Bullet RPM range - worth knowing ?

[jelenko]

displacement per turn, and effective in that it changes from the moment a bullet is released from a barrel

Do you have a reference on 'effective twist rate' that I could read? I'm not familiar with that term/effect.

 

[Bottom Fodder]

My head is now spinning faster than “A speeding bulllet”

 

[dcali]

Yeah, I'm an astronaut shooting in space.
It's the only occasion where my velocity is a constant..

We buy OUR barrels in Inches per turn. OUR bullet requirements are stated in inches per turn.
But I suppose you could use whatever you like for displacement.

You missed my point. Velocity can be thought of a constant in terms of its impact on stability for practical purposes. It doesn't matter if you're shooting a .308 or a .30 '06. The difference in velocity doesn't really matter when it comes to the required twist. subsonic blackout vs a 300 jumbo mag? Yeah, then it's not the same. But usually, you can ignore velocity when selecting a barrel for a bullet.

And the units of twist are totally irrelevant. inches per turn, calibers per turn, it's all the same thing. calibers is a lot more convenient from an engineering perspective.

 

[jackieschmidt]

I am glad that we have those that are much more proficient in mathematics and engineering to figure all of this out so that us that are limited in those fields can simply read their material and follow suite.

I believe that is called “learning”, which none of us are too old to do.

 

[big5ifty]

30 Cal. 187 Gr bullet 14 twist velocity 3050 FPS 156,857 RPM's

What altitude is this at ?

 

[big5ifty]

Every now & then someone searching for a shortcut invents the notion of stability being tied to RPMs.
But it's a helluva stretch to imply direct connection, and failing every test beyond such an effort.

I'm not seeing how bullet stability is not tied to RPM.

Longer bullets need either tighter twist, or substantially higher velocity at lesser twist to maintain stability.

RPM is a product of twist and velocity, does that not mean it's all about the RPM ?

Or is there something else. Like where the center of gravity of the bullet is ? If the CoG is in the middle of the bullet, would it need less RPM to stabilise in flight than a bullet where the CoG is towards the rear ?

 

[DaveTooley]

What altitude is this at ?

From Quantico VA maybe 50' to Byers CO close to 5,000'

 

[dcali]

I'm not seeing how bullet stability is not tied to RPM.

Longer bullets need either tighter twist, or substantially higher velocity at lesser twist to maintain stability.

RPM is a product of twist and velocity, does that not mean it's all about the RPM ?

Or is there something else. Like where the center of gravity of the bullet is ? If the CoG is in the middle of the bullet, would it need less RPM to stabilise in flight than a bullet where the CoG is towards the rear ?

It is about RPM, in part. It's just an awkward way to go about it.

And yes, there is a lot more to it than rpm - air density, the speed of sound, the weight distribution of the bullet, the shape of the bullet, the velocity. It all matters. We tend to think that length, spin and velocity are all that matter, but those are just approximations of a more complex situation. I blame the Miller formula for this - becuase it is so amazingly good. It's better than it has any right to be and a credit to Don Miller's mind. But in reality, he just glossed over a lot of the complexity that you can't actually ignore.

All that said, in my experience, both the full blown theory and the miller simplification tend to be conservative. I find that they tend to predict slightly faster twist rates than are actually required.

 

[mikecr]

You missed my point.

Yes I did, and I agree with your point.
That velocity won't bail out a poor twist rate.

I'm not seeing how bullet stability is not tied to RPM.
Longer bullets need either tighter twist, or substantially higher velocity at lesser twist to maintain stability.
RPM is a product of twist and velocity, does that not mean it's all about the RPM ?

You could connect RPM with a single result. You could also connect recoil and NASDAQ closing to the same single result. But you couldn't predict well, because that correlation is poor.
Look over the table I posted earlier. Could you have predicted the results in a rough & broad sense?
When you understand my assertions here, you can predict and validate every bit of it.

For your CoG change, twist requirements do change, but Revolutions per Time(RPM) is not a stability requirement. What changes is the overturning moment amount, per drag, and so the maximum displacement each turn can overcome.
The twist rate of a bullet -which is separate and different than velocity or time.
Like damoncali described, twist rate (in displacement per time) does not practically change with velocity or time. When the barrel turns a bullet once per 8 inches, that's the bullet's initial twist rate, which does not change with ANY velocity/time.
AT 5,000fps, or 900fps, the bullet is still turning once in 8 inches, the same twist rate.
And the bullet may need that, regardless of velocity.

 

[mikecr]

The Miller rule of thumb, like Greenhill, is useful in a limited sense. That's all, and no more credit should be assigned. Shoot bullets backwards and it fails to predict anything.

 

[urbanrifleman]

@padom

 

[M-61]

AT 5,000fps, or 900fps, the bullet is still turning once in 8 inches, the same twist rate.

Sure is. However once in every 8 is correct in your example. How fast that once it 8 occurs is the question regarding the bullets RPM’s. The RPM number is crazy high. But too slow is the kiss of death for accuracy. Too fast, in a light jacket bullet creates that puff of white smoke 50 feet from the muzzle as the bullet disintegrates.
If the bullet can stay together I don’t think there is an adverse effect on accuracy. Too slow?
I have a 1-14 222Remm. 69 grain bullet. Guarantees horrible accuracy with at least one bullet key holing.
I also believe that the loss of RPM’s over the range is extremely slow.
The above is strictly what I think, maybe not close to reality.

 

[big5ifty]

When the barrel turns a bullet once per 8 inches, that's the bullet's initial twist rate, which does not change with ANY velocity/time.
AT 5,000fps, or 900fps, the bullet is still turning once in 8 inches, the same twist rate.
And the bullet may need that, regardless of velocity.

The twist rate is not used describe any bullet property in flight.

