I just ordered a 338 RUM in the rem XCR II for fun of shooting and western hunting. With a 1:10 twist what bullet weight should I start with? I really only shoot out to 600 yds max. I don't know if that qualifies as long range or not. And while we are on the topic I have a 7 Wby with a 1:10 twist - any weight recs fot that. Thanks for the help. Jack

I think you will be good up to 250 grain cup and core bullets and possibly heavier. My Rem Sendero in 338RUM had a 1:9.25 twist rate and was a real bug shooter.

160gr for the wby. You dont say for what?

Stability and twist rates are actually dictated by the length of the bullet not the weight although for a given caliber, length and weight do correlate as long as the bullet construction is the same. That being said, in my somewhat limited experience, twist rate seems to be more critical in smaller calibers so in your case the factory twists should stabilize just about any bullet available.

accurateshooter.net/Blog/millerformula.xls


http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

http://www.jbmballistics.com/b...ller_stability_1.pdf

http://www.jbmballistics.com/b...ller_stability_2.pdf

Thanks for those links. I have a much better understanding of stability. Not so simple actually.

quote:

Originally posted by Blacktailer:
Stability and twist rates are actually dictated by the length of the bullet not the weight although for a given caliber, length and weight do correlate as long as the bullet construction is the same. That being said, in my somewhat limited experience, twist rate seems to be more critical in smaller calibers so in your case the factory twists should stabilize just about any bullet available.

He said it very well

In the Greenhill formula we see that length is used in order to calculate the required twist rate to stabilize a projectile, based on the specific gravity of the bullet material used, and at the time (1879) it was stated at 10.9 for a jacketed lead-core bullet. Needless to say that if a lighter or a heavier bullet material is being used that the required twist rate would also change.

Thus the Greenhill formula is a simple approximation based on a watered down version of the stability criterion for ease of use. The inherit shortcoming in the Greenhill formula is that it assumes a specific gravity of 10.9 for the bullet based on a shape of a RN ogived lead-core bullet and so does not cater for long low-drag bullets that are boat-tailed with sharp ogived nose shapes that are often plastic tipped or having hollow point noses.



The Gyro theorem was developed much later and describes the condition of stability (SF = stability Factor) more precisely. In the Gyro theorem we see that the stability of a spinning body is related to a number of conditions and parameters, and they are:

1. The axial moment of inertia (Ja)
2. The transverse moment of inertia ( Jq)
3. Spin rate or angular velocity of the projectile
4. Bullet diameter - the presenting reference area of the projectile in motion
5. Bullet velocity
6. Air density - the mass density of the medium in which the bullet is moving
7. The overturning moment coefficient

Here we see that length of the bullet per se is not an absolute factor, but rather primarily bullet mass and its relationship of the moments of inertia (axial & transverse) of the spinning bullet.

Thus we see, the shorter the bullet, the greater the stability and vice versa the longer the bullet is. This arises form the relationship of transverse moment of inertia ( Jq ) being equal to ½ the product of mass times the square of the radius of the of the bullet through the CG. Thus making the radius of the bullet bigger has a huge influence, as does increasing the mass of the bullet.

The energy needed to overturn the bullet is directly related to the transverse moment of inertia (Jq); if Jq is small then it’s easier for the bullet to overturn. (SF = 1.0 to be just air stabilized)

Warrior