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Boresighting is a method of adjustment to an optical firearm sight or iron sights, to align the firearm barrel and sights. This method is usually used to pre-align the sights, which makes zeroing much faster. The process can be accomplished with the naked eye, or with a specialized device called a boresighter. It consists of an optical head and a bore-diameter arbor which is inserted into the muzzle of the rifle. The optical head is then attached to the protruding end of the rod. A grid pattern on the optical head is then used to align the sights with the barrel.
Traditional boresighting, as the name suggests, involves removing the bolt and exposing the breech, then sighting down the bore of the gun to a fixed point. While the rifle is fixed in place, the scope or irons can then be adjusted to also aim at the same distant object. The primary purpose of this practice is to use the bore axis as an initial point of reference and get the shots "on paper", which will make subsequent calibrations easier.
Using the naked eye to boresight can be difficult due to the tunnel vision and lack of brightness, especially with longer-barreled guns. A more advanced method of boresighting uses a collimator, an optical attachment similar to a scope sight, which fits onto the end of the barrel. Using this method, the normal sight (which is fixed to the receiver) and the collimator (which is fixed to the barrel) can be sighted to match. Most collimators have grid patterns for rechecking the zero after the barrel is sighted.
A more modern method of boresighting is to use a laser pointer rather than by visual inspection to illuminate the distant point of aim. This method is preferable because it allows more movement in the gun, as the laser dot will not move relative to the barrel, and is a method of boresighting which does not require the removal of the bolt.
As laser technology has become less expensive, laser boresighters have become popular for sighting in rifles. One type of laser boresighter is shaped like a cartridge, and when inserted into the chamber projects a laser beam through the barrel onto the target. The user then adjusts the rifle scope until the crosshairs are on the projected laser dot. Another type of laser boresighter is more universal and is attached to the muzzle of the barrel, either by inserting into the bore ("arbor" type) or via a magnet. It is held in alignment with the barrel and projects a laser beam onto the target. Again, the user aligns the crosshairs to the laser dot on the target.
No matter which method of bore sighting is used, the result is to align the cross-hairs of the scope to the spot where the barrel is pointing at a particular distance. Because of variations in the trajectory of ammunition and other factors the bore-sighted rifle will probably not shoot to the exact spot that the cross-hairs indicate, and live ammunition will need to be fired to fine-tune the sighting process.
Accuracy is the measure of how well the sighted object is represented. It can be measured from a specific decision-making circumstance, like the orientation of notches of a gun barrel. Alternatively the device could be designed to accommodate a range of circumstances and still be sufficiently accurate.