Bullet RPM range - worth knowing ?

[big5ifty]

I have searched the forum, and of the many previous posts on bullet RPM, and without going into every single one, they all seem to be concerned with in-flight fragmentation, which this is not about.

Is there a reason why the manufacturers do not specify the minimum and maximum RPM for a given bullet ?

Times have changed a bit since a bullet was made for a specific cartridge, which was always usually with a particular twist, and could achieve a particular velocity.

I've got a spreadsheet of the Miller formula for bullet stability. I've modified it to show the bullet RPM.

I've done this because I'm trying to quantify the good results on the target where there should not be good results because of too slow twist. I'm seeing that twist rate can decrease a bit if velocity is substantially higher, and still keep stability. For example, a heavy .308 bullet in a .30 cal at 300 fps faster doesn't need a 10 twist, but the manufacturer will say it does.

Instead of having to practically guess at bullet suitability, it would be useful if the manufacturer could label the box and say 'Minimum RPM xxx 000'.

Or am I missing something ?

 

[RegionRat]

You bring up some good points, but I doubt you will be happy with the answers since we are into an area that isn't clear.

When discussing gyroscopic stability of small rifle bullets, The Miller Formula (and the others) is only a guide to help with a rule of thumb.

The difference is that some things in nature are difficult or impossible to tame with math, while others can be pretty exact. Twist stability is one of those things like you see in fluid dynamics that can only be parametrically described but where you cannot calculate a force balance or exact break point.

Once you are into those things like turbulence or fluid flow, you accept that we say roughly where something good or bad starts to happen, but there are grey zones that are risky.

Well, bullet stability is one of those things where we say to keep the SG greater than 1.5 but that is only a rule of thumb, not a hard wall. There is a huge grey zone where things transition with some working, then not working.

At SG of 1.0 we can pretty much be sure there will be keyholes.
At SG of 1.5 we can pretty much be sure there will be stability with some margin against climate conditions. In between is where your question lives.

If the manufacturers went any closer to SG of say 1.25, then folks would cry when a 1.3 tips over.

I am not trying to defend or condemn them, but I am just laying it out from an engineering design guideline view.

It isn't the same as claiming it is impossible to go below 1.5, but if you go there and it doesn't work in all cases then it is on you.

Many of us have seen folks routinely go below 1.5 and do very well. But by the same token, I think it would be very rare to hear of a case that was unstable with a 1.5 or more.

The higher you live (lower DA) the more you will get away with going below 1.5. The lower you live, the more likely you will see the problem if you explore the values below 1.5.

On that Max end, I wish there was a way to estimate the limits. When the 223 grew up and faster twists for longer bullets became a lot more common, it was easy to blow up light/fast varmint bullets.

We took the 7 twists and shot very fast 40 grain varmint ammo just to watch the puff of smoke out in front of the gun.

In engineering there are different levels of difficulty. One of them can be described as entry level where you learn to design by staying a safety margin away from textbook failure.

However, once you attempt to design things to break on command, you quickly find out how statistics and probabilities work. You find out all those engineering constants and material properties are not really what folks think they are but are only probabilistic.

A sophomore can design something to take a load without failing, but then ask them to closely predict when it will break and you are into graduate territory.

Now imagine trying to predict where bullets blow up. If you try to calculate it, you get a value that is higher than what you find in a lab where the bullets fail before they should. The calculations often do not reflect the damage that heat, pressure, rough rifling, friction, fouling, etc., can do to the bullet. The math gets ugly very fast.

Sometimes it is easier to just run that math knowing it will over predict, but then go into the lab and learn to offset those results based on parametric studies. Trying to wrap up all the other variables that contribute to the bullet blowing up is like herding cats.

For example, we might say the real results are typically 20% worse than the calculations and call that a rule of thumb. It will have some dispersion around it, but you can usually say you would get a margin of roughly 100 fps before trouble started on average once you learned the magic speed that blows them up.

So, while it would be nice to give out rough numbers of where you could expect very fast bullets to blow up, the variations in the damage done by rifling are significant enough to where the values for the fast rough barrels would be real different than the smooth slow twist ones. The minute you tried to put a number on it, someone would grab a rougher (or smoother) barrel and ask you to move the number.

Anyway, other folks will chime in with opinions. YMMV

 

[Intheshop]

Just built a vertical impact test machine for mainly use in the loading room. Don't really need math or physics to figure out cast bullet behavior. Using it has cleared up some,how you say... wives tales. So on the RPM thing,I appreciate ReigonRats engineering appropriate answer right about now,thanks.