It is a physical property of the barrel.

The twist rate, with the velocity, determines RPM, and nothing else does.

Your bullet is turning once every 8 inches on exiting the muzzle, which on it's own cannot tell you what the rotational speed of the bullet is, unless you factor in the velocity.

It's my understanding that the bullet RPM stays relatively constant till the point of impact. The only thing that can slow the rotational speed after muzzle exit is air resistance.

addendum :

Which is why I posed the question, why not tell us what the required RPM for the bullet is. We already so a lot of math to get to precision at range, one more calculation if required is not a problem for good results.

If the bullet is stated by the manufacturer to need a 1 in 8 twist barrel, they assume you're launching at the standard velocity range for the caliber they made the bullet for. If you launch that bullet at 900 fps, the twist alone is not going to stabilise it, because you're not at the required RPM, so the velocity is very important.

 

[RegionRat]

If the bullet can stay together I don’t think there is an adverse effect on accuracy.

Just an opinion but in instances where I have taken this to just below the point where they disintegrate, I have seen the performance fall apart. The high speed video and radar showed that the axis was beginning to wobble and the projectiles were likely right at the edge of failure but just didn't burst. My theory was the inertial axes were distorted enough to ruin the CG but not enough to burst.

Friends have taken indirect observations that also show you want to stay a slight margin away from the limit. For example, when playing with the 20 cal Berger 55 using a parent 22-250 case, it isn't difficult to blow up bullet somewhere up above 3800 fps, but if you stay up near that speed the groups fall disproportionately apart. Drop the speed a little and the groups were at least decent till you found the tune and they snapped right in.

So I agree with you about over-stabilization, but there is a margin below the failure point where the distortion just below the failure can ruin performance. Poor axis symmetry with a bad CG will never do well, so I get your point.

 

[M-61]

Just an opinion but in instances where I have taken this to just below the point where they disintegrate, I have seen the performance fall apart. The high speed video and radar showed that the axis was beginning to wobble and the projectiles were likely right at the edge of failure but just didn't burst. My theory was the inertial axes were distorted enough to ruin the CG but not enough to burst.

Friends have taken indirect observations that also show you want to stay a slight margin away from the limit. For example, when playing with the 20 cal Berger 55 using a parent 22-250 case, it isn't difficult to blow up bullet somewhere up above 3800 fps, but if you stay up near that speed the groups fall disproportionately apart. Drop the speed a little and the groups were at least decent till you found the tune and they snapped right in.

So I agree with you about over-stabilization, but there is a margin below the failure point where the distortion just below the failure can ruin performance. Poor axis symmetry with a bad CG will never do well, so I get your point.

Certainly makes sense. Fits into what has happened to me. I’m only looking for accuracy. Somewhere very close to a load that’s really too hot the accuracy deteriorates. Back it down and generally I find my most accurate loads. This was really prevalent in my 222. That’s with 52 gr. Sierra’s. My first BR rifle I had made in 1975. So I’ve had plenty of time to experiment. Still have it. Sleeved action, SHBF and 3piece laminated stock. And it still holds its own.

 

[urbanrifleman]

RPM is a function of time.

The only portion of RPM that ties it to time is the velocity, which is the other function of time. So, of course velocity matters.

Twist rate has no time function. Twist rate does not determine anything by itself relative to stabilizing a bullet. It's just basic math. The time has to come from somewhere.

 

[dcali]

Which is why I posed the question, why not tell us what the required RPM for the bullet is. We already so a lot of math to get to precision at range, one more calculation if required is not a problem for good results.

Because RPM alone is not any better than twist rate alone to estimate real world stability requirements. It's really that simple.

 

[Rank Amateur]

OK, so the bulk of this thread has been discussion of what isn't adequate to estimate stability requirements. @mikecr seems to support relative twist rate, others seem to favor RPM-plus-something, and others seem to be satisfied with what we can all acknowledge as oversimplifications (RPM by itself, physical barrel twist rate by itself, twist rate without a time function, etc.). May I ask what IS an appropriate indicator to predict stability (ignoring the other part of the discussion which is the prediction of jacket integrity failure)? For this, I would request inclusion of not just the terminology, but how it is measured/calculated (unless it's obvious), AND, how the result is interpreted?

Thanks in advance!

 

[RegionRat]

The Miller Equation is more or less public domain and you can play with web sites that take much of the work out if it.
This calculates a stability factor, SG, that is on a scale where the recommendation is to stay above 1.4 or 1.5.

A few inputs are required, but nothing difficult.

The Berger Web Site is free and easy to play with.
https://bergerbullets.com/twist-rate-calculator/

On that web site, if your results land below 1.5, you will get alternative recommendations.

It is clearly not a hard wall where if you go below 1.5 things instantly break, however, going below 1.5 means you must have your homework done.

As far as other interpretations, I think you mean how does one know as a hobbyist?
Gyroscopic Instability shows up on the target as a sudden loss of performance which can even turn into keyhole behavior. Marginal instances may not tip over, but will perform poorly and show a poor BC.

On ballistic test ranges, projectile stability is tracked in 6 degrees of freedom using optical and radar methods.

 

[big5ifty]

Because RPM alone is not any better than twist rate alone to estimate real world stability requirements. It's really that simple.

RPM is a product of twist and velocity.

If the manufacturer were to give a RPM number, they will be giving you the velocity you need to have for your twist.

How is that not a better number to have than just a twist rate where you have to thumb suck the velocity ?

We are no longer in a time where a bullet was made for a particular cartridge, and the velocity it would be launched at was a standard number. Then the twist alone is adequate.

Today we have many cartridges using the same bullets with wildly different powder capacities, and custom barrels all over the place. Just a twist number is not sufficient anymore.