Bullet stability

[Laguna Freak]

This thread is proving very helpful and may be indicating the presence of flawed paradigms in some of today's conventional wisdom that is often expressed.

 

[MajorSpittle]

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.

Yes, and that is how I read the calculator because there is no way to know what the SD value will be for my gun with a particular bullet and it changes with how that bullet is loaded.

It is good to know that you can actually measure the off axis spin when testing to come up with the calculations for the application.

I assume that since you can do this you would have some great insight into what can be done minimize instability or what rifle/barrel manufactures are creating products that show better stability at lower twist rates? Perhaps with regards to the design of the throats, rifling, actions, crowning, manufacturing process, bedding design...... or if instability can be helped when developing a load by lessening muzzle blast, neck sizing, touching the lands or using a particular muzzle break design?

 

[FEENIX]

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

Me too! I used to just chill with a Scotch and prepare myself for the learning process.

 

[josip89]

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.

Is this because tipped bullet is actualy "shorter" (tip is very light) and when your calculator says that you have 1.5 with berger You actually have more stability with tipped bullet off same lenght?

 

[Mikecr]

I'll try with my understanding of a plastic tipped bullet being more stable than a FMJ given same weight bullet. The reason is increased rotational momentum

To keep a bullet weighing the same while a plastic tip is displacing lead, that lead previously in the nose (at smaller diameter than cal) is moved backward to extend bearing (which is at cal diameter). That's a bit more near cal diameter (bearing) lead spinning, with less under cal diameter(nose) lead spinning.
In other words, that amount of weight is moved further out from the spinning axis.
So while Cg is moved backwards with Cp pretty much the same(de-stabilizing), the extra angular momentum of the longer bearing per weight bullet more than overcomes this.

Hope I got this right

 

[Laguna Freak]

And that results in a shorter yaw moment than the same bullet with lead (or more mass) to the tip???

Effectively reducing the leverage that the Cp can impart against the Cg and centrifugal force rotating around Cg, resulting in relative dynamic force advantage to Cg???

Is that an accurate understanding?

 

[josip89]

More mass farther from the center is better and more stable?

 

[Pdvdh]

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

It was predictable.
A combination of factors, including growth in numbers, increased difficulties of crowd control, and the influence of money. There's only so much time, patience, and effort available to any Forum Administrator. They eventually short cut it, in their efforts to maintain some semblance of crowd control.
Consequently, some invested members move on... No longer invested.

 

[gohring3006]

It was predictable.
A combination of factors, including growth in numbers, increased difficulties of crowd control, and the influence of money. There's only so much time, patience, and effort available to any Forum Administrator. They eventually short cut it, in their efforts to maintain some semblance of crowd control.
Consequently, some invested members move on... No longer invested.

hey phorwath glad to see your back!

 

[MajorSpittle]

More mass farther from the center is better and more stable?

That is what I am guessing because it moves the CG forward so there is less leverage against the CG from the front of the bullet.

So you have two identical bullets with the last 1/3 of the nose cut off (same CG):

You can either make the last 1/3 of the bullet with Lead or make the last 1/3 with Poly that weighs nothing.

If you make it with Lead the CG moves closer to the tip, If you make it with Poly the CG doesn't move.

Now you take both bullets from being level and stationary and drop them, which bullet will turn to fall backend first faster? (poly tip)

Now if you spin the bullets on axis to 200,000 rpm while being suspended by magic, which bullet will resist more force applied to the very tip to knock the bullet off axis? (i suspect the lead tip because it's tip is closer to the CG)

This is how I picture it working anyway.

Another way would be two identical Tops spun at the same speed but one of them has an extended tip that makes it stand twice as tall. Which top will go longer before falling over.