Abstract: An analysis of the forces acting on the SA80 assault rifle during firing was undertaken to investigate barrel rotation whilst the bullet was travelling in the barrel. Bullet displacement and velocity from initial movement to the point when the bullet left the barrel were first calculated. Two separate numerical methods were used to calculate barrel angular displacement and angular velocity against time caused by the propellant pressure acting on the breech face and the angular acceleration of the bullet. The two different methods gave good agreement, giving a barrel angular displacement of 0.066 rad and an angular velocity of 3.75 x 10-4 rad/s respectively.