Bullet RPM range - worth knowing ?
- Thread starter big5ifty
- Start date
dellet
Gold $$ Contributor
Basically what you’re asking is simply
“Why doesn’t everyone provide the same information that Berger bullets provides”.
The answer is,
that’s part of what sets Berger apart from the crowd.
Berger also shows why simple calculators can get you in trouble. As far as I know Berger still tests all bullets for stability using radar and actual live fire. And they also publish minimum twist rates for all their bullets.
bergerbullets.com
If you use the 30 caliber, 230 Hybrid for an example, some of the problems start to show up. For this particular bullet, Berger recommends a 1/10 twist. If you choose the 230 Hybrid into Berger’s own provided twist calculator, which uses the Miller formula, you get an SG of 1.33 at 2650 fps, a warning that the bullet is marginally stable, and recommends a 1/9.25 twist rate. The calculator also generates a list of bullets more suitable for a 1/10 twist. Berger seems to contradict themselves, or maybe just shows the weakness of the calculator.
bergerbullets.com
You raised the question of slowing the velocity and changing twist rates. This can also be seen easily with the information Berger provides. If you plug in 1000 fps for the same 230 Hybrid, you need an 1/8.5 twist to achieve the same SG of 1.33. In the grand scheme of things, not a lot of change in twist needed, 15% to achieve the same stability as 260% increase in velocity.
The numbers would line up better if you used a drag/twist calculator, but it would require more information than Berger provides, such as base diameter, metplate diameter, and the hardest one to measure, ogive radius. Bullet shape matters
jbmballistics.com
When you compare bullet spin rates, 2650 fps in a 1/10 twist @ 190,000 rpm against 1000 fps in a 1/8 twist @ 90,000 rpm you start to see where relying on spin rate alone to calculate stability falls apart.
I can assure you that a 230 Hybrid is perfectly stable at 1000 fps from a 1/8 twist barrel out to 300 yards from personal experience. And that it also has noticeably less drop at the same distance at the same 1000 fps from a 1/5 twist. The difference is spinning fast enough not to wobble and leave round holes in paper, and fast enough to achieve maximum BC.
On a side note Nosler 110 Varmageddon bullets blow up at only 2200 fps in a 1/5 twist. It would be nice if they included the max spin rate in a hot barrel, damn things can’t handle 350,000 rpm.
Using a different drag calculator you can see twist required and different velocities based on Mach reference points, I had to guess on some of the bullet dimensions, but it will be close enough to get the point across.
“Why doesn’t everyone provide the same information that Berger bullets provides”.
The answer is,
that’s part of what sets Berger apart from the crowd.
Berger also shows why simple calculators can get you in trouble. As far as I know Berger still tests all bullets for stability using radar and actual live fire. And they also publish minimum twist rates for all their bullets.
Bullet Reference Charts
DESCRIPTION LEGEND FB: Flat Base High BC FB: High BC Flat Base BR Column: Bench Rest Flat Base BT: Boat Tail Long Range BT/Juggernaut: Long Range Boat Tail VLD: Very Low Drag Hybrid: Combination Secant and Tangent Ogive LR Hybrid: Long Range Hybrid AR Hybrid/Classic Hunter: Short Nose Hybrid...
bergerbullets.com
If you use the 30 caliber, 230 Hybrid for an example, some of the problems start to show up. For this particular bullet, Berger recommends a 1/10 twist. If you choose the 230 Hybrid into Berger’s own provided twist calculator, which uses the Miller formula, you get an SG of 1.33 at 2650 fps, a warning that the bullet is marginally stable, and recommends a 1/9.25 twist rate. The calculator also generates a list of bullets more suitable for a 1/10 twist. Berger seems to contradict themselves, or maybe just shows the weakness of the calculator.
Twist Rate Stability Calculator
Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.
bergerbullets.com
You raised the question of slowing the velocity and changing twist rates. This can also be seen easily with the information Berger provides. If you plug in 1000 fps for the same 230 Hybrid, you need an 1/8.5 twist to achieve the same SG of 1.33. In the grand scheme of things, not a lot of change in twist needed, 15% to achieve the same stability as 260% increase in velocity.
The numbers would line up better if you used a drag/twist calculator, but it would require more information than Berger provides, such as base diameter, metplate diameter, and the hardest one to measure, ogive radius. Bullet shape matters
JBM - Calculations - Drag/Twist
When you compare bullet spin rates, 2650 fps in a 1/10 twist @ 190,000 rpm against 1000 fps in a 1/8 twist @ 90,000 rpm you start to see where relying on spin rate alone to calculate stability falls apart.
I can assure you that a 230 Hybrid is perfectly stable at 1000 fps from a 1/8 twist barrel out to 300 yards from personal experience. And that it also has noticeably less drop at the same distance at the same 1000 fps from a 1/5 twist. The difference is spinning fast enough not to wobble and leave round holes in paper, and fast enough to achieve maximum BC.
On a side note Nosler 110 Varmageddon bullets blow up at only 2200 fps in a 1/5 twist. It would be nice if they included the max spin rate in a hot barrel, damn things can’t handle 350,000 rpm.
