Greenhill
Formula for Rifling Twists
The
Timehonored Greenhill formula was originally used for determining
twist rates in the new rifled artillery in the 19th Century..
It
is
important to note that it is projectile *length*, not *weight*
which is
the determining factor when considering the best twist rate for
your
intended use. Weight can vary for a specific bullet length from
use of
different material and also is determined to some degree by projectile
shape. Often longrange match shooters utilizing low drag or
VLD
projectiles will voice a preference for "lazy" twists;
i.e. just enough
to stabilize the bullet. The requirements of a Shooter who is utilizing
both high velocity and Subsonic ammunition are quite different.
When
utilizing subsonic ammunition there is another factor to take
into consideration. The projectile not only has
only (about) 1/3 of the
forward velocity of standard ammunition in Rifle calibers, it
also has
1/3 of the *rotational* velocity...think "RPM's". This
makes for a less
gyroscopicallystable projectile, so a faster rate of twist is
indicated
than by simply applying Greenhill alone. This wisdom was imparted
to me
in a conversation with the great barrelmaker Boots Obermeyer.
It makes
perfect sense, and doubly so coming from him, so we'll take it
as Gospel.
Greenhill Formula For Rifling Twists T*B=150*Sqrt (Density of Lead/Density of Bullet)
T=Twist
B= Bullet length
Both units have to be in 'Calibers', density portion is optional
**********
To
determine the *approximate* twist rate for a given projectile
(this
varies a bit by shape):
Using
the a 190 gr Sierra MatchKing bullet #2210M (.,30 cal) as an
example. If we measure the bullet's length it is 1.375" long .
B=Bullet
length in *Calibers* (Length/Diameter), hence 1.375/0.308= 4.46
*calibers* long
From
formula we have T=150/4.46=33.6. This figure is now in *calibers*,
so we convert the twist from calibers to inches. So
T=33.6 x
0.308=10.34".
This
is why most commercial rifles in .300 Win. Magnum come with a
1:10" twist barrel.
***********
One
can go further and get involved in formulas for density, but
for
most conventional lead/gilding metal projectiles, the above will
suffice. Note that this is for a normal High Velocity loading.
Subsonics generally require a somewhat faster twist rate. If one
plans
to use both High Velocity and Subsonic loads in the same platform,
a
good compromise which works acceptably for both must be found.
If the
platform is a "dedicated" gun in which subsonic ammunition
alone will be
used, then a faster rate can be utilized in order to be able to
accurately shoot longer (and thus heavier) projectiles subsonically
and
with good accuracy.
Example:
Twist rates for 7.62mm/.308:
High
Velocity only: 1:111/8" to 1:12"
Both: 1:10"
Subsonic ONLY: 1:8"
Bear
in mind that "dedicated" back at the shop might
not be "dedicated" in the field. Most knowledgeable
end users prefer to have the option of
utilizing both loads so as to be able to address changing tactical
requirements on the spot.
