What is the lowest velocity for 6mm 95 gr VLD from 10" twist?
- Thread starter Clark
- Start date
In theory you could start loading down slowly, firing them at 50 yards to see at what point they start to keyhole, then you could punch your normal load into your ballistics program and see at what yardage they are at that keyhole speed and you'll know your max range.
For example, if the keyhole speed is 2400 FPS and you're running it at 3100 FPS muzzle it's at 2417 FPS at 400 yards.
This is all in theory though, and I think it's probably flawed, as the FPS slows down after firing it's hard to say if the bullet RPM is slowing down at the same rate, faster or slower or maybe not at all.
I shoot 75 VLDs and Amaxes out of my 9 twist AR15, the VLDs keyhole under 2650 FPS muzzle, using the previous mentioned theory I should start keyholing at around 150 yards but it holds sub-moa great to 600 yards suggesting that bullet RPM doesn't slow at nearly the same rate as velocity. The only thing creating resistance to spin once in flight are the grooves put in the bullet by the rifling but the airflow over that same rifling acts as a weather vain of sorts I assume so the effect is reduced.
I think shooting your particular load at distance is the only real way to find the answer to your query.
Wayne
For example, if the keyhole speed is 2400 FPS and you're running it at 3100 FPS muzzle it's at 2417 FPS at 400 yards.
This is all in theory though, and I think it's probably flawed, as the FPS slows down after firing it's hard to say if the bullet RPM is slowing down at the same rate, faster or slower or maybe not at all.
I shoot 75 VLDs and Amaxes out of my 9 twist AR15, the VLDs keyhole under 2650 FPS muzzle, using the previous mentioned theory I should start keyholing at around 150 yards but it holds sub-moa great to 600 yards suggesting that bullet RPM doesn't slow at nearly the same rate as velocity. The only thing creating resistance to spin once in flight are the grooves put in the bullet by the rifling but the airflow over that same rifling acts as a weather vain of sorts I assume so the effect is reduced.
I think shooting your particular load at distance is the only real way to find the answer to your query.
Wayne
mikecr said:
Mike,
The statement above is not strictly true. If the bullet were grossly unstable, it would show it immediately and tumble, as you said. If the bullet were marginally unstable, it would enter a period of increasing precession after muzzle exit, eventually becoming sufficiently unstable to tumble. This is why a bullet can show a reasonable group at 100 yards and a random pattern of oblong holes at 300. Most long-range bullets are not fully gyroscopically stable upon exit. They enter a period of diminishing precession, eventually becoming fully stable. The attachment is a classic example from Hatcher's Notebook. Hatcher test fired a Springfield rifle into oak planks at 50 feet and 200 yards. The penetration was much less at 50 feet because the bullet was still in precession and yawed, limiting penetration.
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Dynamic stability is another matter, as is terminal performance. Neither can be predicted, and must be tested for.
But a bullet's either gyroscopically stable, or it isn't.
And while a bullet can be considered 'marginally' stable or marginally unstable, it is still stable or unstable respectively.
There is an absolute line here, because a gyroscopically unstable bullet always tumbles immediately.
Flipside, if it makes it 50yds without tumbling, it will remain gyroscopically stable as Sg only goes up from there. Ever increasing, all the way to transonic(target).
You cannot punch downrange velocity into the 'muzzle velocity' field of a stability program and arrive at the correct result -without adjusting for the downrange 'effective' twist rate..
For example, the 95VLD in 10tw launched at 2Kfps may have an Sg of 1.0x, and depending on the quality of it's release, this might be enough to keep it from tumbling. If it's Sg here is 0.9x it WILL tumble, immediately on release.
Let's say it doesn't tumble..
By 100yds, it's Sg has already climbed to 1.2x because of all the velocity loss with very little RPM loss. At 1Kyds, Sg has reached well over 5.x, with an effective twist rate of 4.x inches per turn.
So if a bullet is tumbling down range, it's not due to it's gyroscopic stability.
And the 95VLD is not known to have dynamic stability issues.
But a bullet's either gyroscopically stable, or it isn't.
And while a bullet can be considered 'marginally' stable or marginally unstable, it is still stable or unstable respectively.
There is an absolute line here, because a gyroscopically unstable bullet always tumbles immediately.
Flipside, if it makes it 50yds without tumbling, it will remain gyroscopically stable as Sg only goes up from there. Ever increasing, all the way to transonic(target).
