| T=150 x D --- R here: T is the twist required (number of inches for one revolution), 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 f.p.s. or greater. At 2800 f.p.s. the constant can be changed to 180 with good results. Note that it is bullet LENGTH, not weight that is important. Greenhill works well with all lead/lead-alloys commonly used for bullets. the above was written by Normal F. Johnson Minimum twist is one of the many calculators included in the Lee Shooter Program. |
