Bullet stability

[MajorSpittle]

I imagine SD/SG as a QB throwing a football. SG is how fast it is spinning and SD is how tight the "spiral/wobble" is on the ball when it leaves his hand. They both play on each other. Enough SG (spin) will keep the ball from tumbling if it isn't a tight spiral (SD) or a tight enough spiral and you don't need as much spin to keep it stable.

Not sure how correct I am but I can sleep at night and feel better about how one rifle can stabilize a heavier bullet but a second rifle that is identical can't with the same muzzle velocity. I also think this would explain why with less air pressure you need less SG because there is less force applied on the nose of the bullet working against the SG. Also why less SG is need as the bullet slows because the force on the nose of the bullet lessens with speed. Also why you need less SG with a more blunt nose on a bullet....

I have watched 7.62 tracer rounds go down range and cork screw through the air about 2 feet up and 2 feet right only to finish the corkscrew and be relatively on target by 600 meters. I would assume this is an example of great SG and poor SD.

 

[Mikecr]

I don't like rules of thumb (miller approach), but in this case, due to the plastic tip and actual pressure & temp, I would favor it's results.
The Litz numbers would be closest for a non plastic tipped bullet.

 

[Laguna Freak]

I don't like rules of thumb (miller approach), but in this case, due to the plastic tip and actual pressure & temp, I would favor it's results.
The Litz numbers would be closest for a non plastic tipped bullet.

What rules of thumb, in layman's terms, does Miller tout? I've perused the math but it didn't take more than 3 sq root equations for me to lose interest.

 

[MajorSpittle]

Thread Resurrection;

I'm resurrecting this 3-year-old thread in hope of gaining some new insight and well-reasoned answers to a central question I have regarding two respected online Sg calculators.

My quest for information was initiated yesterday after I started a thread in the Long Range Hunting forum seeking opinions about the applicability of Nosler's 142 gr Accubond Long Range bullet in a 26 inch 264 Win Mag barrel with 1 : 9 twist. While all of the responses were helpful, none of them were able to satisfactorily address a central question I had. This central question regards the delta in results that I received from Berger's Bryan Litz Twist Rate Stability Calculator vs JBM Ballistics' Miller/Courtney Sg Calculator. In short the delta between these two calculators is the difference between an Sg of 1.27 (Litz) and 1.651 (Miller/Courtney). Essentially no-go vs go.

My question; Which is the "most accurate" calculator?

I believe the answer lies in the relevance of the data collected by each calculator. Each calculator asks for the same input data parameters with the exception that Berger's Litz asks for Altitude (Drag coefficient), while JBM's Miller/Courtney asks for barometric pressure and length of plastic ballistic tip (Drag coefficient and CG parameters).

I've studied up a little bit and have developed a basic understanding of Sg and its influence on the elusive Sd, and their collective potential transonic stability implications.

I've begun to develop an opinion about which calculator I am inclined to believe is "most accurate". But I don't want to pollute the thought pool with my primordial opinion.

Which one do you think is the "most accurate" Sg calculator?
Why?

Thanks to everyone who plays along with a well-reasoned response.



Berger Litz:

Stability Analysis

Your bullet is MARGINALLY STABLE.
Your bullet stability is marginal. You may shoot good groups under these conditions, but the BC of the bullet will not be optimized.
​

SG = 1.27 Bullet BC (G1): 0.625 Adjusted BC for 1 in 9" Twist: 0.582
Your BC is being compromised by:7%
Minimum Twist Recommended: 1 in 8.25"


JBM Miller/Courtney:

Stability Input Data Caliber: 0.264 in Bullet Weight: 142.0 gr Bullet Length: 1.450 in Plastic Tip Length: 0.190 in Muzzle Velocity: 3050.0 ft/s Barrel Twist: 9.0 in Temperature: 80.0 °F Pressure: 29.59 in Hg Output Data Stability: 1.651

31-Jul-20 17:10, JBM/jbmstab-5.1.cgi

I think that the only relevance that "length of plastic ballistic tip (Drag coefficient and CG parameters)" has is in calculating the center of gravity of the bullet. Basically a bullet with a better center of gravity (more forward) has a better chance of launching with better SD "less wobble" and it will slightly reduce the distance the tip is from the CG so there is less leverage against the CG per X amount of SD. As far as Drag coefficient it doesn't matter what the tip is made of because air pressure will act upon the surface the same and that air pressure is what the SG has to over come.