You cannot punch downrange velocity into the 'muzzle velocity' field of a stability program and arrive at the correct result -without adjusting for the downrange 'effective' twist rate..
For example, the 95VLD in 10tw launched at 2Kfps may have an Sg of 1.0x, and depending on the quality of it's release, this might be enough to keep it from tumbling. If it's Sg here is 0.9x it WILL tumble, immediately on release.
Let's say it doesn't tumble..
By 100yds, it's Sg has already climbed to 1.2x because of all the velocity loss with very little RPM loss. At 1Kyds, Sg has reached well over 5.x, with an effective twist rate of 4.x inches per turn.
So if a bullet is tumbling down range, it's not due to it's gyroscopic stability.
And the 95VLD is not known to have dynamic stability issues.
mikecr said:
Almost everything you stated is incorrect, but the statements above are particularly egregious. No matter, believe what you like. I'm going to the range to shoot some marginally stable bullets.
All bullets precess, to a greater or lesser extent, when leaving the muzzle. Some may eventually become fully stable, some may not. If you carefully read chapter 10 of Bryan Litz's "Applied Ballistics For Long Range Shooting", the controlling factors are clearly explained.
For one, neither I, nor thread starter, were ever talking about marginal stability. It's not even part of the subject -yet. But in my defense:
"If the rigidity of the spinning mass is greater than the overturning aerodynamic torque, then Sg will be greater than 1.0, and the bullet will be gyroscopiclly stable. However, if overturning aerodynamic torque is stronger than the rigidity of overturning mass, then Sg will be less than 1.0 and the bullet will not be stable."
Also "If the spin is fast enough, the bullet is said to have adequate gyroscopic stability. If the spin is not fast enough, the bullet will lose it's orientation and tumble. Both Applied Ballistics, Ch10, pg 148.
And further: "Technically speaking Sg only has to be greater than 1.0, so why aim for a higher value?" "If you aim for an Sg of 1.0 and conditions exist to drop it to 0.99, you have a problem" pg. 151
This is the 'line' I'm referring to, while putting aside lesser conditions in external ballistics(like bullets wobbling until dampened out by gyroscopic stability). Yes, "adequate" stability is greater than marginal, but both ARE stable and that relates to my point.. If YOUR point relates to the fact that a 95VLD from 10tw is marginally stable, and less than desirable, then I'm in total agreement. But it doesn't really address the misunderstanding thread starter has about stability. Well, it could have, had he not mentioned the 'known' condition of 75 V-Maxs going unstable at some downrange velocity. Doesn't work that way...
And even the 95VLD in 10tw would not likely go 'unstable' from any cartridge that propels it.
It'll make it 50yds, and so beyond(just won't perform well).
And last in my defense(since I've just read it thank you):
"As a bullet flies downrange, it's forward velocity declines much faster than it's rotational velocity. This means the aerodynamic overturning torque diminishes at a faster rate than the rigidity of spinning mass, so gyroscopic stability actually increases over it's flight. A bullet that was launched with an Sg of 1.5 at the muzzle can have it's Sg grow to 4.0, or 5.0, or more by the time it reaches the target" pg. 150
This supports my notions well enough.
"If the rigidity of the spinning mass is greater than the overturning aerodynamic torque, then Sg will be greater than 1.0, and the bullet will be gyroscopiclly stable. However, if overturning aerodynamic torque is stronger than the rigidity of overturning mass, then Sg will be less than 1.0 and the bullet will not be stable."
Also "If the spin is fast enough, the bullet is said to have adequate gyroscopic stability. If the spin is not fast enough, the bullet will lose it's orientation and tumble. Both Applied Ballistics, Ch10, pg 148.
And further: "Technically speaking Sg only has to be greater than 1.0, so why aim for a higher value?" "If you aim for an Sg of 1.0 and conditions exist to drop it to 0.99, you have a problem" pg. 151
This is the 'line' I'm referring to, while putting aside lesser conditions in external ballistics(like bullets wobbling until dampened out by gyroscopic stability). Yes, "adequate" stability is greater than marginal, but both ARE stable and that relates to my point.. If YOUR point relates to the fact that a 95VLD from 10tw is marginally stable, and less than desirable, then I'm in total agreement. But it doesn't really address the misunderstanding thread starter has about stability. Well, it could have, had he not mentioned the 'known' condition of 75 V-Maxs going unstable at some downrange velocity. Doesn't work that way...
And even the 95VLD in 10tw would not likely go 'unstable' from any cartridge that propels it.
It'll make it 50yds, and so beyond(just won't perform well).
