Twist Rate and stability

[BrentM]

Looking at Bergers site I play with this calculator. I have been wondering about my 204 bullets and how they match up. Current set up is under 1, which according to their info is not good enough.

How much stock do you put into this number?

 

[Mikecr]

Sg at 1.0 means instant tumble

 

[BrentM]

Hmmm. Tell me what you think please.

3800 fps
1/12 twist
40 grain Vmax. Measures .75 tip to base.

Using those numbers at 30F and 4000 ft I get .93. I shot a 4 shot group at 500 yards that measured 2". I had 1 shot that I pulled another 2" left by accident. My finger was so cold I was playing with the trigger for feel and BANG. LOL

 

[MontanaRifleman]

I think with the small bullets variables like plastic tips become much more important. When I run the numbers for the 53 gr Vmax out of a measured 14" twist 22-250, it spits out a .86 Sg and they seem to stalize just fine out of my rifle.

I'm guessing the Berger calculator might be tweaked to reflect the stability of their cup and core HP VLD bullets.

There are a lot of variables in the stability formula such as specific gravity, length and shape of boat tail if any, length and shape of nose, specific gravity of the bullet, etc., that Berger doesn't include in the inputs.

The plastic tip alters the specific gravity of the bullet and also pushes the CG back

 

[bigngreen]

If you measure the metal part of the bullet from base to where the tip starts I can give you a number that take a polymer tip into account , bet your in the 1.2 range though.

 

[BrentM]

Their system doesn't help or hinder their bullet line. It is just a plug and play unit that is not brand specific. I see your point about design etc though. I am sure more goes into a bullet design then I will ever understand. I just find it interesting that this bullet seems to fly just fine even though the computer says it should not

I was kind of thinking about trying the 55 grain 204 at .381 bc from Berger. If I can get it to fly around 3500 fps it will retain a boat load of energy compared to the lower BC 40 grain bullet. But when I look at the numbers of the stability issue I was unsure what to do.

It puts out as much energy at 700 yards as a 223 does at 400. I would shoot dogs at 400 with a 223 no problem.

Tinkering is fun. Just not sure what to think of the issue.

 

[MontanaRifleman]

Their system doesn't help or hinder their bullet line. It is just a plug and play unit that is not brand specific.

Not sure about this one way or another. I suspect it is "type" specific vs "brand" specific. I don't think it's intended to promote their line, but just an aid in determining the best bullet for your application and conditions

 

[Mikecr]

The plastic tip alters the specific gravity of the bullet and also pushes the CG back

Moving the CG backward is destabilizing though.
I've never understood why plastic tipped bullets are more stable than same form of non-tipped.

The Miller stability formula won't work with these.

 

[Deleted member 46119]

Moving the CG backward is destabilizing though.
I've never understood why plastic tipped bullets are more stable than same form of non-tipped.

The Miller stability formula won't work with these.

If I understand it correctly, the lower density of plastic does not generate a destabilizing mass forward of the CG. Takes less to 'go to sleep'.

Just grabbing the last JBM bullet flight times on my page.

~200,000 to 250,000 rpm
~20,000 to 25,000 revolutions in 100 yards. (.089)
~200,000 to 250,000 revolutions in 800 yards. (1.038)

Made me think about the going to sleep part. Like a thrown foot ball that starts a little wobbly then smooths out. I know this is a poor analogy but can't think of anything closer.

 

[BrentM]

If you measure the metal part of the bullet from base to where the tip starts I can give you a number that take a polymer tip into account , bet your in the 1.2 range though.

Tip to base is .75. I have not measured without the tip. .68 is a guess.

 

[bigngreen]

Tip to base is .75. I have not measured without the tip. .68 is a guess.

That puts you at 1.12 Stability Factor when figuring in the poly tip. Bottom line if they shoot, they shoot!

 

[BrentM]

What program are you using? Shooter and Berger are lower, I realize it doesn't matter that much if they shoot fine. Just trying to decide if this factor is even worth looking at any more. I using like to look at it for fun and listen to people discuss the subject about over or under stabilizing the bullet.