So I would go with Berger while keeping in mind that basically both stability calculators are based on; you need X amount of RPMs in a bullet of X diameter that is X weight to overcome X amount of off axis force on a surface X amount of distance from the CG.

Berger uses a static "X amount of off axis force per Air Pressure"

JBM uses a dynamic "X amount of off axis force per Air Pressure" because it attempts to account for leverage with distance to CG and could possibly increase the RPMs needed with longer poly tipped bullets due to resistance being farther from the CG.

But I am just guessing and have been drinking a lot tonight.

 

[Laguna Freak]

I think that the only relevance that "length of plastic ballistic tip (Drag coefficient and CG parameters)" has is in calculating the center of gravity of the bullet. Basically a bullet with a better center of gravity (more forward) has a better chance of launching with better SD "less wobble" and it will slightly reduce the distance the tip is from the CG so there is less leverage against the CG per X amount of SD. As far as Drag coefficient it doesn't matter what the tip is made of because air pressure will act upon the surface the same and that air pressure is what the SG has to over come.

So I would go with Berger while keeping in mind that basically both stability calculators are based on; you need X amount of RPMs in a bullet of X diameter that is X weight to overcome X amount of off axis force on a surface X amount of distance from the CG.

Berger uses a static "X amount of off axis force per Air Pressure"

JBM uses a dynamic "X amount of off axis force per Air Pressure" because it attempts to account for leverage with distance to CG and could possibly increase the RPMs needed with longer poly tipped bullets due to resistance being farther from the CG.

But I am just guessing and have been drinking a lot tonight.

Aight! So I've been drinking too but it reads to me like you are talking about the relative force that drag imparts on CG's restraint of yaw moment (momentum for the un-initiated). Shorter yaw moment = greater Sd... Yea or nay?

 

[MajorSpittle]

Aight! So I've been drinking too but it reads to me like you are talking about the relative force that drag imparts on CG's restraint of yaw moment (momentum for the un-initiated). Shorter yaw moment = greater Sd... Yea or nay?

I don't know anymore because I switched to BV and coke. I think it was something about a soccer ball flys better then a football that flys better then a tennis ball with a dixie cup stuck to it...... think I will get off the interweb now.

 

[josip89]

I think we need better twist rate calculator for sure.

Because it is not same from what meterial is bullet made and many many other things.

But my personal opinion is that gain twist with thighter twist is the best.
Let say for 300 magnum in 26" barrel You have starting twist of 15" and end is 9" or 8".

 

[DocUSMCRetired]

Just to touch on a couple of things in here:

1) Our calculator uses Station Pressure (altitude is irrelevant) for the calculation.

2) Something to consider. When you think about stability you are somewhat concerned with the overturning moment (end over end). This has a lot to do with the Center of Pressure vs Center of Gravity. So a shorter fatter bullet will be naturally more stable needing less twist. Make a bullet longer, and you increase the distance between the CP and CG which in turns means you need to spin it faster to prevent that overturning moment and yaw due to instability.

3) With the above understood, a bullet loses forward velocity much faster than it loses rotational velocity. So it is safe to assume that you have higher stability down range.

4) A bullet that isn't stable from the start will not later become stable. So you need the bullet to leave the muzzle with the stability you want for it to transition into subsonic without incident. Bullets stabilize in the first couple procession cycles. So in essence it never gets better if you don't start off stable, an unstable bullet will not clean up at say 200 yards etc.

4) A +/- 0.1 error when using the Miller formula is generally accepted.

