Do you need (relative) velocity to achieve accuracy with heavy-for-caliber bullets?
I ask this because in answer to some queries posters say “that bullet needs higher velocity to stabilise to be accurate.”
Can one not just use the appropriate twist? For example, could I achieve excellent accuracy with a .338 Federal and 250gr bullets at a very modest velocity (say, 2100fps) as long as I ordered my rifle with the appropriate twist. How low can the velocity go?
This is a hypothetical question and I concede that accuracy may fall off after 150 or so yards.

evry bullet reguardless caliber and/or weight will have it's own "sweetspot" where the relationship between twist and velocity best stabilizes the projectile. that's why we have to try severak powders and charge weights. because the rate of spin is controled by a constant, the rate of twist in the rifling, which remains constant through-out the bullet's flight, the sweetspot is found by varying the velocity. there are rules which guide how much twist will generally work correctly for a given weight of bullet and the velocity you decide to drive it and barrel makers usually offer at least a couple rates of twist to accomidate light or heavy bullets. it is then the game of load devopment that finds the velocity that is the right balance between spin rate and polar moment of the bullet that stabilizes it. one good rule of thumb is to look at what the bullet is or was most popularily loaded in by the factory, it will give you a good idea of the velocity you might need. that's also a good reason to have a chronograph. if you find you're driving a heavy bullet pretty hard and it's not groupng, you probably don't have a fast enough twist for the bullet. a large diameter bullet will have a shorter polar moment and will need less twist than a small diameter heavy bullet with a long polar moment that will need to be spun faster.
there's no definitive answer to your question besides... try it and see. because it involves dynamic couple, 50 or 100 fps might make all the difference in the world.

quote:

For example, could I achieve excellent accuracy with a .338 Federal and 250gr bullets at a very modest velocity (say, 2100fps) as long as I ordered my rifle with the appropriate twist.

yes, you only need to maintain the RPMs of the bullet by using a faster twist if you intend to shoot slower.

However.....you might want to ask yourself if the bullet performance will suffer at the lower velocity.....that's up to you!!

I'd use a 1-9" twist for 250 grain .338 bullets at 2,100 FPS

I've used the Greenhill formula to pick the twist for a couple of new barrel jobs. It worked OK as the rifles I was puting together were rather standard caliber and velocity. It basically confirmed the twist that several manufacturers use.

But...My question is:

What if I were farther "outside the envelop" -

Is there a method to calculate the RPM required to stablize a bullet given the diameter and length for velocities much lower 2,200 or higher 3,200 fps?

For example; subsonic rifle loads or high velocity varmint loads.

This is an excellent question. The answer is complicated but suffice to say that matching twist to bullet length is the key componet, not velocity. However, we don't bullets on length, i.e. I'm going to the store to buy some 2-inch .338 bullets. We buy bullets on weight which is the practical approach. Accuracy starts with case prep and bullet twist match. I've heard of accuracy improving at longer distance when the bullet goes to "sleep" but that is a bit different discussion.

If you are looking at a new custom barrel - the barrel manufacture should be able to answer your question.

quote:

Originally posted by Karoo:
Do you need (relative) velocity to achieve accuracy with heavy-for-caliber bullets?
I ask this because in answer to some queries posters say “that bullet needs higher velocity to stabilise to be accurate.”
Can one not just use the appropriate twist? For example, could I achieve excellent accuracy with a .338 Federal and 250gr bullets at a very modest velocity (say, 2100fps) as long as I ordered my rifle with the appropriate twist. How low can the velocity go?
This is a hypothetical question and I concede that accuracy may fall off after 150 or so yards.