And last in my defense(since I've just read it thank you):
"As a bullet flies downrange, it's forward velocity declines much faster than it's rotational velocity. This means the aerodynamic overturning torque diminishes at a faster rate than the rigidity of spinning mass, so gyroscopic stability actually increases over it's flight. A bullet that was launched with an Sg of 1.5 at the muzzle can have it's Sg grow to 4.0, or 5.0, or more by the time it reaches the target" pg. 150
This supports my notions well enough.
OK, I used that web page
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
I put a 1 in 14" take off benchrest barrel on a Sav99, reamed to 6mmBrR, and it shot 0.1" at 50y and keyholed at 100y with wimpy 75 gr loads.
if 75 gr Vmax 14" twist 3050 and 3100 fps some stable at 50 and 100 some only at 50y.
bullet length .935"
3050 muzzle
2900 50y stable a .786
2750 100y not stable at .772
The 1.15" long 95 gr bullet 10" twist to get .786 stability, needs at least 1150 fps
If I can get the VLD going 3250 fps, with a 10" twist it should be stable out to 1450 yards.
But that would be if thing scaled.
Maybe 1450 yards is very different than 50 yards for making bad things happen.
I have never killed a deer past 510 yards, but I would like to have 600 yard capability.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
I put a 1 in 14" take off benchrest barrel on a Sav99, reamed to 6mmBrR, and it shot 0.1" at 50y and keyholed at 100y with wimpy 75 gr loads.
if 75 gr Vmax 14" twist 3050 and 3100 fps some stable at 50 and 100 some only at 50y.
bullet length .935"
3050 muzzle
2900 50y stable a .786
2750 100y not stable at .772
The 1.15" long 95 gr bullet 10" twist to get .786 stability, needs at least 1150 fps
If I can get the VLD going 3250 fps, with a 10" twist it should be stable out to 1450 yards.
But that would be if thing scaled.
Maybe 1450 yards is very different than 50 yards for making bad things happen.
I have never killed a deer past 510 yards, but I would like to have 600 yard capability.
In his book, (Ch. 10), Bryan Litz is very specific that if Sg is below 1.4 (not 1.0) the bullet has marginal gyroscopic stability and accuracy will suffer. In fact, it will deteriorate with distance. I ran a long series of tests on this, using 30 caliber bullets up to 200 grains in a 1:13" twist, you can see those results here:
http://riflemansjournal.blogspot.com/2010/06/ballistics-heavy-bullets-in-113-twist.html
Even though the heavy bullets shot fabulously at 100 yd., by 500 yd. they were opening up. That is the effect of marginal gyroscopic stability.
The program in Litz's book is excellent for these calculations and if you don't have it, we host an online version of it (with Bryan's permission) at www.desertsharpshooters.com
http://riflemansjournal.blogspot.com/2010/06/ballistics-heavy-bullets-in-113-twist.html
Even though the heavy bullets shot fabulously at 100 yd., by 500 yd. they were opening up. That is the effect of marginal gyroscopic stability.
The program in Litz's book is excellent for these calculations and if you don't have it, we host an online version of it (with Bryan's permission) at www.desertsharpshooters.com
I agree that it's better to be adequately stabilized for accuracy's sake. But this varies with the shooting system(it's not a line like Sg1.0). In other words, whats 'adequate' for Joe, may not be for Jim.
Bryan recommends 1.4 or greater for safety margin, and this is nothing new. But In your article you wrote: "unlike the bullet with adequate gyroscopic stability (Sg at least 1.4) which gets more stable, the marginally stabilized bullet will become less stable as it travels downrange"
I doubt Bryan said this...
And it's not necessarily true (actually, it's unusual).
Also in your article: "The other element of ballistic stability, known as dynamic stability, deals principally with the effect on a bullet of going into the transonic/subsonic region"
This is not true either..
A bullet can be dynamically unstable from the moment it leaves the muzzle. It can have dynamic stability issues at any point in it's flight path. This is not gyroscopic stability.
The only bullet I know of like this is the 168smk. But maybe you're showing a little with the 200smk..
What Bryan did imply was that marginally stabilized bullets may take longer to damp initial yaw & fly true, and so their effective BC can suffer. Of course, overall accuracy could suffer as well, but maybe it just didn't seem to show in your 100yd shooting..
Maybe your shooting at 500yds the next day, didn't show it either.. Maybe a 200smk just isn't accurate from that barrel.
