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The bore axis of a
firearm A firearm is any type of gun designed to be readily carried and used by an individual. The term is legally defined further in different countries (see Legal definitions). The first firearms originated in 10th-century China, when bamboo tubes ...
is the longitudinal axis through the
geometric center In mathematics and physics, the centroid, also known as geometric center or center of figure, of a plane figure or solid figure is the arithmetic mean position of all the points in the surface of the figure. The same definition extends to any o ...
of the
gun barrel A gun barrel is a crucial part of gun-type weapons such as small firearms, artillery pieces, and air guns. It is the straight shooting tube, usually made of rigid high-strength metal, through which a contained rapid expansion of high-pres ...
. In a
rifled In firearms, rifling is machining helical grooves into the internal (bore) surface of a gun's barrel for the purpose of exerting torque and thus imparting a spin to a projectile around its longitudinal axis during shooting to stabilize the ...
barrel, the
projectile A projectile is an object that is propelled by the application of an external force and then moves freely under the influence of gravity and air resistance. Although any objects in motion through space are projectiles, they are commonly found i ...
(
bullet A bullet is a kinetic projectile, a component of firearm ammunition that is shot from a gun barrel. Bullets are made of a variety of materials, such as copper, lead, steel, polymer, rubber and even wax. Bullets are made in various shapes and co ...
/
ball A ball is a round object (usually spherical, but can sometimes be ovoid) with several uses. It is used in ball games, where the play of the game follows the state of the ball as it is hit, kicked or thrown by players. Balls can also be used f ...
, pellet or
slug Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a ...
) 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 An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light pro ...
of a sighting device), also known as the ''sight height'', is an important factor to consider due to
parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby object ...
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 In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
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 Recoil (often called knockback, kickback or simply kick) is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, as according to Newton's third law the force r ...
from a fired cartridge (and the action movement) exerts a rearward impulse along the bore axis, which is commonly above the
center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
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 Couple or couples may refer to : Basic meaning *Couple (app), a mobile app which provides a mobile messaging service for two people *Couple (mechanics), a system of forces with a resultant moment but no resultant force *Couple (relationship), tw ...
creates a
rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
al
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
around the mass center and make the gun pitch upwards after each shot — a phenomenon known as ''
muzzle rise Muzzle rise, muzzle flip or muzzle climb refers to the tendency of a firearm's or airgun's Muzzle (firearms), muzzle (front end of the Gun barrel, barrel) to rise up after firing. It more specifically refers to the seemingly unpredictable "jump" ...
''. This is more prominent in shorter-barreled weapons such as
handguns A handgun is a short-barrelled gun, typically a firearm, that is designed to be usable with only one hand. It is distinguished from a long gun (i.e. rifle, shotgun or machine gun, etc.), which needs to be held by both hands and also braced a ...
due to less frontal weight available to counter the upward lifting of the muzzle, 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 a
buttstock A gunstock or often simply stock, the back portion of which is also known as a shoulder stock, a buttstock or simply a butt, is a part of a long gun that provides structural support, to which the barrel, action, and firing mechanism are attache ...
in a
long gun A long gun is a category of firearms with long barrels. In small arms, a ''long gun'' or longarm is generally designed to be held by both hands and braced against the shoulder, in contrast to a handgun, which can be fired being held with a single ...
will also help negate the effect of muzzle rise by shifting the pivot of the muzzle rise back towards the butt end, thus utilizing the entire gun's weight with a longer
lever A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or '' fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is d ...
to counter the rise.


Bore axis in pistols

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.{{Cite web, url=https://www.guntweaks.com/beyond-bore-axis-decreasing-recoil-in-pistols.html, title=Handgun Recoil Characteristics and Lowering Felt Recoil, website=GunTweaks.com Handguns such as the CZ-75, which combine a heavy steel frame with a low bore axis (relative to other hammer-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.


See also

* Boresight (firearm)


References


External links


GunTweaks.com - Pistol Bore Axis: Comparison and ExplanationThe Truth About Guns - The Truth About Muzzle Flip and Bore Height
Firearm terminology