Archers had used spiral fletching on arrows for
centuries before firearms were invented. They may not
have understood the physics involved but the arrow
makers learned early on that selecting all the feathers
from the same wing and gluing them at an angle imparted
a spin that improved accuracy. There were even some
crossbows that shot a bolt with no fletching that got
its spin by being oval in cross section, being twisted
and being fired through a tight fitting aperture in a
plate at the “muzzle” of the crossbow.
In answer to the earlier question about why bother with
straight rifling—In a smooth bore WITH A LOOSE FITTING
BALL you can shoot either a knuckle ball or a curve
ball. A curve ball happens when random drag between one
side of the ball and the barrel (at the muzzle) imparts
a spin across the axis of the balls travel. This spin
generates different pressure on the sides of the ball
and causes it to curve just like the deliberate curve a
pitcher can throw. Problem with the curve on a musket
ball is it can break up, down, right or left and to
different degrees between shots.
A tightly patched round ball in a smooth bore is
unlikely to become a curve ball but a patched ball in a
straight rifled barrel can never become a curve ball. It
is a knuckle ball and has that amount of variation in
flight so it is not as accurate a spiral rifling but it
is more accurate than no rifling and has the added
flexibility be being used to fire shot without
disrupting the pattern the way spiral rifling does.
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