Using a different drag calculator you can see twist required and different velocities based on Mach reference points, I had to guess on some of the bullet dimensions, but it will be close enough to get the point across.
Last edited:
Orr89rocz
Silver $$ Contributor
Interesting but doesnt address the question
Rank Amateur
Gold $$ Contributor
Obviously, quality bulletmakers and NASA engineers such as Damon and Randy know what they are talking about (sorry, I just don't know the backgrounds of most of the other folks contributing to this thread, but I have certainly purchased bullets made by both of these gentlemen). Knowing the Sg (or calculating it using the tools on Damon's site or at Berger) seems to be "good enough" to answer the "is it going to be stable" question in most cases (especially if you include some margin for error).
Quoting the first paragraph of Don Miller's 2005 article: "For a bullet to fly point-forward, it must spin fast enough to be stable. For stability, the "gyroscopic stability factor" s, which depends on the spin rate, must be greater than 1.0. Because the spin in flight declines considerably more slowly than the forward velocity, the bullet's stability factor increases as it flies downrange, and it actually becomes more stable. Consequently, the stability factor at the muzzle is the most significant. Since the spin rate at the muzzle depends on rifling twist, twist is an important part of the stability factor."
Is the point that RPM is simply a measurable byproduct of other factors (barrel twist, muzzle velocity), but it's calibers per turn (without consideration of time) that actually determines stability? If so, is that true only when a specific set of conditions are true? I think I'm tracking as long as the bullet is traveling point on, there is SOME velocity, there is minimal yaw, etc? So, as long as the bullet is launched point on, it's stability (perhaps defined as its tendency to maintain that point-on attitude?) once it leaves the barrel is most dependent upon it possessing a minimum specific spin/twist described by calibers per turn?
Is all this confusion (including mine) a result of a hidden conversion in that we all speak of barrel twist in terms of inches per twist, while the calculation of gyroscopic stability uses a twist rate with units of calibers per turn?
The bullet's velocity (and therefore RPM) IS relevant to stability, but that factor is contained in the value of the overturning moment. Miller's formula uses substitutes, based on bullet length, for a directly measured overturning moment to calculate a stability factor. So, RPM is just as valid as barrel twist in discussing bullet stability, but ONLY in that both of those can be used to back-calculate to the constants/variables used to calculate gyroscopic stability. Gentlemen, am I starting to understand this? Thanks!!!
Quoting the first paragraph of Don Miller's 2005 article: "For a bullet to fly point-forward, it must spin fast enough to be stable. For stability, the "gyroscopic stability factor" s, which depends on the spin rate, must be greater than 1.0. Because the spin in flight declines considerably more slowly than the forward velocity, the bullet's stability factor increases as it flies downrange, and it actually becomes more stable. Consequently, the stability factor at the muzzle is the most significant. Since the spin rate at the muzzle depends on rifling twist, twist is an important part of the stability factor."
Is the point that RPM is simply a measurable byproduct of other factors (barrel twist, muzzle velocity), but it's calibers per turn (without consideration of time) that actually determines stability? If so, is that true only when a specific set of conditions are true? I think I'm tracking as long as the bullet is traveling point on, there is SOME velocity, there is minimal yaw, etc? So, as long as the bullet is launched point on, it's stability (perhaps defined as its tendency to maintain that point-on attitude?) once it leaves the barrel is most dependent upon it possessing a minimum specific spin/twist described by calibers per turn?
Is all this confusion (including mine) a result of a hidden conversion in that we all speak of barrel twist in terms of inches per twist, while the calculation of gyroscopic stability uses a twist rate with units of calibers per turn?
The bullet's velocity (and therefore RPM) IS relevant to stability, but that factor is contained in the value of the overturning moment. Miller's formula uses substitutes, based on bullet length, for a directly measured overturning moment to calculate a stability factor. So, RPM is just as valid as barrel twist in discussing bullet stability, but ONLY in that both of those can be used to back-calculate to the constants/variables used to calculate gyroscopic stability. Gentlemen, am I starting to understand this? Thanks!!!
urbanrifleman
Site $$ Sponsor
Bullets don't shoot well when they're trying to fly apart. Even if they make it to the target without exploding at terminal RPMs they begin to deform, and take good bullets and turn them into bad bullets.
I only experienced problems with too much RPM with Berger 80 gr Varmint in a 243win with a 8" twist.
Problem was those bullets have very thin jackets and in the summer heat going through a hot barrel some go POOF in a cloud of lead dust.
In the winter sub freezing temps they shot awesome and never came apart due to the barrel not getting too hot.
In the same barrel I've shot lighter bullets in the summer that never came apart, but accuracy was not that impressive.
So I only shoot the Berg's 80 gr Varmint bullets in freezing temps. Otherwise I stick to 105 / 107 gr bullets.
Problem was those bullets have very thin jackets and in the summer heat going through a hot barrel some go POOF in a cloud of lead dust.
In the winter sub freezing temps they shot awesome and never came apart due to the barrel not getting too hot.
