External ballistics

Question 1

Centre of pressure (CoP)

Answer 1

• The point where the aerodynamic forces act after adding them all up
• Should be close to the centre of mass (CoM) for the best stability
• Normal bullets will have their CoP infront of the CoM, this makes the bullet want to tumble - this is why the bullet needs gyroscopic stabilisation to overcome the want to tumble and keep the bullet pointing towards the impact point

Question 2

Drag stabilisation - how is it achieved and what does it do

Answer 2

• Achieved by adding fins to the projectile, which creates additional drag forces at the rear of the projectile
• This brings the CoP rearward of the CoM, leading to a smaller and less influential turning moment
• This results in a stable flight path without the need for any gyroscopic stabilisation

Question 3

Drag coefficient (Cd)

Answer 3

The total drag force that is experienced by the projectile

Question 4

Ballistic coefficient (Cb)

Answer 4

The measure of the aerodynamic forces exerted on a particular bullet in flight

Question 5

Yaw

Answer 5

• The lateral movement of the nose of the bullet away from the line of flight
• Caused by: poorly cast bullet or bad loading causing the bullet to be off centre in the cartridge case neck, irregular rifling or non-optimal spin rate

Question 6

Precession

Answer 6

The rotation of the bullet around the centre of mass

Question 7

Nutation

Answer 7

The small circular movement at the bullet tip due to the tip not being perfectly round

Question 8

Gyroscopic stability

Answer 8

• The resistance of a rotating body to a change in its plane of rotation
• A bullet is gyroscopically stable if it responds to an external wind force with nutation and precession

Question 9

The Greenhill formula

What does it mean

Answer 9

It is the optimum twist rate in metres

Question 10

Aerodynamic lift

Answer 10

• Where a ‘boundary laye effect’ occurs whenever air passes over a curved surface
• For example, air moving over the curved upper wing surface travels further than the air passing under the flat lower surface. This creates a pressure drop over the wing and generates lift
• The amount of lift is proportional to the air speed over the surface and the rate of curvature

Question 11

Wind deflection

Two types

Answer 11

1. Aerodynamic: caused by wind flow over the projectile in flight, generating more lift on one side of the bullet due to spin induced pressure difference
2. Windage: deflection caused by a constant wind pressure during projectile flight - as more of an effect on the overall trajectory