The Greenhill formula

[Russell3812]

THE GREENHILL FORMULA

Twist = 150 x Diameter squared / Length

So... I've been reading the ABCs of reloading by Rodney James and now found something else to drive me crazy. I've just started purchasing reloading equipment, have my single stage press and a good set of dies. So now it's time to buy some bullets and after reading this I've come to a road block.

If I am understanding this correctly the greenhill formula will tell me what size/length of bullet would be optimal for the rifle I own which is a 308 with a 1-10" twist. According to the author if the bullet is too long or short it will not group well unless matched with the proper rifle twist. I see a lot of information regarding regarding the bullets weight and ballistic coefficients but nothing from the manufacturer about the bullets length. Am I reading this incorrectly? Does the length of the bullet matter and where is that information? Thank you.

 

[bohem]

Re: The Greenhill formula

The greenhill formula breaks down with bullets that are not largely shaped like a simple cylinder, meaning that almost everything you're likely to shoot now is not going to work well in it.

Look at the Miller Stability formula to estimate stability.

JBM has a Miller calculator available and it is much more accurate than the Greenhill solution will be for modern rifle projectiles.

 

[Russell3812]

Re: The Greenhill formula

Thank you. I found an example of this at:

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

But even that formula requires the bullet length as part of the equation.

Do you consider the bullet length in relation to the rifling when purchasing bullets?

 

[Russell3812]

Re: The Greenhill formula

Well after looking at JBMs web site I found someone has published a list of bullet lengths matched to caliber.

http://www.jbmballistics.com/ballistics/lengths/lengths.shtml

Still questioning how important the length of the bullet is in comparison to the rifling. If anyone has experience with this please reply.

Thank you bohem for directing me to JBM I really appreciate it

 

[Longshot38]

Re: The Greenhill formula

Put simply those formulas only give any idea of how a bullet will preform. This is because of variations in bullet design. An example is that a monolithic bore rider design has different grove requirements then say a swagged VLD style bullet. This is largely due to bearing surface but a couple of other factors come into play.

A simple answer to your question is that a 10 twist barrel is plenty capable of stabilizing any swagged 30 caliber bullet on the market today. Also the best source for information about stability factors with a given bullet is with the manufacturer. Those companies spent a lot of time and money developing their products and they really do have a clue.