The rate of twist is more significant than the MV in achieving bullet stability. And twist is matched to bullet LENGTH, not weight. Use the Greenhill formula to calculate required twist, and note that too fast a twist is much better than too slow a twist.

quote:

Originally posted by El Deguello:

quote:

Originally posted by Karoo:
Do you need (relative) velocity to achieve accuracy with heavy-for-caliber bullets?
I ask this because in answer to some queries posters say “that bullet needs higher velocity to stabilise to be accurate.”
Can one not just use the appropriate twist? For example, could I achieve excellent accuracy with a .338 Federal and 250gr bullets at a very modest velocity (say, 2100fps) as long as I ordered my rifle with the appropriate twist. How low can the velocity go?
This is a hypothetical question and I concede that accuracy may fall off after 150 or so yards.

The rate of twist is more significant than the MV in achieving bullet stability. And twist is matched to bullet LENGTH, not weight. Use the Greenhill formula to calculate required twist, and note that too fast a twist is much better than too slow a twist.

Proper twist rate is the key at the desired velocity. The 300 Whisper is very accurate out to even 1,000 yards with the 240 grain SMK (The bullet the Whisper was designed around) even at the sedate 1100 FPS muzzle velocity

quote:

Originally posted by jwp475:

Proper twist rate is the key at the desired velocity. The 300 Whisper is very accurate out to even 1,000 yards with the 240 grain SMK (The bullet the Whisper was designed around) even at the sedate 1100 FPS muzzle velocity

That's what I'm talking about!

So what twist rate does the 300 Whis use for 240 SMKs at 1,100 fps?

1 in 8 twist with 240 SMKs is the twist used by SSK Industries

Thanks for the technical explanations. I am not planning a project just yet, but the question has intrigued me for a while.

quote:

Originally posted by jwp475:
1 in 8 twist with 240 SMKs is the twist used by SSK Industries

Thank you jwp.



But...That just raises more questions!

If I run a .308/240 SMK at a bullet length of 1.7" thru the Greenhill formula it predicts you would need a twist of 1:8.4" to just stablize that bullet at or below 2,800 fps velocity. This seems in good agreement with SSK manufacturer's barrel spec!

However, there's a big difference in bullet RPMs from a 1:8 barrel twist at difference velocities! For example:

• at 2,800 fps the RPMs at muzzle would be 252,000
  
• at 2,200 fps the RPMs at muzzle drop to 198,000
  
• but at 1,100 fps the RPMs at muzzle be down to 99,000
  

That sub-sonic load has 2.5 times less spin than the same bullet shot at 2,800 fps. How can the RPMs be this different and both be stable?

Basically, the faster the bullet, the more stabilizing it needs. Even so, sometimes enough velocity is required to stabilise some bullets sometimes. I managed to stabilise a 55gr boat tail bullet in my 1 in 16 twist hornet by driving it faster. Never that accurate but nose first on the target as opposed to sideways!

quote:

Originally posted by 303Guy:
Basically, the faster the bullet, the more stabilizing it needs. .....

But which is the chicken and which is the egg?

quote:

Originally posted by RaySendero:

quote:

Originally posted by 303Guy:
Basically, the faster the bullet, the more stabilizing it needs. .....

But which is the chicken and which is the egg?

The higher the velocity the less twist that is required to stabilize a bullet. The slower the velocity the faster the twist must be.

The Greenhill Formular is a good rule of thumb but is by no means absolute.

quote:

The higher the velocity the less twist that is required to stabilize a bullet. The slower the velocity the faster the twist must be.

I came across something somewhere which said not quite. It was something along the lines that more twist was needed over a certain velocity range but I do not remember the details. I shall have to look for it!

Here's a quote from The Los Angeles Silhouette Club http://www.lasc.us/Brennan_2-3_Twist.htm

quote:

Over stabilization

Let's take another Greenhill example: a .224" diameter bullet .84" long will be stabilized by a barrel with a 9" twist. Many 22 caliber bullets are shorter than .84". How does this fast twist affect them? Would a much shorter bullet be "over stabilized"? Some shooters say so. My experience, mostly with 30 caliber rifles, is that nothing VERY bad happens when I shoot short bullets in a 10" twist .308" barrel or a 9" twist .224" barrel. Perhaps the better shooters with better equipment can see the difference; I haven't yet.