 

[Ned Ludd]

Here is the reason bullet manufacturers use barrel twist rate instead of bullet RPM:

Pop..pop....pop..pop..........pop...pop...pop,pop,pop....pop....pop....pop....pop,pop...pop
...pop.....pop....pop..pop..........pop...pop........pop...pop....pop....pop....pop....pop,pop...

That is the sound of customer's heads exploding across the world if the bullet manufacturers switched over to using RPM instead of twist rate.

Nonetheless, you're not wrong, stating a suitable RPM range would work...just as stating a suitable gyroscopic stability coefficient (Sg) range would work. For that reason, I would agree with you that the RPM value is worth knowing. The problem with such parameters is that they will require further hand calculations by the end user, plugging numbers into an online calculator, or something similar. Some will not put in the extra effort. I guess the bullet manufacturers figure going with a minimum suitable twist rate is the easy button. As you noted, that doesn't necessarily mean it's the best method.

Asd I see it, the biggest problem with this area of shooting is that it's relatively poorly defined. Bryan Litz used to recommend using an Sg of 1.4 or greater to achieve full gyroscopic stability and attain the full intrinsic BC of the bullet. He later increased that recommendation to a minimum Sg of 1.5. The bullets of shooters that had previously been running the recommended Sg of 1.4 didn't suddenly fall out of the sky once the new recommendation was issued. In fact, many shooters might never even notice the slight difference in BC achieved with an Sg of 1.3 and 1.4, or even 1.5. It's typically not until the Sg drops to around 1.1-ish that keyholing and instability start to become noticeable. So there is a pretty wide range of Sgs, maybe 1.3 to 1.5+ or so, that for many shooter's purposes differ only slightly in terms of attained BC for their bullet of choice. Slight differences in BC can easily be lost in the noise unless someone is looking really hard to discriminate those differences.

At the high end of the spectrum, I have been told by both bullet manufacturers and barrel makers of a [theoretical] maximum RPM value of 300K. In fact, I've heard that value thrown about quite a bit. Nonetheless, even that is not written in stone. A perfect example of this is that several years ago, jacket failures were not uncommon among F-TR shooters using 30"+ long barrels and the tighter 0.218"/0.224" configuration with 88/90 gr bullets. Loads pushing these bullets at velocities in the neighborhood of 2850 fps generally worked ok out of a 7-twist barrel. But increasing the twist rate to 6.8, or 6.7, or faster would often be sufficient to cause jacket failures, even though all else remained the same. However, if one simply goes to a 0.219"/0.224" barrel configuration, it becomes possible to run twist rates of 6.7, 6.5, or even faster without the same propensity for jacket failure using 88s and 90s, even though RPM values exceed the "magic 300K barrier". The bottom line is that there are numerous contributing factors for jacket failure including barrel length, twist rate, bore dimensions, land configuration/geometry, bullet velocity, bullet bearing surface length, and probably a few more I'm not even aware of. It's mostly about friction, but other contributing factiors can markedly affect where the actual point of failure occurs. So no matter what units or parameters they choose to supply, the best a bullet manufacturer can do is provide a reasonable range over which their bullet performance should be acceptable, if not necessarily optimal. As I stated earlier, I guess they feel like twist rate is the easy button in that regard.

Along the same line of thinking, even just a few decades ago, most every bullet manufacturer reported BCs based on the G1 standard. Bryan Litz started banging the G7 BC drum several years ago and many manufacturers now also offer G7 BC values for their products. Change can happen if the reasons behind it are solid and enough people make themselves heard, but it usually doesn't happen fast.

 

[mikecr]

With stability, overturning bullet drag is countered through a turn worth of inertia.
Given this, stability is expressed in displacement per turn, NOT time per turn.
Example 8:1 twist rate is 8" of relative displacement (relative to air density/drag) per turn.
There is no TIME in this requirement.
You're just trying to fool yourself

 

[DaveTooley]

Here's examples of two extremes I have.
17 Cal. 30 gr. bullet 10 twist velocity 4100 FPS 302,400 RPM's
30 Cal. 187 Gr bullet 14 twist velocity 3050 FPS 156,857 RPM's

The 17 Cal would start blowing up bullets after 7 or 8 shots and need cleaning.
The 30 Cal with an Sg of about 1.1 won me a lot of fake wood in 1K comps.

