The secret behind the system is based on mirrors. Mechanical systems moves a mirror at the same speed as the projectile. The angle of reflection allows the camera to film the shell at high speed.
Systems like this have been used for many years to film incredibly short-lived events, such as the first moments of a nuclear explosion.
This revolutionary C4 camera system was developed in the UK by British scientists after World War II. It consisted of a rotating mirror weighing over two tons. It held the speed record at the time of photographing 7,000,000 frames a second.