The bore axis of a firearm is the longitudinal axis through the geometric center of the gun barrel. In a rifled barrel, the projectile (bullet/ball, pellet or slug) will spin around the bore axis as it goes through the barrel. Boresighting is a process of placing one's line of sight down along the bore axis. Bore-to-sight distance The distance between the bore axis and the sight axis (the optical axis of a sighting device), also known as the sight height, is an important factor to consider due to parallax principles. It is normally desirable to keep the sights of a firearm as close to the bore as possible, since a firearm with a large bore-to-sight distance will require more compensatory sight adjustment when shifting between shooting at different targets at close ranges (due to foreshortening). On the other hand, a firearm with a short bore-to-sight distance will need less sight adjustment when changing between targets at close ranges. At longer ranges the bore-to-sight distance will be of less importance, since gravity has affected the projectile so much that the height difference between the bore axis and the sight axis has far less impact compared to the bullet drop. Traditional bolt hunting rifles usually have had the optical center of about above the bore, while AR-type rifles and modern bolt-action rifles usually have their optical center about above the bore. On firearms with a picatinny rail, the distance from the bore to the optical axis can be divided into distance from bore axis to top of rail, and distance from top of rail to center of the optical axis. For example on an AR-type rifle, the mil-spec distance from center of bore to top of rail is 1.211 inches, or approximately . The mount height itself, that is the distance from the top of rail to center of scope, varies, but typically lies around (higher and lower mounts are also readily available). In total, this gives a typical scoped AR-style rifle a height over bore of around . Recoil control The recoil from a fired cartridge (and the action movement) exerts a rearward impulse along the bore axis, which is commonly above the center of mass of the gun. Meanwhile, the gripped areas of the gun, where the user will exert a reactional forward push countering the recoil, is almost always below the gun's center of mass. This force couple creates