Bullet stability

[RockyMtnMT]

It was pointed out to me that bullets become more stable as they slow down. It appears that this is indeed true. I can not find out how much stability increases relative to the velocity of the bullet though. The Hornady calculator show huge increases in stability as the bullet slows down in flight. This does not make sense to me for a couple of reasons. It does not line up with range test results and it does not make sense with the findings of Litz showing the need for a min of 1.5sg for optimal bc. The Hornady calculator shows a 1.4sg bullet becoming higher than 1.5sg inside of 100y of slowing down and over 5.0 at 1500y. So if the Hornady data is correct the 1.5sg minimum would be of very little factor at long range as related to bc.

Turns out that there is also dynamic stability (sd) to go along with gyroscopic stability (sg). So far what I have found is that sd may be the larger contributor to unstable bullet performance down range. I think this has something to do with why we see the same bullet give different results from one rifle to the next for the bc value that lines up with the drops.

Just when I thought I had a grip on stability now I need to learn about sd and what makes it good or bad.

We have done a lot of testing on bullets and their terminal performance relative to sg. Our results have shown that bullets that are below a 1.5sg will have trouble giving satisfactory terminal performance. From our testing a bullet should have at minimum a 1.5sg for hunting. The faster the rotational spin of a bullet is the longer it stays point oriented after contact, resulting in bullets performing as they are designed. Less than 1.5sg increases the chance of bullets failing to expand. We do our testing at short range for the convenience of it, but I have witnessed marginal stability resulting in poor terminal performance on animals, but according to the Hornady model that bullet would have been well over 1.5sg by the time it impacted the animal.

At this point I am going to stick with what I have seen with test results and the need for 1.5sg at the muzzle for good flight ballistics and terminal performance. In fact this year I am changing to lighter bullets with higher sg for hunting and adjusting my max range accordingly to ensure better terminal performance. I have made the decision that better terminal performance out weighs the extended range of the higher bc bullets that are not as high for sg.

Steve

 

[HARPERC]

I won't say I understand it, but I operate under the general rule of buying barrels with as much twist as I can get.
Given the choice, (can't rebarrel them all every time a new bullet shows up) I'd much rather increase twist, and use the heavy for caliber bullets, but can see the validity of your move to lighter bullets. Especially given the bullets you are using, not sure I could/would say that for other bullet types.

 

[RockyMtnMT]

This guy has some good info on his site.

http://bisonballistics.com/articles/bullet-stability

Steve

 

[Mikecr]

As far as I know, dynamic stability cannot be predicted. It's tested for.
But you can sleep well with an Sg of 1.5 at the muzzle. Good plan

 

[tbrice23]

Have you checked Sierra's BC listing? I just looked, according to them it appears that the BC varies depending on which type of their bullet is being tested.

It seems their Match king decreases consistently throughout all calibers, iirc.

BTW, I'm not familiar with their testing protocol, procedures or parameters compared to Hornady.

 

[dfanonymous]

It was pointed out to me that bullets become more stable as they slow down. It appears that this is indeed true. I can not find out how much stability increases relative to the velocity of the bullet though. The Hornady calculator show huge increases in stability as the bullet slows down in flight. This does not make sense to me for a couple of reasons. It does not line up with range test results and it does not make sense with the findings of Litz showing the need for a min of 1.5sg for optimal bc. The Hornady calculator shows a 1.4sg bullet becoming higher than 1.5sg inside of 100y of slowing down and over 5.0 at 1500y. So if the Hornady data is correct the 1.5sg minimum would be of very little factor at long range as related to bc.

Turns out that there is also dynamic stability (sd) to go along with gyroscopic stability (sg). So far what I have found is that sd may be the larger contributor to unstable bullet performance down range. I think this has something to do with why we see the same bullet give different results from one rifle to the next for the bc value that lines up with the drops.

Just when I thought I had a grip on stability now I need to learn about sd and what makes it good or bad.

We have done a lot of testing on bullets and their terminal performance relative to sg. Our results have shown that bullets that are below a 1.5sg will have trouble giving satisfactory terminal performance. From our testing a bullet should have at minimum a 1.5sg for hunting. The faster the rotational spin of a bullet is the longer it stays point oriented after contact, resulting in bullets performing as they are designed. Less than 1.5sg increases the chance of bullets failing to expand. We do our testing at short range for the convenience of it, but I have witnessed marginal stability resulting in poor terminal performance on animals, but according to the Hornady model that bullet would have been well over 1.5sg by the time it impacted the animal.