Your tests didn't necessarily show a 190 shooting better than a 200 because of stability. It showed less accuracy from completely different bullets/loads at BOTH ranges(the 190 bad at 100yd, and the 200 bad at 500yd).
It is quite likely that there are 13tw barrels out there that could release a 200smk so cleanly that neither stability, nor accuracy would be an issue at Sg 1.33. Or, maybe what your test was indicating, was a dynamic stability issue, instead of gyroscopic stability issues. Did you shoot the 185 Berger(the other Sg 1.33 bullet) at 500yds? I didn't see that to validate the notion that things fell apart for bullets at Sg 1.33, while being ok for Sg 1.39.
I'm sceptical of the article's implications because I shoot a gun that is very accurate with a running Sg ~1.3, and I don't see accuracy drop off much at 500yds. I certainly wouldn't change anything based on it's stability.
Now I'll concede that a boatail in the same weight did not shoot as well from the same gun (it's Sg ~1.1). But there again, it's accuracy did not get appreciably worse with distance. It just shot a fraction worse all the way around.
So I'm sure none of my bullets have dynamic stability issues, and that marginal Sg (by your standard), is adequate -for me.
Anyway, thread starters example is well outside any of this discussion.
Bryan recommends 1.4 or greater for safety margin, and this is nothing new. But In your article you wrote: "unlike the bullet with adequate gyroscopic stability (Sg at least 1.4) which gets more stable, the marginally stabilized bullet will become less stable as it travels downrange"
I doubt Bryan said this...
And it's not necessarily true (actually, it's unusual).
Also in your article: "The other element of ballistic stability, known as dynamic stability, deals principally with the effect on a bullet of going into the transonic/subsonic region"
This is not true either..
A bullet can be dynamically unstable from the moment it leaves the muzzle. It can have dynamic stability issues at any point in it's flight path. This is not gyroscopic stability.
The only bullet I know of like this is the 168smk. But maybe you're showing a little with the 200smk..
What Bryan did imply was that marginally stabilized bullets may take longer to damp initial yaw & fly true, and so their effective BC can suffer. Of course, overall accuracy could suffer as well, but maybe it just didn't seem to show in your 100yd shooting..
Maybe your shooting at 500yds the next day, didn't show it either.. Maybe a 200smk just isn't accurate from that barrel.
Your tests didn't necessarily show a 190 shooting better than a 200 because of stability. It showed less accuracy from completely different bullets/loads at BOTH ranges(the 190 bad at 100yd, and the 200 bad at 500yd).
It is quite likely that there are 13tw barrels out there that could release a 200smk so cleanly that neither stability, nor accuracy would be an issue at Sg 1.33. Or, maybe what your test was indicating, was a dynamic stability issue, instead of gyroscopic stability issues. Did you shoot the 185 Berger(the other Sg 1.33 bullet) at 500yds? I didn't see that to validate the notion that things fell apart for bullets at Sg 1.33, while being ok for Sg 1.39.
I'm sceptical of the article's implications because I shoot a gun that is very accurate with a running Sg ~1.3, and I don't see accuracy drop off much at 500yds. I certainly wouldn't change anything based on it's stability.
Now I'll concede that a boatail in the same weight did not shoot as well from the same gun (it's Sg ~1.1). But there again, it's accuracy did not get appreciably worse with distance. It just shot a fraction worse all the way around.
So I'm sure none of my bullets have dynamic stability issues, and that marginal Sg (by your standard), is adequate -for me.
Anyway, thread starters example is well outside any of this discussion.
Why, when it is far simpler and more effective to run a bullet weight/length that will effectively stabilize at a given velocity in a barrel with X amount of twist rate, would you choose to use a bullet of marginal stability? It makes little sense to me and is pointless to argue as it can in no way produce an actual gain in accuracy when compared to a bullet used in an optimal twist rate for the given bullet weight/length and velocity.
I don't care how perfectly a bullet leaves the barrel, if it has marginal stability it will never perform better than a bullet that is launched at an optimal RPM so as to achieve ideal gyroscopic stability.
Anyway, the SG of 1.4 is a GUIDELINE, it in no way states that it's impossible to achieve acceptable results at 1.3. Only that at 1.4 you will most likely have a more stable bullet in most cases. I have found this system to work very well for me, as I have a 6-284 with a 1-10 twist and bullets with an SG of 1.4 or above at the velocity my rifle launches them at are more accurate and consistent than are bullets that have an SG of 1.2.
