A particular weight bullet will shoot well in a particular twist rate barrel. Is it the length of the bearing surface? Or the actual length of the bullet? Bullet construction, conventional vs. monometal will affect both.
Re: the 375 350 gr. thread.
IIRC Robertson wrote that the 350 out of a 375 stabilized sooner and gave better straight line penetration at close range.
What effect does bullet shape, flat nose vs. spitzer, and construction, lead core vs. monometal (longer to achieve weight) have on stabilization? (assuming bullets are the same weight and velocity)

Thanks, Scott

Generally speaking longer bullets need a faster twist. Compare a 55 grain .223 to a 77 grain, 1 in 16 to 1 in 8. 90's need a 6.5.

It is bullet length and to a significantly lesser extent velocity. Flat nosed bullets will be shorter for a given weight than a spitzer bullet and thus easier to stabilize. A lead bullet will be shorter than monometal bullets (unless you're using depleted uranium)for a given weight and thus easier to stabilize.

I have found the bearing surface of the bullet to be the determining factor for bullet stabilization. One case I have found is when flat based bullets won't stabilize but boat tail bullets of the same weight worked fine.

It is not the length of the bullet per se that is the determinant nor the bearing surface length that determines stability in spin stabilized projectiles.

It is the spatial distribution of mass within form that is the determinant of stability. ie the spatial distribution of mass relative to the projectile's centre of gravity ( or centre of mass if the frame of reference is intertial mass and not gravitational mass).

It is the very same principle that governs why Sectional Density is valid and current in ballistics because it is founded in the same philosophical field of physics.

Length may be used in some approximated formulae such as Greenhill but it then cannot account for bullets made up of materials of mixed density nor projectiles with shape and form where mass may be disproportionately distributed such as load bearing munitions.

Here's some readin for the technophobes, and a simple Miller Stability Factor calculator for us mortals.

quote:

Calculates the Miller stability factor. This formula was derived by Don Miller and published in Precision Shooting. This formula is much better than the antiquated Greenhill's formula. Stability value should be in the range of 1.3 to 2.0 to ensure bullet stability. Don Miller and Dave Brennan (editor of Precision Shooting) have also been kind enough to let me host his stability paper on my exterior ballistics bibliograpy page. The paper is titled A New Rule for Estimating Rifling Twist An Aid to Choosing Bullets and Rifles.

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

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


http://www.jbmballistics.com/b...rs/calculators.shtml

Miller works with jacketed lead bullets but does not work well with copper or brass monos. Use this one, based on Bob McCoy's work for that.

http://www.border-barrels.com/barrel_twist.htm

Molitz and Strobel " Exterior ballistics" in German Aussere Balllistic published by Springer 1963. it is available as a E-book.

It exlains gyrsoscopic stability based on the principles of the effects of mass in motion, both linear and rotational.