A boresighter is a very helpful, tool when sighting in a rifle to get to a “starting point”, which usually means on paper, or on the target. Once the firearm is hitting the paper, adjustments can be made to the scope to get the bullets hitting where they should be. Without the help of a boresighter, this can be a very time – and ammo – consuming task. The most common type of boresighter is a colimator, which is an optical instrument mounted to the bore of the rifle (or pistol, or shotgun for that matter) and has an internal grid to use for adjusting the scope crosshairs. Most use an arbor of the correct caliber to fit in the bore of the gun to align the colimator. The arbors are seldom real tight fits, and can fall to one side or the other at just the wrong moment. The colimator must be the same height as the scope for accurate adjustment, which is sometimes difficult since scope mounts can be of different heights and even scope diameters are not all the same.
A magnetic colimator uses a strong magnet to attach to the bore, and will not usually move during the sighting process. Proper height can be set by measuring the scope height and placing the colimator accordingly. The Bushnell Magnetic Boresighter I used to get started with my muzzle loader works very well, and is reasonably priced. Leupold also markets a magnetic version.
I was able to get “on paper” at 25 yards with the first shot using the Bushnell Magnetic Boresighter, which has not always been the case for me when using arbor type colimators.