Boresight (firearm)

Boresight 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 (zero drop at XX distance) much faster. A device called a bore sighter or collimator is used to accomplish this. 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.