 

[Rank Amateur]

Seems to me that the physics involved are immutable. The thresholds of stability and jacket failure are blurred by the influence of numerous variables, specific to each event. That said, the values we're discussing seem to all be dependent upon each other, right?

Twist rate influences stability and structural integrity, but only relative to bullet velocity. I.E., the same bullet accelerated to a given velocity in a slow-twist barrel will have a lower muzzle (maximum) RPM than one accelerated through a fast-twist barrel. A bullet accelerated more quickly to a given velocity and RPM will deal with more frictional heating in the barrel than one accelerated more slowly. More stress is placed on bullet components when accelerated quickly (heat, mechanical distortion) or in tighter bores than those accelerating more slowly or traveling through looser bores. Bullets traveling in flight with optimal spin-stability and initial SG values become unstable as they slow regardless of twist rate, muzzle RPM, or displacement. In other words, you can't consider stability/structural integrity based on a single value (RPM, twist rate, etc.) because the effect of that value is immutably linked to others.

Manufacturers (and we) use shorthand (such as minimum twist rates for stability) in our considerations. But they are only shorthand. When we say that a 10 twist is optimal for a given bullet size/cartridge, we ASSUME that we are all discussing a relatively standard load/velocity/bore/smoothness/bullet composition/bullet structure/etc./etc./etc. Any one of us can define numerous fringe cases where "the twist rate is fine, except if..."

 

[mikecr]

Bullets traveling in flight with optimal spin-stability and initial SG values become unstable as they slow regardless of twist rate, muzzle RPM, or displacement.

That's not true.
They become unstable at impact, or possibly dynamically unstable when dropping mach, but before these, Sg climbs with bullets traveling downrange.
This is because velocity (displacement) drops way faster than turn rate, so effective twist rate climbs.

 

[jelenko]

That's not true.
They become unstable at impact, or possibly dynamically unstable when dropping mach, but before these, Sg climbs with bullets traveling downrange.
This is because velocity (displacement) drops way faster than turn rate, so effective twist rate climbs.

Ummmm. Once the bullet has left the barrel, the only thing affecting it's stability is the environment, it's BC and spin rate. It's velocity doesn't enter into it.

 

[Rank Amateur]

That's not true.
They become unstable at impact, or possibly dynamically unstable when dropping mach, but before these, Sg climbs with bullets traveling downrange.
This is because velocity (displacement) drops way faster than turn rate, so effective twist rate climbs.

No argument to effective twist rate, but that does not make my statement untrue. As you wrote: "...dynamically unstable when dropping mach...". All I wrote was that they become unstable.

 

[Rank Amateur]

Ummmm. Once the bullet has left the barrel, the only thing affecting it's stability is the environment, it's BC and spin rate. It's velocity doesn't enter into it.

Isn't BC dependent upon and vary with velocity? Again, I would suggest that many/most of the factors influencing both stability and integrity are interdependent.

 

[Rank Amateur]

By the way, the level of this discourse, and the contribution of real experts participating here (I absolutely do not put myself anywhere near that category), is what makes this forum stand out. I participate in 5 other shooting related forums and none compare. Thanks to you all for the quality you bring!

 

[mikecr]

become unstable as they slow regardless of twist rate, muzzle RPM, or displacement.

That's what you said, right there.
I'm just qualifying a declaration that could be taken too broadly.
REALITY: Very few bullets become unstable before impact.

Once the bullet has left the barrel, the only thing affecting it's stability is the environment, it's BC and spin rate. It's velocity doesn't enter into it.

Bullet stability does in fact change with distance, and not just because of changing drag.
Effective twist rate is changing as well, and this is dominant over anything else.
Twist rate is tied to velocity because displacement per turn is changing. The velocity slows faster than revolution rate.

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.
In the past I've run the math to show folks. If you carefully study this example/table, you can see a lot of truths that pass all tests.

 

[Rank Amateur]

That's what you said, right there.
I'm just qualifying a declaration that could be taken too broadly.
REALITY: Very few bullets become unstable before impact.

Bullet stability does in fact change with distance, and not just because of changing drag.
Effective twist rate is changing as well, and this is dominant over anything else.
Twist rate is tied to velocity because displacement per turn is changing. The velocity slows faster than revolution rate.

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.
In the past I've run the math to show folks. If you carefully study this example/table, you can see a lot of truths that pass all tests.

