Airgun World – July 2019

76 AIRGUN WORLD http://www.airgunshooting.co.uk

TECHNICAL AIRGUN

simply won’t cut it. The easiest of the three to
describe and understand is spin stability, so
we’ll start with that.

SPIN STABILITY
There are two aspects to spin stability, and the
first is the gyroscopic effect, in which the
spinning gyroscopes axis will move so as to try
to point directly into the air flowing over it. As a
result of the gyroscopic stability, the pellet in
flight will be reluctant to deviate from its
attitude when it exited the muzzle and settled
into the airflow. The second effect of spin
stability is equally desirable.
Compared with the pellets we all used a few
decades ago, today’s top pellets are very
consistent in weight and shape, but they are
not perfect. Some imperfections are obvious,
such as dents in the skirt. Another imperfection
manifests itself if you weigh the pellets and find
some are heavier than others, but there’s one
imperfection that’s invisible, and that is an
offset centre of gravity.
If the centre of gravity (CoG) of a pellet shot
through a smoothbore gun is offset from its
axis, the pellet will yaw in flight, so that it will
not be pointing in the direction in which it is
travelling. It will veer to one side, and which
way it will veer will be wholly dependent on
where the offset was (top, bottom, left, right)
when the pellet was loaded into the breech.
This is not just true of pellets; early guns all
suffered the same, with very poor accuracy,
until the projectiles were made to spin in flight,
which happened by chance, not design. Early
gunpowder left sticky residue in the barrel,
which needed a thorough cleaning every few
shots. An unknown gunsmith cut a channel
along the inside of the barrel to act as a
reservoir for the gunge and allow a few more
shots before the barrel needed cleaning, then
another unknown gunsmith cut two channels,
and a third unknown gunsmith cut the

channels in a helix, to make them longer, so

they held even more residue. That was the

accidental birth of rifling.

THE DEVIL IS IN THE DETAIL

It was observed that projectiles from rifled

barrels were more accurate than those from

smoothbore barrels, though nobody at the time

knew why, and the church banned the

manufacture of rifled barrels on the suspicion

that the Devil was sitting astride the bullets and

guiding them to the target! We’re a bit more

enlightened these days, and understand why

projectiles are more accurate if they spin.

When a pellet spins in flight, the axis of the

pellet orbits an offset centre of gravity, which

means that it effectively acts equally all around

the axis, so the pellet does not develop a

permanent yaw in one direction that would

otherwise cause it to veer to one side. Perhaps

it’s worth mentioning that the pellet will wobble

slightly, but for the tiny offsets of centre of

gravity typical of modern pellets, will not veer

by a large distance, but if the offset was large

• for instance, if the pellet was deformed – that
  would cause the pellet to wobble badly in flight,
  and spin stability would not rescue the dismal
  accuracy you’d get from a deformed pellet.

AERODYNAMIC STABILITY

This is the bit most people get wrong, and

where they go wrong is to describe the pellet as

being ‘drag’ stabilised. I have immense

sympathy for those who do believe pellets to be

drag stabilised, because it seems perfectly

logical, and it’s easy to understand or, in this

For accuracy, best stick with quality dome-head pellets

Damage to the muzzle crown can create significant pellet yaw and be ruinous
to accuracy.

If a silencer or muzzle brake is not concentric to the bore, it can cause the

pellet to yaw at the start of its flight