What is the rule for rate of twist in a barrel versus weight of bullet?

The Greenhill Formula works for conventional bullets. See for example:

http://www.loadammo.com/Topics/July01.htm

jim dodd

Use the link below, that website has a twist rate calculator. You have to sign up to use it but it's free. http://www.realguns.com/

I'm not really sure how much leeway you have from "optimum" twist until you have to worry about over/under stabilizing a bullet-- maybe someone more knowledgable than me can advise you.

Use the Greenhill formula, as HunterJim has suggested. Twist is based upon LENGTH of bullet, rather than weight. For example, a long, spitzer boatail of .30 cal. weighing 220 grains needs a faster twist than a shorter .30 round-nose 220-grain bullet...

Here is a link to a Greenhill calculator that you don't have to sign up to use

http://personal.adiis.net/tenx/Greenhill.html

Also, there are several other variants of this, plus one or more completely different estimators.

I would use caution, however, in applying this. Most of the calculators are for people like me, that shoot lead with blackpowder. For this, they work just "okay". But none of them come close to predicting things like a 14 twist for a standard in a .458 Win Mag. We get by just fine with the 18 twist that Greenhill and it's variants predict.

Brent

Here is another page listing various calibers and twist rates recommended for different bullet weights: Bencrest.com twist rates courtesy of Shilen
- mike