These bore sighters or a laser boresighters assists you in sighting in your rifle without firing a shot.  Sighting in your rifle can be costly and even dangerous if the sites are too far out of adjustment.  After you have your rifle sighted in with the bore sighter, it will be necessary to put the final zero on with live ammunition.  Time spent with the bore sighter will save you time, money, and headaches by reducing the number of shots required to sight in.