there are many different borresight units. they all atach to the barrel and you line up your scope with the mark made with the boresighter. then you go to range and shoot the firearm to zero the firearm. the boresighter is only to get you on paper hopefully. if you have any other question as me. without the boresiter you can go to the range and bench closely to see where you are hitting then ajust --old fashon but still works.