In the same barrel I've shot lighter bullets in the summer that never came apart, but accuracy was not that impressive.
So I only shoot the Berg's 80 gr Varmint bullets in freezing temps. Otherwise I stick to 105 / 107 gr bullets.
Simplify it. If your shooting a 6BR like I am for varmint hunting. What twist rate do they use at high level matches for a 65 gr bullet. What twist do the best use for 105-108 gr bullets. Problem solved. Same logic for 7mm or 30 cal. No physics PHD work needed.
Ned Ludd
Silver $$ Contributor
In fairness to Berger, I wouldn't necessarily call this a problem, or even contradictory. As shown below, Berger recommends a minimum 10-twist or faster for the 230 Hybrid, which is not totally out of line with the Twist Rate Calculator. If I were to call them out on anything, it would be the suggestion by the calculator that an Sg of 1.33 denotes "marginal" stability. A bullet flying with an Sg of 1.33 is plenty stable, it just might not attain quite the full intrinsic BC of the bullet....If you use the 30 caliber, 230 Hybrid for an example, some of the problems start to show up. For this particular bullet, Berger recommends a 1/10 twist. If you choose the 230 Hybrid into Berger’s own provided twist calculator, which uses the Miller formula, you get an SG of 1.33 at 2650 fps, a warning that the bullet is marginally stable, and recommends a 1/9.25 twist rate. The calculator also generates a list of bullets more suitable for a 1/10 twist. Berger seems to contradict themselves, or maybe just shows the weakness of the calculator.
...Twist Rate Stability Calculator
Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.
dellet
Gold $$ Contributor
I agree. It’s a small nit to pick.
What I feel is confusing, to someone with no experience using the calculator, is not only that the numbers don’t match, but if you read the instructions for using the calculator, it reinforces the need to be 1.5 or greater, “to optimize BC”.
I get into this argument all the time and it goes something like this
How long is that bullet you’re shooting and what twist?
1.5” in a 1/10. Works fine.
But it’s not stable.
It’s perfectly stable.
But the calculator says it’s SG is 1.4.
I know.
So it’s not stable.
It’s perfectly stable, just has more drop.
But the calculator instructions, and everybody knows it’s got to be at least 1.5, and 1.6-1.9 is even better.
That’s for maximum BC
But it has to have the best BC possible or you’ll never get a good group.
I only shoot this combination 200 yards.
But 1.4 isn’t stable.
See you next month.
The line between what will shoot round holes, and what will have less drop at whatever distance is lost in the translation some how. It’s labeled a “Stability calculator” not “optimized BC calculator”. People plug in the numbers and ignore probably 25% of potentially great bullets, simply because they “are not or are marginally stable”.
Better yet is the subsonic crowd, “1.4, you’ll wipe out your baffles with that bullet”.
Berger kind of addresses this, if you read all the instructions. Problem is most people don’t read the instructions.
Yeah, it’s a pet peeve of mine.
Ned Ludd
Silver $$ Contributor
I'm right there with you. I remember when Bryan Litz used to advocate that an Sg of 1.4 was sufficient to achieve the full intrinsic BC of the bullet. Then he changed it to 1.5 and suddenly all my bullets started falling out of the sky. LOL
I have a 7mm08 with a 9.5" twist and Berger tells me I need a 9" twist to stabilize their 180 VLD.
Yet at 2,620 fps they flew just fine and the dope was pretty close compared to their listed BC ( environmental conditions matter more than 5 % of BC loss, so that's why we WRITE DOWN every load at every condition ).
At the same time I had 140 gr Remington Accutip V ( factory ammo ) come apart every shot.
Going by written down Data On Previous Engagement ( D.O.P.E. ) is still a reliable way to make first round cold bore impacts on small targets as long as the conditions and elevation/DA is the same.
We still use the Applied Ballistics app.
The AB app does gives us better results when shooting cold bore in atmospheric conditions we never shot at. We try to combine the 2 as often as possible to true the AB data.
There is a certain warm feeling we get when we hit a hanging (with fishing line ) golf ball at 400 yards with the first shot of the day... repeatedly on different days regardless weather it's freezing or hot as hell outside.
Yet at 2,620 fps they flew just fine and the dope was pretty close compared to their listed BC ( environmental conditions matter more than 5 % of BC loss, so that's why we WRITE DOWN every load at every condition ).
At the same time I had 140 gr Remington Accutip V ( factory ammo ) come apart every shot.
Going by written down Data On Previous Engagement ( D.O.P.E. ) is still a reliable way to make first round cold bore impacts on small targets as long as the conditions and elevation/DA is the same.
We still use the Applied Ballistics app.
The AB app does gives us better results when shooting cold bore in atmospheric conditions we never shot at. We try to combine the 2 as often as possible to true the AB data.
There is a certain warm feeling we get when we hit a hanging (with fishing line ) golf ball at 400 yards with the first shot of the day... repeatedly on different days regardless weather it's freezing or hot as hell outside.
It's a 30cal barrel, could be anything ordered, as the need is tied to the chosen bullet (not the cartridge).
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