Page 137 - FAT 105 4.17 MOODLE

canted to one side and some of the edges inside the bore within 7” of barrel length.
are beveled. Ratchet rifling is a form of Various twist rates are used for different
canted land rifling. calibers to stabilize various projectiles.
Typically, the longer the projectile, the
Polygonal rifling differs from traditional
faster the rate must be to properly stabi-
rifling significantly. All sharp edges are
lize it (1 in 7”, 1 in 6”). The shorter the
done away with and lands and grooves are
projectile, the slower the twist can be (1
replaced with “hills and valleys.” The hills
in 16”, 1 in 18”). Comparing twist rates
and valleys are a series of arcs inside the
and long and short projectiles to foot-
bore. The transition between the hills and
balls and baseballs (respectively) makes
valleys is smooth and leaves little to no
understanding twist and stabilization
deformation in the bullet.
much easier. When a baseball is thrown,
it requires little to no spin to properly
The type and shape are not the only im-
stabilize in flight. But when a football is
portant factors when discussing rifling;
thrown, if it is not spinning fairly quickly,
“twist rate” can be more important for the
it will wobble in flight.
stabilization of the projectile than rifling.
Twist rate is a relationship between the
Another critical area of the barrel is the
rifling and the distance it spans while
muzzle crown. The shape of the muzzle is
twisting through the barrel. For example,
critical for the accuracy and precision of
a twist rate of 1 in 7” means that the ri-
any firearm. Because the muzzle is the last
fling will complete one full revolution
part of the firearm to touch the projectile,

Figure 11: Various muzzle crowns.

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