A boresighting device to equip a turret provided with a barrel and one or several sight system(s) each with an optical system includes: a deflection target intended to be positioned outside the barrel, at a muzzle brake of the barrel; a housing intended to be positioned outside the barrel, at a shaft of the barrel. The housing includes: a first optics system provided with a deflection camera, the first optics system being used to determine a parallelism error between a firing line from the shaft and that from the muzzle brake; and a second optics system provided with a boresighting camera, the second optics system being used to determine a parallelism error between the firing line from the shaft and an optics line from the sight system(s). The deflection target integrates a geometric figure serving as a reference point for the first optics system.