I don't care how perfectly a bullet leaves the barrel, if it has marginal stability it will never perform better than a bullet that is launched at an optimal RPM so as to achieve ideal gyroscopic stability.
Anyway, the SG of 1.4 is a GUIDELINE, it in no way states that it's impossible to achieve acceptable results at 1.3. Only that at 1.4 you will most likely have a more stable bullet in most cases. I have found this system to work very well for me, as I have a 6-284 with a 1-10 twist and bullets with an SG of 1.4 or above at the velocity my rifle launches them at are more accurate and consistent than are bullets that have an SG of 1.2.
I totally agree with you Kenny.
The thread went from 'when does a bullet go unstable downrange, based on my 75VM results', to the merits of adequate stability.
To connect the two, it's my contention that marginally stable bullets don't normally go unstable downrange as Sg should climb with distance.
The greatest of all disturbances to a bullet(beyond hitting an animal) is muzzle release, and this is right where Sg is at it's lowest. So if a bullet is going to tumble, it will likely happen nearest that event.
And this is just what would happen to a 75VM from a 14tw.
Well, I've tried.
The thread went from 'when does a bullet go unstable downrange, based on my 75VM results', to the merits of adequate stability.
To connect the two, it's my contention that marginally stable bullets don't normally go unstable downrange as Sg should climb with distance.
The greatest of all disturbances to a bullet(beyond hitting an animal) is muzzle release, and this is right where Sg is at it's lowest. So if a bullet is going to tumble, it will likely happen nearest that event.
And this is just what would happen to a 75VM from a 14tw.
Well, I've tried.
I agree that a 14 twist will never stabilize a 75gr V-Max, and I doubt a 10 twist will stabilize the 95gr VLD either.
I am actually quite sure the 95grainer will never stabilize because I tried them in my 10 twist 6-284 and no matter how hard or slow I pushed them, and trust me I was flattening and cratering primers and beating my brass to death in the process, would they perform even decently. I tried fast and slow powders, and all they did was suck up barrel life.
In the 10 twist you need to focus on bullets 1" long or less. Any longer and they just do not shoot as well as the shorter bullets. If you want to shoot heavies, buy an 8 twist.
You will be much better off with a stable bullet than an unstable bullet with a high BC.
I am actually quite sure the 95grainer will never stabilize because I tried them in my 10 twist 6-284 and no matter how hard or slow I pushed them, and trust me I was flattening and cratering primers and beating my brass to death in the process, would they perform even decently. I tried fast and slow powders, and all they did was suck up barrel life.
In the 10 twist you need to focus on bullets 1" long or less. Any longer and they just do not shoot as well as the shorter bullets. If you want to shoot heavies, buy an 8 twist.
You will be much better off with a stable bullet than an unstable bullet with a high BC.
mikecr said:
I checked 4 times in a row with tape on a cleaning rod.
Each time one revolution was exactly 14".
It is a 6mm PPC Shilen 21.5" Used SS Shilen 6PPC benchrest take off barrel. 5.1 pounds
$75 + $12 shipping = $87 off Ebay when the sold barrels
Have put it on Sav 99 and re chambered to 6mmBR
So I know that:
stability = 0.772 makes ~ 1.5 moa groups at 100 yards with 6mm 75 gr Vmax 0.935" long, 14" twist, 2750 fps terminal
stability = 0.786 makes ~ 0.3 moa groups at 50 yards with 6mm 75 gr Vmax 0.935" long, 14" twist 2900 fps terminal
stability = 0.929 makes ~ 0.3 moa groups at 100 yards with 6mm 65 gr Vmax 0.835" long, 14" twist 2792 fps terminal
The question is what will happen with a different barrel?:
7# = 112 ounces 28" Shilen select match 243 one in 10" twist Stainless Steel barrel #7 contour $264 + $4 shipping
http://www.brownells.com/aspx/NS/store/productdetail.aspx?p=12416&st=&s=
Will put it on a Ruger #1 or 98 Mauser in small primer 243 by October.
stability = 1.136, 6mm, 95 gr VLD, 1.150" long, 10" twist, 3450 fps muzzle
stability = 1.121 makes what size group at 50 yards? 95 gr VLD, 1.150" long, 10" twist. 3319 fps terminal?
stability = 1.109 makes what size group at 100 yards? 95 gr VLD, 1.150" long, 10" twist. 3211 fps terminal?
stability = 0.986 makes what size group at 600 yards? 95 gr VLD, 1.150" long, 10" twist. 2261 fps terminal?
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