Help me understand stability and bullet construction

[JPeelen]

There are Intlift and Intlift2 on the JBM website. Which one would you recommend for small arms bullets?

 

[dcali]

There are Intlift and Intlift2 on the JBM website. Which one would you recommend for small arms bullets?

Intlift 2 is the same program with a bug fix and some minor changes in the display code. I used that code to rewrite it in a modern computer language and it seems to work fine within its limitations.

The original program that Ballisticboy created for larger caliber weapons is not on JBM. Bob McCoy used it as a basis for Intlift, although I am not aware of what exactly he did or why. I just know he adapted it for small arms use. The code itself is fairly opaque and without documentation.

 

[DocUSMCRetired]

I don’t know why Berger is saying 13. My BIBs at .972” for what it’s worth. Are you looking at Sg of 1.0? When I put .972” and 118 into Berger at 2850 FPS, with a 13 twist, I get about 2.0 Sg.

I also don’t know why Berger has a field for BC, as it doesn’t matter at all.

The reason for the BC slot is because it calculates the degradation in BC for SG less than 1.5 then spits it out for you. Showing the BC loss from being in the non optimized zone. It is not a part of the stability calculation.

 

[DocUSMCRetired]

https://www.amazon.com/dp/0764338250/?tag=accuratescom-20
Robert McCoy's work is math heavy reading, but also the most comprehensive information on this topic.
There is an appendix of corrections out there (for some of the math).
Much of McCoy's endeavor was put into software by Bill Davis (which also has a few errors/assumptions).
The two go hand in hand to follow. Lija Barrels provided:
http://riflebarrels.com/bullet-design/

You do not need the correction sheet with V2 of McCoys book. The corrections have been implemented into the book.

 

[DocUSMCRetired]

If you plug in a 1/19 twist, which is Bergers’s recommendation for their bullet, it comes out with the SG in the low 9’s and recommends the 1/13 twist. I used your velocity of 2500.

They use 1.5 as a minimum recommended SG. To ensure max BC, and openly state and provide a linke to the recommended twists for their fb bullets to keep knuckleheads like me from asking questions.

This helps refine my question,
It the calculator off, or is it that flat base bullets are more stable at a lower SG and it’s the recommended twist, based on a boat tail, that is off?

The Bergers are live fire tested in the lab and then calculated.

 

[dellet]

The Bergers are live fire tested in the lab and then calculated.

Can you elaborate a bit on how stability is checked with live fire?

The base of my question came from what I was seeing on target vs what the calculators predicted. Vertical drop was no surprise, but expecting keyholes and getting round holes raised the question of how to predict stability with any certainty on the low end.

 

[fyrewall]

Any evaluations or comments on Hornady's 4DF calculations?

Programming with modern computer languages - BASIC, FORTRAN, "C" ? - I don't know much about stability math yet but can program real good - need compiler.

Playing around with the Miller approximator and my modified version for plastic tipped (Courtney-Miller) on Excel I was able to duplicate the stuff for plastic tipped bullets on JBM. Treats all bullets as monolithic. What's the term for aero effects on the light weight plastic tips?

I, also expected sideways bullet hits but got nice round holes with Sg values well under 1.2 using the Miller approximator at 100 at 75 degrees F. The problem was all my rifles have moderately fast twists - no more 1-14, .224's to play around with 90 grain bullets.

 

[dcali]

Any evaluations or comments on Hornady's 4DF calculations?

Programming with modern computer languages - BASIC, FORTRAN, "C" ? - I don't know much about stability math yet but can program real good - need compiler.

Playing around with the Miller approximator and my modified version for plastic tipped (Courtney-Miller) on Excel I was able to duplicate the stuff for plastic tipped bullets on JBM. Treats all bullets as monolithic. What's the term for aero effects on the light weight plastic tips?

I, also expected sideways bullet hits but got nice round holes with Sg values well under 1.2 using the Miller approximator at 100 at 75 degrees F. The problem was all my rifles have moderately fast twists - no more 1-14, .224's to play around with 90 grain bullets.

Last I heard, Hornady calculates the necessary coefficients with PRODAS.

 

[fyrewall]

Thank you - I have much to learn.

 

[DocUSMCRetired]

Can you elaborate a bit on how stability is checked with live fire?

The base of my question came from what I was seeing on target vs what the calculators predicted. Vertical drop was no surprise, but expecting keyholes and getting round holes raised the question of how to predict stability with any certainty on the low end.

By using barrels of different twist rates/directions, and measuring the rotational timing of the bullet in flight near the barrel and down range. I know we published spin testing in the Modern Advancements Series of books.

 

[dellet]

By using barrels of different twist rates/directions, and measuring the rotational timing of the bullet in flight near the barrel and down range. I know we published spin testing in the Modern Advancements Series of books.

I will revisit those chapters, but from memory they were written from the point of achieving maximum BC, which could be an indicator of best stability. I do not remember any mention of a flat base bullet other than the experiments done squaring the heal on a few boat tails.

The Modern Advancement series really is geared toward long range, so understandably does not really address issues of what would be considered a short range bullet design.

Berger really did provide a great service by providing twist rates for all their bullets that don't fit the calculator/Miller formula.
My question is how did they arrive at those numbers?

Ballistic calculators differentiate between G1 and G7, seems like stability calculations should also, or Berger would not have gone the extra mile by publishing those numbers separate for their flat base bullets.

 

[JPeelen]

I am afraid that G1 as well as G7 are provided mostly because many shooters tend to stick with G1. And manufacturers keep publishing G1 ballistic coefficients, because they are higher (for example G1 0.504 vs. G7 0.258 for the same bullet) and therefore look much better on paper.
As a matter of fact, each projectile has its individulal drag curve. Modern designs with a slender ogive are better described by the G7 drag curve than by G1. A range table computed using G1 (measured at short range) gives the user the illusion of higher velocities at longer ranges. So G1 is here to stay.

 

[DocUSMCRetired]

I will revisit those chapters, but from memory they were written from the point of achieving maximum BC, which could be an indicator of best stability. I do not remember any mention of a flat base bullet other than the experiments done squaring the heal on a few boat tails.

The Modern Advancement series really is geared toward long range, so understandably does not really address issues of what would be considered a short range bullet design.

Berger really did provide a great service by providing twist rates for all their bullets that don't fit the calculator/Miller formula.
My question is how did they arrive at those numbers?

Ballistic calculators differentiate between G1 and G7, seems like stability calculations should also, or Berger would not have gone the extra mile by publishing those numbers separate for their flat base bullets.

We do all of the testing for Bergers numbers in our Laboratory. We actually do testing for a number of companies, and consultation work. Some companies you would be surprised to learn about. They came about the numbers through our lab work, where we measure the spin rate, and spin rate decay in our lab. We have tools that can measure the bullets RPM at specific distances and you can use a capture card to determine the pitch/yaw down range to see different levels of stability. Plus other systems.