I don't think it was used for bore-sighting the guns, but bore-sighting the camera itself. It is an ESK 2000 that was used in training units. It was aligned like a machine gun and operated (instead of the machine guns to save ammo) when the pilot pressed the gun button. More information here:
ESK2000
In this case, I don't think it was used for training purposes, but maybe to shoot an educational or propaganda film. Interesting are the lines on the fuselage at different angles starting from the position of the pilot's head. Maybe they filmed at different angles towards a target on the ground.
Cheers,
Roger