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Posted: 9/27/2004 6:45:09 PM EST
Hi Ar'ers.
I'm considering buying another AR for Targer shooting.
I am needing to know some information. Please forgive me if this has been asked before.
what bullet grain weight will the
1:10"
1:9"
1:8"
stablize

what twist would be the best ratio of twist for shooting 50-300 yards? What twist ratio would be the best for shooting 300 - 600 yards? What Twist ratio would be best for 600 and greater yards?

is this correct?
40-55 is 1:10
55-70 is 1:9
70-80 is 1:8
80+ is 1:7
Link Posted: 9/27/2004 7:31:39 PM EST
Never mind! I found it. It's the Greenhill formula:

6. How to Choose the Proper Rifling Twist Rate
by Normal F. Johnson (njohnson@nosc.mil)
The Greenhill formula is an empirical equation that does a good job of establishing the barrel twist necessary so that a bullet of a given length will be adequately stabilized.

Greenhill made easy:



T=150 x D
---
R

here: T is the twist required (number of inches for one revolu- tion),
D is the bullet diameter (in inches)

R is the bullet length to diameter ratio, (length divided by diameter)

Conversely, to find out what length bullet will be stabilized in a given twist, use:



L=150 x D x D
-----
T (that is, 150 x D squared divided by T)

where L= bullet length
The number 150 is a constant used by Greenhill and works well at velocities in the vicinity of 1500 fps or greater. At 2800 fps the constant can be changed to 180 with good results.

Substatute the Constant of 150 (1500 fps) with 180 for a (velosity of 2800 fps)

Thanks anyway!

Brian
Link Posted: 9/27/2004 8:18:43 PM EST
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