The collimator is used in all shotguns to look for the perfect alignment between the eye and the sight (in relation to the rib), both horizontally and vertically.  
 
The collimator must be placed 10 cm from the starting of the barrel, when the sight is perfectly visible from the collimator and the position of the shotgun to the shoulder is correct. It results to be very useful for regulations of the latest ergonomic stocks. Made of aluminium and equipped with a magnet which acts on the rib to avoid movements during the regulation.