5) Dynamic stability is a whole different beast but we do have the tools to measure it and our system isn't as simple on the inside as we make it look on the outside. Our system even accounts for pressure changes and gravities effects as you shoot uphill/downhill just to explain how complicated it is internally.

 

[Mikecr]

Pretty sure the stability software & math available to us are rules of thumb. With this, they are right for trending conditions tested for, and wrong otherwise. Greenhill was right for hunting bullets back in it's day. Miller is way better for bullets used today. There are more comprehensive approaches from Robert McCoy & Bill Davis, but they still do not account for anything possible. They probaby did not test plastic tipped bullets at Aberdeen.
Closing a meplat affects stability. In fact anything changing drag/BC changes stability. Or we can change stability without affecting BC.
Simple rules of thumb(like Miller) do not account for many things. And even comprehensive approaches do not account for CG possibles -like an aluminum bullet with a placed tungsten core, -or firing bullets base first.
I don't know what Berger uses for stability now.

 

[DocUSMCRetired]

Pretty sure the stability software & math available to us are rules of thumb. With this, they are right for trending conditions tested for, and wrong otherwise. Greenhill was right for hunting bullets back in it's day. Miller is way better for bullets used today. There are more comprehensive approaches from Robert McCoy & Bill Davis, but they still do not account for anything possible. They probaby did not test plastic tipped bullets at Aberdeen.
Closing a meplat affects stability. In fact anything changing drag/BC changes stability. Or we can change stability without affecting BC.
Simple rules of thumb(like Miller) do not account for many things. And even comprehensive approaches do not account for CG possibles -like an aluminum bullet with a placed tungsten core, -or firing bullets base first.
I don't know what Berger uses for stability now.

Might be surprised how much of it is verified through testing. We do know that our current system works very well even for tipped bullets.

 

[Mikecr]

Nothing beats validation

 

[gohring3006]

I miss the days when we had these type of technical discussions on here.

 

[MajorSpittle]

Might be surprised how much of it is verified through testing. We do know that our current system works very well even for tipped bullets.

But how do you account for variability of SD with each rifle/cartridge. All you can really do is take a sample among multiple rifles and say X% of rifles tested stabilized this cartridge with this spin rate under these conditions.

So you are always guessing as to what "off axis spin" to use as a baseline for Minimum Twist Rate needed.

So perhaps when software says a 208 LR Hybrid has poor stability in my 1/11 twist 30-06 at sea level launched at 2650fps, it doesn't account for me hand loading them in my rifle and finding the most accurate load that most likely has the least amount of off axis spin when launched which is why it is shooting .5 MOA at 200 yrds.

I don't see anyway to account for this and any calculator will just provide generic guidance on probability of being able to achieve stability or develop a load.

So in my case I get a 1.2 stability rating with a 9% reduction in BC for the above example. I would look at that as the odds are not good that I will find any over the counter box of ammo with that bullet to work well for LR shooting in my rifle at sea level; But I have a good chance to still be able to hand load and find a load that will do better than what the calculator shows because the SD variable used was probably developed by throwing the same load in multiple rifles that may or may not have "liked" the generic load used for testing.

Has anyone ever took a rifle that wouldn't stabilize a generic load with X bullet at sea level and develop a load for it that shoots great at 8,000ft. Then take that load back down to sea level and check to verify if it still wouldn't stabilize? If it does maybe a person could quantify the SD need to stabilize at a certain spin rate/altitude? Say, if you develop a load that shoots >1.2 MOA at 8,000ft with X RPMs, then that load will remain stable all the way to sea level with the reduction of FPS expected for the load at sea level?

 

[DocUSMCRetired]

*** Snip***

1) Below 1.0 is considered unstable.
2) Between 1.0 and 1.5 is stable but not optimized.
3) Above 1.5 is both stable and optimized.

A bullet with an SG of 1.2 has plenty of potential to shoot small groups and show no signs of instability. A lot of benchrest shooters are getting extremely close to 1.1 or even 1.0 and shooting sub .2 MOA.

With that being said, we actually have the equipment to physically test and measure this kind of data. So we aren't guessing.