They also give tables of twist and max bullet length and twist and max bullet weight. Funny thing - the max bullet weight for my 1-in-16 twist hornet is 44grs (Greenhill formula). I shoot 60gr spire points! This could be due to the small volume of the muzzle blast.

Posted by Brian Litz on Snipers Hide

quote:

Actually the window of proper stability is quite large. On the low end, you have to use at least the minimum recommended twist. On the high end, well, most of the adverse affects cited for over stability are myths. The most adverse effect of very high stability levels is extra spin drift. Other than that, elevation is not affected and BC is not affected.
I think most of the myths about over stability stem from imperfect understanding of the sights including: imperfect zero, click adjustments not being true, canted reticle, etc.
I occasionally see cases of extremely high (and low) BC's being reported by those conducting tests. You'll find that in most of those cases, the non-physical results are caused by oversights in the test.
It's a comfort to know that most of the smoke and mirrors surrounding BC is just that; smoke and mirrors. At the end of the day, actual BC's are very well behaved and constant from all rifles across a large range of stability levels. If two bullets have the same shape and one is heavier, the heavier one has a higher BC. If two bullets are the same weight, and one is more streamlined, the more streamlined one has a higher BC. This is common sense but many 'advertised' numbers contradict these basic trends and it causes unnecessary confusion.

Take care and good shooting,
-Bryan
_________________________
Ballistician
Berger Bullets

http://www.snipershide.com/for...=1012389#Post1012389


I agree totally, as long as the jacket is not comprimised by the additional twist then "overstabization" is not a problem

it's fairly simple...
for any given caliber/cartridge combination, as the weight of the bullet goes up, it's velocity goes down and it's length gets longer, therefore it needs a faster twist to stabilize, the reverse is also true.

quote:

Originally posted by jwp475:
Posted by Brian Litz on Snipers Hide

quote:

.....
Take care and good shooting,
-Bryan
_________________________
Ballistician
Berger Bullets

http://www.snipershide.com/for...=1012389#Post1012389


I agree totally, as long as the jacket is not comprimised by the additional twist then "overstabization" is not a problem

jwp,

I've read where some guys specificly get barrels with faster than needed twist to "over stablize" DG solids so that they stay extra stable and penetrate straighter after impact.

quote:

I've read where some guys specificly get barrels with faster than needed twist to "over stablize" DG solids so that they stay extra stable and penetrate straighter after impact.

________
Ray

I have read that as well, can't hurt IMHO

quote:

"over stablize"

OK - just for fun. An overstabilized bullet is one that will retain its original axis orientation. Hopeless for long range!

quote:

Originally posted by 303Guy:
[QUOTE]"over stablize"

OK - just for fun. An overstabilized bullet is one that will retain its original axis orientation. Hopeless for long range![/QUOTE]


Did you read this?


Posted by Brian Litz on Snipers Hide

quote:

Actually the window of proper stability is quite large. On the low end, you have to use at least the minimum recommended twist. On the high end, well, most of the adverse affects cited for over stability are myths. The most adverse effect of very high stability levels is extra spin drift. Other than that, elevation is not affected and BC is not affected.
I think most of the myths about over stability stem from imperfect understanding of the sights including: imperfect zero, click adjustments not being true, canted reticle, etc.
I occasionally see cases of extremely high (and low) BC's being reported by those conducting tests. You'll find that in most of those cases, the non-physical results are caused by oversights in the test.
It's a comfort to know that most of the smoke and mirrors surrounding BC is just that; smoke and mirrors. At the end of the day, actual BC's are very well behaved and constant from all rifles across a large range of stability levels. If two bullets have the same shape and one is heavier, the heavier one has a higher BC. If two bullets are the same weight, and one is more streamlined, the more streamlined one has a higher BC. This is common sense but many 'advertised' numbers contradict these basic trends and it causes unnecessary confusion.

