Thebore axis of afirearm is the longitudinal axis through thegeometric center of thegun barrel. In arifled barrel, theprojectile (bullet/ball,pellet orslug) will spin around the bore axis as it goes through the barrel.

Boresighting is a process of placing one'sline of sight down along the bore axis.

The distance between the bore axis and the sight axis (theoptical axis of asighting device), also known as thesight height, is an important factor to consider due toparallax 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 toforeshortening). 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, sincegravity 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 38 mm (1.5 in) above the bore, while AR-type rifles and modern bolt-action rifles usually have their optical center about 68 mm (2.7 in) above the bore.^[1]

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 31 mm (1.2 in). The mount height itself, that is the distance from the top of rail to center of scope, varies, but typically lies around 36–38 mm (1.4–1.5 in) (higher and lower mounts are also readily available). In total, this gives a typical scopedAR-style rifle a height over bore of around 67–69 mm (2.6–2.7 in).

Therecoil from a firedcartridge (and theaction movement) exerts a rearward impulse along the bore axis, which is commonly above thecenter of mass of the gun. Meanwhile, the gripped areas of the gun, where the user will exert areactional forward push countering the recoil, is almost always below the gun's center of mass. This forcecouple creates arotationaltorque around the mass center and make the gunpitch upwards after each shot — a phenomenon known asmuzzle rise. This is more prominent in shorter-barreled weapons such ashandguns due to less frontal weight available to counter the upward lifting of themuzzle, and how high the bore axis sits above the gripping hand will also affect the degree of muzzle rise. A lower bore axis will align more of the recoil directly into the hand, creating less couple torque and thus less muzzle rise. Among other things that affect muzzle rise and felt recoil is the weight of the pistol frame and tuning of recoil springs. The presence of abuttstock in along gun will also help negate the effect of muzzle rise by shifting thepivot of the muzzle rise back towards the butt end, thus utilizing the entire gun's weight with a longerlever to counter the rise.

Handguns lack a buttstock to aid in recoil control, so design elements that reduce felt recoil or muzzle flip are considered highly desirable and numerous manufacturers claim their handgun design has a low bore axis, although few offer measurements to prove this claim. While a lower bore axis does typically yield reduced felt recoil and muzzle flip, shooters intent on reducing these features as much as possible (such as competitive sport shooters) will typically seek out handguns with heavy steel frames, as increasing the weight of the gun's non-reciprocating mass (the frame) yields greater reductions in felt recoil and muzzle flip than any difference in bore axis height can typically achieve.^[2] Handguns such as theCZ-75, which combine a heavy steel frame with a low bore axis (relative to otherhammer-fired guns) achieve twofold reductions to felt recoil and muzzle flip, and are popular choices in many competitive shooting disciplines.

Striker-fired handgun designs are typically able to achieve a lower bore axis than their hammer-fired counterparts since strikers travel straight forward, rather than up and forward like a typical hammer mechanism.

One pistol which combines a low bore axis, striker fire and weight is theWildebeest pistol.

• Boresight (firearm)

• GunTweaks.com - Pistol Bore Axis: Comparison and Explanation