The Bushnell boresighter is designed to get your rifle scope in approximately correct position so that it can be sighted in. Most guns you buy will come boresighted, but if you buy and install your own scope, a boresighter can make the first steps of sighting in your new scope a lot easier.
Mechanics of the Boresighter
A traditional boresighter consists of a low-magnification lens which is attached to a stabilizing mechanism that fits into the bore of the rifle. The boresighter is mounted on top of the barrel, with the small lens pointing at the target area. The fitting is then installed into the bore and adjusted to the point of perfect alignment and then tightened into place. The lens is then pointed directly at the target. The rifle scope can then be adjusted so that the crosshairs are lined up at the center of the lens.
The newer magnetic boresighters available from Bushnell do not require actuall fitting into the bore but attach magnetically. Once you have attached the magnetic boresighter to the barrel, align the crosshairs of the scope to the center of the grid on the boresighter lens by adjusting the windage knobs on the side and top of the scope.
Using the Boresighter
The boresighter is the first step of sighting in the rifle. The purpose of a boresighter is to bypass the long and complicated process of boresighting yourself, which consists of strapping the gun in a vise and looking down the barrel yourself. The boresighter eliminates the need for that extra work, but it does not sight in the rifle. After you have boresighted the rifle, you must still take it to the range to do final sighting in. After boresighting, your rifle is actually likely to shoot well off of zeroed position. Boresighting will sight it in well enough to hit a target at 25 yards, but the fine-tuning must still be done by hand.