Take care and good shooting,
-Bryan
_________________________
Ballistician
Berger Bullets

quote:

Did you read this?

Yes I did. I think what folks mean when they speak of 'over stabilitazion' is simply more stabilization than needed, often to the detriment of accuracy, due mostly to bullet deformation and disintegration. I wonder how often an 'overspun' bullet is actually being fired in a rifle designed for longer, heavier bullets and is thus entering the bore skew due to incorrect throat design.

/

quote:

This is valid for flat fire trajectories and trajectories with departure angles less than 70 degrees.

If I may ask, what is meant by 'flat tyre trajectories'? Does 'departure angle less than 70 degrees' mean that the projectile will only remain at departure angle above that? That is surprizingly steep! (To me). Thanks!

OK, this is interesting.

Hypothetic question:

Lets say you have a .224cal, 60gr V-Max fired from a 1-14" 222rem barrel at 2950-3000fps which fails to stabilise - insufficent twist at that velocity.

Can that same bullet be stabilised fired by a 22/250 at (for example) 3450 fps through a similar 1-14 barrel??

/

quote:

Can that same bullet be stabilised fired by a 22/250 at (for example) 3450 fps through a similar 1-14 barrel??

Yes....this is possible!....and the reason is the increased velocity will also increase the RPMs of the bullet!

quote:

Originally posted by vapodog:

quote:

Can that same bullet be stabilised fired by a 22/250 at (for example) 3450 fps through a similar 1-14 barrel??

Yes....this is possible!....and the reason is the increased velocity will also increase the RPMs of the bullet!

Thanks, that was my understanding: given twist x higher velocity = more RPm's = stabilisation.

quote:

Yes....this is possible!....and the reason is the increased velocity will also increase the RPMs of the bullet!

Not according to ALF's explanation. Well, it is possible, but my understanding is that it's because the required spin rate is a little lower at higher velocity for some bullets. I've been trying to find the source of that information. I suspect it is due not to some irregularities in physics but rather to the way the bullet is destabilized at the muzzle!

quote:

Originally posted by 303Guy:

quote:

Yes....this is possible!....and the reason is the increased velocity will also increase the RPMs of the bullet!

Not according to ALF's explanation. Well, it is possible, but my understanding is that it's because the required spin rate is a little lower at higher velocity for some bullets. I've been trying to find the source of that information. I suspect it is due not to some irregularities in physics but rather to the way the bullet is destabilized at the muzzle!

The increased muzzle velocity does indeed increase the rotational velocity

quote:

The increased muzzle velocity does indeed increase the rotational velocity

Indeed it does! I was meaning the 'rate' of spin does not change but that the 'required' rate of spin might change. Putting it simply, the faster the bullet exits the muzzle, the faster the spin needs to be to stabilize it.

From what I have been able to find out, overstabilization is a very real possibility - no myth at all! Bullets that do not nose down sufficiently at long range are overstabilized.

quote:

Not according to ALF's explanation

If you can understand ALF then you're a far better man than I am.....much of what he posts is pure unintelligible!

quote:

Putting it simply, the faster the bullet exits the muzzle, the faster the spin needs to be to stabilize it.

This is not true.....

/

Ever spin a top? What happens when it slows down?

Same principle. It wobbles and yaw is increased.

Faster MV increases spin by speed alone in same twist gun. Same bullet, less speed at muzzle, more twist needed. You need a sweet spot in RPM range at the muzzle.

I ALWAYS, repeat ALWAYS rather a twist some say is OVERLY fast, as you might lose a tad Primo target accuracy but I am a hunter first, I want to know I can shoot any and all bullet wts. for a caliber and I want it nose forward during flight, and after impact.

