Bullet stability question

[Banshee]

Do the formulas for calculating twist rates factor in the polymer tip on modern bullets? It appears to be old when polymer tips are "modern".

The tip weighs little to nothing but is a significant length component, should the calculations be made for the length of the lead/copper only?

 

[RiverRider]

As I understand it, the Greenhill formula was specifically pertinent to lead bullets, but it's still useful in gaining a general ballpark approximation as to what's required.

I would guess that if your bullet is so close to the ragged edge of instability because of the length of the polycarbonate tip, you need to change it up a little. If you arrive at this conclusion by simply calculating, I would shoot it and see. The target doesn't lie.

 

[Desert Fox]

Do the formulas for calculating twist rates factor in the polymer tip on modern bullets? It appears to be old when polymer tips are "modern".

The tip weighs little to nothing but is a significant length component, should the calculations be made for the length of the lead/copper only?

It does have a bearing when using the old Greenhills formula, although probably insignificant. The bullet make-up, their density and their specific gravity can have more impact when choosing the right barrel twist for a particular bullet. Greenhills though as old as the formula is, like the G1 drag model, still works or at least can be use as a base line for todays modern projectiles.

For those interested, I borrowed these from Robert Rinkers "Understanding Ballistics" good book by the way. I recommended everyone to read it - Very informative!

Greenhills formula is base on his role, "The twist required in calibers equals 150 divided by the lenght of bullet in calibers". This is fairly limited in it's application for todays modern bullet since it was based on a jacketed bullet with fairly blunt nose and has a specific gravity of 10.9, Remember Greenhill came up with this formula at the turn of the century.

It's funny, what a coincedence this thread is. I happened to look at this same questions when I switched bullet with my 300 Weatherby, from 180 grains Barnes TSX to !80 TTSX. I noticed that their is a significant difference in bullet lenght when you compared the two. I was wondering at the time if this will affect the stability hence the accuracy of my rifle. Barnes bullet, being pure copper are already built longer than lead core bullet of the same weight as it is. So I was wondering, if this will have an effect on the stability of the bullet for the 300, which has the standard 10 twist. Being an accuracy freak, I left nothing to chance so I did my own little researched. One phone call to Barnes would have answered my questions but my curiosity always got the best or worst of me .

So here's what I got:

Bullet lenght:

1.473" TTSX

Applying Greenhills formula

1.473 divided by.308 caliber = 4.78 calibers long

Now devide 150 by 4.78 = 31.38 caliber

Multiply the bullet diameter, in this case .308 to convert it into inches

31.38 X .308 = 9.66" this is your barrel twist

Compared to the 180 grain TSX using the same formula, I came up with 10.26" twist.

To sum it up, the 10 twist barrel of my 300 Weatherby is just about right for the TSX but a little slow for the TTSX. Can I see the difference in accuracy? Don't know yet! but I suspect It won't affect much.