Boresighting aligns the optical sight on top of the gun barrel with the axis of your bore, and should be the first priority after mounting your scope. This ensures that your first shot will be on a large piece of target paper at a distance of about fifty yards. Boresighting not only gives you a reference point from which to actually start sighting in your gun, but saves time, ammunition expense, and occasionally your shoulder from recoil.
Bore sighting will NOT sight in your gun. This must be done by firing a specific type of ammunition at a certain distance.