator is only an estimator providing a ratio, like over 1.0 having neutral stability & 1.5 having adequate stability.
The Miller method is widely accepted and is used by Berger & JBM. Berger uses an altitude pressure table vs. actual barometric pressures. It (Miller) does not consider large cavities in bullets like the 107 grain 6.5 Sierra hpbt that appears to have close to 20% void at tip (material density). Plastic tips are factored in by an operation like (H25 - H26 / H23) ^2, as shown in the calculation spread sheet.
Sg values increase down range as bullet velocity decreases rapidly and effects such as yawing & pitching decrease while bullet revs decrease very slowly with TOF being short (spinning like crazy, lots of RPM's like over 300K in some cases)
From Hornady;
"The pitch and yaw angle is continually getting smaller because the bullet is dynamically stable."
This makes Sg values increase as velocity decreases.
Velocity only has a minor effect on Sg values, like:
Note the Sg value creeps up from 1.65 to 1.68 with a 150 fps velocity increase - no big deal.
Form factor is another important consideration giving rise to lighter bullets that can be driven faster, like:
I like the .224 75 grain ELDM in my 7.7 twist .22-.250's because it provides good ballistics at low cost, like under $30 for 100 & powder charges in the mid 30 grain range. The .224 80 grain ELDM has higher G7 BC values but cannot be driven as fast, however the 80's give slightly better numbers using Hornady's 4DOF program with ranges up to 1000 with 100 fps slower MV's.
I have no plans to shoot some real big beast with my little .22-.250 with 75 grain ELDM bullets but I have plans for some programming compiler on my computer so I can quit messing with spread sheets like (H25 - H26 / H23) ^2
The Miller method is widely accepted and is used by Berger & JBM. Berger uses an altitude pressure table vs. actual barometric pressures. It (Miller) does not consider large cavities in bullets like the 107 grain 6.5 Sierra hpbt that appears to have close to 20% void at tip (material density). Plastic tips are factored in by an operation like (H25 - H26 / H23) ^2, as shown in the calculation spread sheet.
Sg values increase down range as bullet velocity decreases rapidly and effects such as yawing & pitching decrease while bullet revs decrease very slowly with TOF being short (spinning like crazy, lots of RPM's like over 300K in some cases)
From Hornady;
"The pitch and yaw angle is continually getting smaller because the bullet is dynamically stable."
This makes Sg values increase as velocity decreases.
Velocity only has a minor effect on Sg values, like:
Note the Sg value creeps up from 1.65 to 1.68 with a 150 fps velocity increase - no big deal.
Form factor is another important consideration giving rise to lighter bullets that can be driven faster, like:
I like the .224 75 grain ELDM in my 7.7 twist .22-.250's because it provides good ballistics at low cost, like under $30 for 100 & powder charges in the mid 30 grain range. The .224 80 grain ELDM has higher G7 BC values but cannot be driven as fast, however the 80's give slightly better numbers using Hornady's 4DOF program with ranges up to 1000 with 100 fps slower MV's.
I have no plans to shoot some real big beast with my little .22-.250 with 75 grain ELDM bullets but I have plans for some programming compiler on my computer so I can quit messing with spread sheets like (H25 - H26 / H23) ^2
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Mikecr
Well-Known Member
What do you mean [exactly] by 'marginally stable' for that scenario?
Consider a hypothetical muzzle velocity affect to a Sierra 111 DTAC, ICAO air density, 9:1 twist:
3,000fps Sg = 1.16
3,500fps Sg = 1.19
4,000fps Sg = 1.21
10,000fps Sg = 1.34
17,513fps Sg = 1.50 fully stable
All of these Sg's under 1.5 are considered 'marginal'.
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scpaul
Well-Known Member
Now all I have to do is load it a "little" hot, 17,513 fps. Just joking but it wouldn't surprise me if some "novice" were to try it. I wonder how big the brass would have to be and how long of a barrel. Haha
RockyMtnMT
Official LRH Sponsor
I know this is technically right on. On the other hand we see long for caliber bullets holding sub half moa but then swapping ends at 1300y. Speed still well away from transonic with stability technically getting higher the farther out the bullet travels but then hitting a point the heavier end of the bullet passes the lighter front end. Which is where it wants to be and we stop it from happening by spinning the bullet. There comes a point where dynamic stability trumps ballistic stability and then it is all thrown out on its ear. It is most definitely a head scratcher.