First, I may be weird, but I find this discussion a lot of fun. Then, I submit that you are both over-interpreting the statements (I stand by my statement as you reproduced above), and introducing the concept of effective twist rate (please correct me, but I'm interpreting this to be the barrel twist rate that would produce the observed RPM at the observed velocity?). An interesting concept, but if I'm interpreting the term correctly, it's another value that is derived/calculated/dependent on other values already under discussion. If it is a way of pulling together the barrel's physical twist rate and the characteristics of a specific projectile/ cartridge, then it might be especially useful in pulling those things together to predict stability. On the other hand, the structural integrity of the projectile may not be appropriately tied to this value as the maximum spin rate/centrifugal force on the jacket will never be faster/greater than that calculated from the actual twist rate (as opposed to the effective) at the muzzle. So, again if I'm understanding, the effective twist rate at the muzzle is 1 x the barrel twist rate. So, effective twist rate may be useful in describing stability, but not any more useful than barrel twist rate when discussing bullet structural integrity.

Moreover, it seems that your objection to my statement about stability is couched by your reality statement. It may be true that few bullets become unstable before impact, but that limits the concept. No one inserted the distance to the target in the original discussion. Instability certainly affects 30 cal bullets fired from .308 Win cartridges when the target is a plate 1750 yards out. That's a key reason that cartridge isn't used for that target. In the limit, velocity is always a factor in stability/spin discussions. To be ridiculous, a super-high-BC bullet thrown by hand at a target is unstable. The velocity/spin/BC/etc. are simply not significant enough to maintain an aerodynamic profile - I.E., it's just a chunk of lead tumbling through the air.

You are certainly correct when you say that RPM alone is not sufficient to describe the behavior of the projectile. Of course, I never did so. Aerodynamic stability of a bullet IS tied to RPM as the centrifugal force aids in maintaining the aerodynamic orientation of the bullet to the airstream. Insufficient RPM (centrifugal force) permits the bullet to pitch and yaw which rapidly devolves to tumble. Air displacement of two identical projectiles traveling at identical speeds through identical media is identical. If one of those projectiles is traveling point-on and the other is pitched in some other orientation, only one of those is aerodynamically stable despite identical displacement/RPM values.

My point is to both violently (grin) agree with you that any attempt to oversimplify the physics of the bullet's flight/structural integrity will yield inaccurate predictions vs observed behavior, and to suggest that effective twist rate is another oversimplification (although it may be better than others). Again, please correct my misunderstanding of your terminology.

 

[dcali]

Twist and velocity are simple terms that get you to exactly the same place, and are easier for most shooters to focus on. In reality, velocity can be thought of as a constant for the vast majority of cases. And if you're doing something weird enough to be an exception, you know it.

Side note: the miller formula is a clever approximation, but it's only so good. In cases that deviate from the bullets used to develop the simplifications it can fall apart completely.

The actual gyroscopic stability is a function of:

air density
twist rate (calibers per turn)
frontal area
transverse and logitudinal inertia
the bullet's overturning moment coefficient, which is dependent on velocity (more precisely - the mach number)

You'd have to reconfigure the industry standards going back before ww2 if you wanted to switch, and there's no compelling reason to do so. They did it the way they did for a reason - it's the most convenient form.

 

[mikecr]

velocity can be thought of as a constant for the vast majority of cases. And if you're doing something weird enough to be an exception, you know it.

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

twist rate (calibers per turn)

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.

introducing the concept of effective twist rate (please correct me, but I'm interpreting this to be the barrel twist rate that would produce the observed RPM at the observed velocity?).

No
I'm talking about bullet twist rate, not barrel.
It's displacement per turn, and effective in that it changes from the moment a bullet is released from a barrel.

 

[RegionRat]

Aerodynamic stability of a bullet IS tied to RPM as the centrifugal force aids in maintaining the aerodynamic orientation of the bullet to the airstream.

BTW, Centrifugal force and Gyroscopic moments are not the same things.
Not important here, but just to make sure we stop it before it propagates. Carry on.

 

[Rank Amateur]

No
I'm talking about bullet twist rate, not barrel.
It's displacement per turn, and effective in that it changes from the moment a bullet is released from a barrel.

Units? Thanks.

BTW, Centrifugal force and Gyroscopic moments are not the same things.
Not important here, but just to make sure we stop it before it propagates. Carry on.

Understood, and of course correct. Please forgive my imprecision.

 

[Old Navy]

My head hurts! I am going to go have a nap.

 

[RegionRat]

Understood, and of course correct. Please forgive my imprecision.

No worries. I realize it isn't everyone who has had to run a slide rule to the point of starting a fire....