Greenhill Formula

K

Kenneth L. Walters

New member
As I understand it, the Greenhill formula allows you to compute from a bullet's diameter and length the rate of twist for which it would be ideally suited.

Suppose that a particular bullet was suited for a rate of twist of 1 in 20 but your rifle has a rate of twist of 1 in 10. What exactly does that mean? Will the bullet be adequately stabilized, overstabilized or what?
 
Theoretically, the bullet would be overstabilized.

Then formula is most easily remembered and worked with in this form:

rate of twist = 150 x diameter squared / length of bullet

I understand that the formula is most accurate for a pure lead bullet.

You might want to search for bullets for your particular rifle based on their length as selection criteria. Just manipulate the formula to:

length of bullet = 150 x diameter squared / rate of twist

Hope this helps answer the question.

-RR-


Ooops! I think now that I see your question was just as much about rate of twist as it was about the Greenhill formula.

A "one-in-ten" rate of twist means that the bullet will rotate one time for each ten inches of travel in the bore.

There. I think we got it now!
 
??

At least some 308's have a twist of 1 in 10. Suppose a cast bullet would be stabilized by a rate of twist of 1 in 20. What I'm confused about is would this bullet be adequately stabilized in this particular 308?
 
I'll try not to confuse the issue further. :)

A 1:20 twist actually, way too slow for any .308 bullet) would stabilize a very short bullet. Very short = very light, as in round ball, maybe. Staying away from the issue of velocity, note that the old black powder guns which used a round lead ball had a twist rate around 1:40.

In a 30-caliber rifle, if all you were going to shoot were 110-grain bullets, use a 1:12 twist. For 150-only, maybe 1:10. 180- and 220-grain bullets, maybe 1:9. I'm "ballparking", is all.

You're trying to throw a spiral with a tiny football. The longer it is, the faster ya gotta spin it to have stability, so the faster the rate of twist.

Bullets which spin either too fast or too slow will tumble and thus be erratic. Well, you probably already know that.

I recommend http://www.povn.com/~4n6/index.htm for some info. "How Bullets Fly", with a minimum of exotic equations and such...

Hope this helped a little, anyhow.

:), Art
 
Greenhill's formula is also further broken down by calibre. The constants are different for large and small bullets (I can't remember where I got the formula from in program form, but I think it was Sniper Country. It was a program that you entered calibre, length, and weight, and got twist. The breakpoint for .308 for 1-in-10 was around 171-172 grains. run a search on the net for this program...best of luck!!!
 
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