At this point I am going to stick with what I have seen with test results and the need for 1.5sg at the muzzle for good flight ballistics and terminal performance. In fact this year I am changing to lighter bullets with higher sg for hunting and adjusting my max range accordingly to ensure better terminal performance. I have made the decision that better terminal performance out weighs the extended range of the higher bc bullets that are not as high for sg.

Steve

miller stability calculator

 

[RockyMtnMT]

miller stability calculator

Yes that is what we use to design our bullets. Which explains my ignorance of what is going on after the bullet leaves the rifle. Bullet stability increasing as the down range vel decreases was never in my mind. It just goes against 'my' conventional thinking. Most bullets show a decrease in bc with lower vel. So as they travel from beginning to end the bc will tend to fall off down range as the vel decreases. Some designs show increases as they slow down but not most. So generally a faster launch will result in an over all higher bc.

For now we will stick with what works very well. Our design that is super accurate and sg's that are at 1.5 or higher. Sometimes it is better to be lucky than good.

Steve

 

[bigngreen]

In all my bullet test I've seen many bullets stable out to 600 yards but fall completely apart by 1000 yards, the idea of once stable always stable is completely false!!

 

[Mikecr]

Gyroscopic stability is easier to understand if you look at from a perspective of displacement per turn -to overcome that displacement.
That is, displacement(drag) is an overturning force, and each turn gyroscopically counters this.

This is how stability requirements are declared. For example: 8:1, is 8" of displacement per turn, and that displacement is tied to an atmospheric standard(usually same as BC is tied to).
Because velocity slows more than rotation rate, Sg usually goes up downrange as the bullet is displacing less and less overturning drag per each turn.

Normally, muzzle release is the biggest challenge to stability(muzzle pressure slapping the back of bullets where Sg is lowest). So if the bullet makes it 10yds without tumbling, it's usually good to go.
The next aerodynamic challenge is transonic. This is an issue, if dynamic stability is worse than gyroscopic stability can control. If I remember right, the 155smk is a known offender here.
Notice no bullet makers list or guarantee BC/Sg numbers at or below transonic. There are no public ballistic calculators that credit numbers at or below transonic as valid. This is because dynamic stability is an unknown.

 

[RockyMtnMT]

I think the center of gravity of a projectile has a lot to do with down range stability. Keeping the sd and sg simultaneously good though out the entire range of flight.

Steve

 

[gohring3006]

You hear guys talk about loads falling apart, and you got guys who shoot sub Moa groups at 1500 yards with bullets that are marginally stabile. The 215 hybrid comes to mind. So I believe there is some other force affecting flight, and bullet upset.
Like what Mike said about muzzle exit pressures and Steves center of gravity statement.

 

[RockyMtnMT]

The effects on the bullet as it leaves the muzzle is where I think we got lucky. We created our design that we patented it was not with the thought of how it would exit the bore. We were trying to create a better way to minimize bore contact yet keep enough contact with the rifling to not have trouble with spinning out. Also keep seating depth adjustment freedom. The only thing that we can come up with that explains the extreme ease of load development in virtually every rifle, has to do with how the bullet leaves the bore. That is what I mean when I say we got lucky. We started with the fully tangent ogive as it is the most forgiving. As we mess with other designs nothing seems to change when it comes to ease of accuracy. They all seem to act the same. Just got a call from a customer yesterday that is shooting a 25 Nosler with a 10" twist. Very difficult to get a very heavy bullet with solid copper that will not tumble. I have him shooting our 116g Sledge Hammer as it is the heaviest that we have that will stabilize in the 10" twist. He called to let me know that he was at 3700fps and shooting sub 1" at 200y. This bullet is 1.2" long and almost all baring surface. No boat tail and a very short nose with a large dia hollow point. Just as heavy as we could make a 25cal and have it be bullet shaped to stay stable in the 10" twist. Not what I would call a visually attractive bullet but shoots lights out.

Steve

 

[dfanonymous]

Yes that is what we use to design our bullets. Which explains my ignorance of what is going on after the bullet leaves the rifle. Bullet stability increasing as the down range vel decreases was never in my mind. It just goes against 'my' conventional thinking. Most bullets show a decrease in bc with lower vel. So as they travel from beginning to end the bc will tend to fall off down range as the vel decreases. Some designs show increases as they slow down but not most. So generally a faster launch will result in an over all higher bc.

For now we will stick with what works very well. Our design that is super accurate and sg's that are at 1.5 or higher. Sometimes it is better to be lucky than good.

Steve

stability is more like it doesnt, then it does, then it doesnt.
All long body bullets yaw when they first leave the muzzle. to say what distance a projectile stabilizes depends on quite a few different things. if you want to get into the weeds with it, look up stuff on magnus effect of bullets.

 

[Laguna Freak]

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