Is too stable a problem?

[HARPERC]

....Torquing of the shooting system is another reason to limit spin to that actually needed......All matters of recoil matter for each shot.....Also, if it didn't matter then all barrels would come in 6tw.......

I think this may be true for certain applications, but not others. Twist rates have changed to accommodate projectile construction, and purpose. It may well effect a bench rest shooter. In a long range hunting rifle, good barrel, good bullets, I don't think say going from 1-12" to a 1-9" hurting a .30 caliber build.

In jacketed bullets, driven hard, a 6 twist has been not beneficial, and in some cases unworkable. With the newer bullets I think the jury is out regarding how long bullets can get, and how hard they can be driven.

Depending on where the accuracy standard is set, may change the answer to the question. Achieving 0.1" matching the barrel to the projectile specifically, one might need to be conservative regarding twist. If 0.5" groups from a wider range of useable hunting bullets is the goal I don't think a more aggressive twist will be a limiting factor.

More specificity in the question could be useful.

 

[Mikecr]

I'm speaking in a general sense here. There is a price for everything. There is a best and worst for everything (a scale). If there were not, all barrels could simply be taken to tightest viable twist (which would only eliminate a handful of ridiculous cartridges).
I do know that it doesn't 'hurt' to go conservative on twist, if not an extreme.

From my experience, I don't understand why 14tw is used for many 22cal guns, given that 12tw would always cover 14tw, and allowing use for the majority of 22cal bullets.
I ran into this(14tw) with a Cooper M21 in 223rem. Even tiny 50gr boat tails ended up marginally stable with this, as seen by grouping opening with higher air densities. Going to flat base bullets (any) in that weight cut grouping in half -under all conditions.
I believe 12tw, while conservative, would have been a better choice. But I do not believe a 7tw would be a better choice. The bullets would hold together in this cartridge, but the gun would then torque a lot in recoil, and that can't help anything.

 

[Edd]

What would be the disadvantage in using a 10" twist in a 35 Whelen?

With a 250 gr Swift A-Frame, the SG will be about 4.0.

 

[SapperRIP]

Agreed, yes there is such a thing as too fast a twist. Voted no because it doesn't have anything to do with SG.

It doesn't mention twist in the posed question, so not sure where you're going on that.... http://www.bergerbullets.com/twist-rate-calculator/
But it definitely factors in twist of barrel on SG...

 

[Hand Skills]

Just agreeing with what was said in posts #8, #9, #10, #11, #12,...

Happy to answer any questions you may have.

To put it another way, unless you are shooting in a vacuum, you will encounter mechanical failure if you try to push SG too high. Accelerating a projectile from 0-400,000rpm in a couple thousandths of a second puts extreme stress on the bullet, and the shooting system.

There are two ways to increase SG for a given projectile.

-Increase RPM's
-Decrease air density

@HARPERC makes a very good point in that more specificity in the question could be useful.

Without specifying your conditions, you might as well be asking; "Does shooting at high altitude negatively affect your shooting?"

SG is a derivative. It has no limit.

Bullets are physical, and have physical limits.

@Canhunter35 post #5

 

[jasent]

I would agree there is a point of too fast. Mainly due to bullet or jacket strength if that's the right word

 

[Hand Skills]

But is there such a thing as too much SG?

Take berger's default example in the twist calculator @SapperRIP linked;

At sea level the SG is 2.42

At 30,000ft, the SG is 8.14

Same bullet, same gun. The only variable that changed was air density.

Of course, no one is going to use a .308 at 30,000ft, but there are guns that operate up there...

Is there such thing as too much twist? Yes, we all seem to agree on that.

I'm not sure why we are so divided on SG however. Perhaps there is misunderstanding in the question or the terms.

 

[Mikecr]

There are two ways to increase SG for a given projectile.
-Increase RPM's
-Decrease air density

NOT RPMs.
Revolutions per time are meaningless to stability.
This is why bullet twist requirements are never stated in RPMs.

The requirement as declared comes down to bullet displacement per turn(to gyroscopically overcome the overturning moments with that displacement).
The displacement is air, and so air density changes Sg at a given twist rate. So does bullet BC, which is velocity dependent, or any other thing affecting drag. That drag applies to the offset arm between center of gravity and center of pressure. So if you make the arm longer another way, say with a longer bullet, then Sg will lower with this given all else equal.
Basically, there is no way to generalize gyroscopic stability, and dynamic stability cannot even be predicted (must be tested for).

Just felt it needed to be pointed out that RPM notions fail tests for Sg.
Anyone who attempts to pull away from a poor stability issue, using higher MV, to bump RPMs, will find that it does not work to alleviate that issue.

 

[Hand Skills]

Yes RPM's.

The reason you don't see it in Miller's formula is because he uses a constant (30) which represents 2800fps at standard atmosphere.

Contrast that with Greenhills formula, and RPM's (specifically angular momentum in radians per second) is very much a factor in stability.

You're absolutly right about the complications that result from chasing stability with muzzle velocity - but take it the other way, as muzzle velocity decreases, twist rate must increase to maintain stability.

300AAC's use 1:7 and 1:8 twists for this reason. Seekins is well acknowledged for testing different twist rates and verifying experimentally their results (ie. they didn't just do it because some formula told them to).

Miller, in his work, notes several corrective equations that can be used.

 

[HARPERC]

.....Is there such thing as too much twist? Yes, we all seem to agree on that.
.......I'm not sure why we are so divided on SG however. Perhaps there is misunderstanding in the question or the terms.

