Dear Student,
Recoiling of gun is an application of Newton's third law, which states that for every action there is equal and opposite reaction.So,when a bullet is fired from a gun, the gun exerts a force on the bullet in the forward direction. This is the action force. The bullet also exerts an equal force on the gun in the backward direction. This is the reaction force. Due to the large mass of the gun it moves only a little distance backward by giving a jerk at the shoulder of the gunman. The backward movement of the gun is called the recoil of the gun.
Regards