A boresighting device to equip a turret provided with a barrel and one or several sight system(s) each with an optical system, said device comprising: - a deflection target intended to be positioned outside the barrel, at the muzzle brake of said barrel, - a housing intended to be positioned outside the barrel, at the shaft of said barrel, said housing including: - a first optics system provided with a deflection camera, said first system being used to determine a parallelism error between a firing line from the shaft and that from the muzzle brake, - a second optics system provided with a boresighting camera, said second 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 device being characterized in that the deflection target is devoid of mirror and integrates a geometric figure serving as a reference point for the first optics system, said device being destinated to implement a method of boresighting including the following steps: - Calculating the displacement ΔX and ΔY of the geometric figure relative to a reference position of said figure, said calculation being done based on image processing from the deflection camera, the parallelism error between the firing line from the shaft and that from the muzzle brake next being determined via a mathematical model based on calculated ΔX and ΔY values, - Comparing an image taken by the boresighting camera with an image taken by an optical system of the sight system(s) in order to determine the parallelism error between the firing line from the shaft and the optics line from the sight system(s), - Accumulation of two parallelism errors and displacement of said optics line accordingly. - a deflection target intended to be positioned outside the barrel, at the muzzle brake of said barrel, - a housing intended to be positioned outside the barrel, at the shaft of said barrel, said housing including: - a first optics system provided with a deflection camera, said first system being used to determine a parallelism error between a firing line from the shaft and that from the muzzle brake, - a second optics system provided with a boresighting camera, said second 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 device being characterized in that the deflection target is devoid of mirror and integrates a geometric figure serving as a reference point for the first optics system, said device being destinated to implement a method of boresighting including the following steps: - Calculating the displacement ΔX and ΔY of the geometric figure relative to a reference position of said figure, said calculation being done based on image processing from the deflection camera, the parallelism error between the firing line from the shaft and that from the muzzle brake next being determined via a mathematical model based on calculated ΔX and ΔY values, - Comparing an image taken by the boresighting camera with an image taken by an optical system of the sight system(s) in order to determine the parallelism error between the firing line from the shaft and the optics line from the sight system(s), - Accumulation of two parallelism errors and displacement of said optics line accordingly. - Calculating the displacement ΔX and ΔY of the geometric figure relative to a reference position of said figure, said calculation being done based on image processing from the deflection camera, the parallelism error between the firing line from the shaft and that from the muzzle brake next being determined via a mathematical model based on calculated ΔX and ΔY values, - Comparing an image taken by the boresighting camera with an image taken by an optical system of the sight system(s) in order to determine the parallelism error between the firing line from the shaft and the optics line from the sight system(s), - Accumulation of two parallelism errors and displacement of said optics line accordingly.