Some accuracy buffs slow them down to bare minimal RPM window for gilt edged accuracy. No problem for paper punching and competition, when you know what bullet wt. you are willing to stick to, but I have had many more problems with 'marginal twist rate' rifles than the other way around.

Just my experience and thoughts, worth .02

quote:

This is not true.....

May I ask what makes you say that? I can think of one or two instances that supports that but I'm not sure to what extent. There is likely to be a good reason why we seem to be seeing things differently. It could be the effect of bullet design, meaning that maybe a simple lead round nose bullet follows a straightforward linear stability curve while a spitzer or boat tail follows an exponential curve due its shape and centre of mass position. I could test that proposition simply enough with cast bullets in my hornet. Not saying I will...

quote:

Faster MV increases spin by speed alone in same twist gun. Same bullet, less speed at muzzle, more twist needed. You need a sweet spot in RPM range at the muzzle.

This is true and is actually in agreement with my post. But that may be due 'other' factors that aggrevate the stability issue. What Alf says is that the spin requirement is proportional to the speed of the bullet. That does not take into account adverse muzzle conditions. I'm not sure whether there is a 'favourable' muzzle condition, though - just less adverse. I am thinking that what happens at lower velocity is the bullet cannot escape the destabilizing effects of the muzzle blast quickly enough but the higher the velocity the quicker it gets out of the destabilizing zone. I have seen it written that suppressors can sometimes improve accuracy by stripping away the destabilizing muzzle blast. It might also be due to barrel vibration dampening. We could put this to the test by substituting a weight for a suppressor if we could find someone with a gun that showed accuracy improvement.

Do 223's and 22-250's stabilize 60gr spire points in 1-in-14 twists (I don't suppose there are any in 1-in-16). Do they adequately stabilize 55gr BT's? I know my 1-in-10 Mini-14 did not like 60gr spire points. Likely not twist rate related. Well, vapodog has already said a 22-250 will stabilize a 60gr bullet, but will the 223?

Right, I have been able to find some stuff on the 'net which quite honestly I don't quite understand! Did you folks know there was a thing called Magnus force? Anyway, a small snippet of the paper;

quote:

Now let us consider the most general case of a bullet having a yaw angle. By saying so, the ballistician means that the direction of motion of the bullet' s CG deviates from the direction into which the bullet's axis of symmetry points. Innumerable experimental observations have shown that an initial yaw angle is principally unavoidable and is caused by perturbations like barrel vibrations and muzzle blast disturbances.

Best you folks who are interested have a look at the site - http://www.fulton-armory.com/fly/fomo.htm It's quite a read!

quote:

Now let us consider the two forces F1 and F2. It can be shown that this couple is a free vector, which is called the aerodynamic moment of the wind force or, for short, the overturning moment MW. The overturning moment tries to rotate the bullet around an axis, which passes through the CG and is perpendicular to the bullet's axis of form, just as indicated in the figure.

Luck!

On the other hand, now that I have been able to connect with the link jwp475 gave above;

quote:

The most common misconception is that a bullet that's spun too fast will fail to trace with the trajectory. In other words, it will fly point high on the downrange leg of the trajectory. Not true.

Mmmm.... conflicting reports! Some say bullets can fly nose high at long ranges.

When I build a custom rifle, I already know what bullets I plan on using and what the approximate velocities will be.

I usually order a barrel with the slowest twist that will stabilize the longest bullet I plan on using. This tends to produce the best combination of velocity and accuracy, everything else being equal.

For example, my 7-08AI has a 1:11 twist Shilen barrel. That is perfect for 140gr bullets, and that is all I shoot in the rifle. Velocity and accuracy are excellent. If I need more horsepower than that combination affords me, I have other rifles to take over where that one leaves off.

You must understand, however, that reselling that rifle with something other than the "standard twist" for that caliber will limit the number of potential buyers at resale. If there is a chance you will sell the rifle at some point in the future, err on the side of more twist to increase versatility for using longer bullets and the marketability of the rifle.