I understand the question a little differently now than I did at the beginning. I think the different understanding of the question though has been educational.

 

[Mikecr]

Hand skills, both Greenhill and Miller's new greenhill are rule of thumb approaches to loosely define only majority(common) situations. They are not correlating directly with ANY local reality, and fail tests beyond limitations of design.
As an example, Bullet BC is not directly accounted for in either rule of thumb. Yet BC changes Sg. So does center of gravity, which can be set anywhere in a solid bullet design (again, not accounted for).
To see more tests pass you need to go to Robert McCoy's math.

You may know that Sg goes way up as a bullet travels down range. This, even while the bullet's RPMs slow a little.
Sg goes up here because the bullet slows in velocity way more than revolutions slow, meaning there is less & less displacement per turn (tighter effective twist rate), supporting what I told you earlier.
Your RPM test failed here, as Sg goes up while RPMs slowed slightly.

2800fps was chosen because it is a linear place in drag curves, and a chosen average of velocities through normal shooting distances. It is also a useful velocity to assign an average BC (as Berger does). But BC can be adjusted with meplat pointing, regardless of velocity.
At any rate, 2800fps has nothing to do with RPMs, or this would simply be a part of the rules of thumb,, and barrel ordering. Right? That's easier.
But if we tried to pick barrels based on RPMs, we would often fail to reach the correct twist rate.

I could lay out examples of this, but you can run the numbers yourself to see it.
Watch how big velocity (or RPMs if you wish) has to be adjusted to bail you from unstable through marginally stable, to fully stable. And then consider the idea that it never was about RPMs.
I'm just tryin to help. It's a bad idea to propagate notions of RPMs bailing people out of wrong barrel or bullet choices. When it doesn't.

 

[Hand Skills]

I understand the question a little differently now than I did at the beginning. I think the different understanding of the question though has been educational.

My sincere thanks for pointing that out.

I thought I understood the question, but now fear I'm losing the plot!

LOL, evidently there is some subtext here I don't comprehend.

On the upside my viewpoint is ahead in the poll, and I've thus far resisted turning into a troll... or at least I think I have..?

 

[Hand Skills]

To see more tests pass you need to go to Robert McCoy's math.

You may know that Sg goes way up as a bullet travels down range. This, even while the bullet's RPMs slow a little.
Sg goes up here because the bullet slows in velocity way more than revolutions slow, meaning there is less & less displacement per turn (tighter effective twist rate), supporting what I told you earlier.
Your RPM test failed here, as Sg goes up while RPMs slowed slightly.

2800fps was chosen because it is a linear place in drag curves, and a chosen average of velocities through normal shooting distances. It is also a useful velocity to assign an average BC (as Berger does). But BC can be adjusted with meplat pointing, regardless of velocity.
At any rate, 2800fps has nothing to do with RPMs, or this would simply be a part of the rules of thumb,, and barrel ordering. Right? That's easier.
But if we tried to pick barrels based on RPMs, we would often fail to reach the correct twist rate.

I could lay out examples of this, but you can run the numbers yourself to see it.
Watch how big velocity (or RPMs if you wish) has to be adjusted to bail you from unstable through marginally stable, to fully stable. And then consider the idea that it never was about RPMs.
I'm just tryin to help. It's a bad idea to propagate notions of RPMs bailing people out of wrong barrel or bullet choices. When it doesn't.

I appreciate what you're saying @Mikecr - and likewise, am just trying to help.

I think you're reading me wrong bro', I'll give it one more try here (for those who aren't confused enough)!

Read my post carefully, nowhere did I advise increasing muzzle velocity... in my example, muzzle velocity remains constant.

I asserted 2 ways of increasing SG for a GIVEN SET OF CONDIONS.

1. Increase RPM's

2. Decrease Air density


You are making a scarecrow out of me, because at MV: 2650fps the only way to increase the rate at which the bullet spins (RPM) is to increase RATE OF TWIST in the barrel.

We could talk about time. I love talking about time.

We have rotations per time (RPM)

And we have distance per time (velocity)

Combining these terms into displacement (rotations per distance) is cool. Not only do we remove time from the equation, but we get the reciprocal of the TWIST RATE in our barrel (distance per rotation). So next you'll be saying 'velocity has nothing to do with SG'? No argument here. It's a true statement when time gets erased from the equation. It all comes back to RATE OF TWIST.

Yay, we can both do alphabet math. I get all that, but for me this is neither the time, nor the venue for such a discussion. It will only serve to obfuscate the point and alienate ourselves from everyone else in this thread.

Let's get back to the real world, where BC and SG are dynamic quantities, where we are sending bullets into the future, and where we agree on terms to further understanding.

Thank you for suggesting McCoy, I will most certainly look into his work.

I encourage everyone interested in this subject to experiment with berger's stability calculator. If you hunt in the snow, be sure to use a realistic temperature (0deg F? Colder?).

While we can't seem to agree on whether bullets can be too stable, we certainly agree on the fact that inadequate stability is a real problem, especially with high BC and monolithic projectiles.

 

[6MM06AI]

Built myself a 22-250 with 1 in 8 twist, was shooting vmax bullets at around 3800. They started coming apart because of RPM's. (cost me a coyote). Had to switch to a better bullet 52gr smk to handle the spin rate. Also have noticed it seems to me that a higher spin rate seems to kill animals better than a slow twist. 200k rpm vs 325k rpm , if I did the math right. The faster the bullet is spinning when it hits becomes more violent when knocked out of perfect rotation.