Boresighting is a method of visually pre-aligning a

firearm A firearm is any type of gun that uses an explosive charge and is designed to be readily carried and operated by an individual. The term is legally defined further in different countries (see legal definitions). The first firearms originate ...

barrel A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden stave (wood), staves and bound by wooden or metal hoops. The word vat is often used for large containers ...

bore axis The bore axis of a firearm is the longitudinal axis through the geometric center of the gun barrel. In a rifling, rifled barrel, the projectile (bullet/round shot, ball, pellet (air gun), pellet or shotgun slug, slug) will spin around the bore axis ...

with the target, in order to more easily

zero 0 (zero) is a number representing an empty quantity. Adding (or subtracting) 0 to any number leaves that number unchanged; in mathematical terminology, 0 is the additive identity of the integers, rational numbers, real numbers, and compl ...

the gunsight (

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...

iron sight Iron sights are a system of physical alignment markers used as a sighting device to assist the accurate aiming of ranged weapons such as firearms, airguns, crossbows, and bows, or less commonly as a primitive finder sight for optical telescope ...

s). The process is usually performed on a

rifle A rifle is a long gun, long-barreled firearm designed for accurate shooting and higher stopping power, with a gun barrel, barrel that has a helical or spiralling pattern of grooves (rifling) cut into the bore wall. In keeping with their focus o ...

, and can be accomplished either with the naked eye, or with a specialized device called a boresighter.

Optical boresighting

Optical boresighting, also referred to as "eyeball boresighting", as the name suggests, involves using the naked eye and the bore axis of a firearm to dial in the sights of a firearm. Based on the type of firearm the procedure for optical bore sighting will vary. For AR style rifles the upper receiver can be dropped from the lower receiver and the bolt carrier group (BCG) can be removed exposing a clear line of sight through the barrel. With the upper receiver fixed in place you can then co-witness through the barrel and the sights on the top of the rifle, while doing this you can visually align the bore axis with the sights on a desired target. For bolt actions rifles the process is the same aside from the ability to remove different receivers, but the bolt can still be removed to view through the barrel. Once you are able to align the bore axis and the sights on the firearm you should then be able to at least get shots on or close to the target. With the rifle reassembled you should be able to put shots down range and make more fine-tuned adjustments. A drawback of using the naked eye to boresight is that it can be difficult due to the

tunnel vision Tunnel vision is the loss of peripheral vision with retention of central vision, resulting in a constricted circular tunnel-like field of vision. Causes Tunnel vision can be caused by: Eyeglass users Eyeglass users experience tunnel vision ...

and lack of brightness, especially with longer-barreled guns. A more advanced method of boresighting uses a

collimator A collimator is a device which narrows a beam of particles or waves. To narrow can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., make collimated light or parallel rays), or to cause the spat ...

boresighter, an optical attachment similar to a scope sight, which fits onto the muzzle end of the barrel via a bore-diameter arbor and projects a grid pattern onto the sight picture. 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.

Laser boresighting

A more modern method of boresighting is to use a

laser pointer A laser pointer or laser pen is a (typically battery-powered) handheld device that uses a laser diode to emit a narrow low-power visible laser beam (i.e. Coherence (physics), coherent light) to highlight something of interest with a small brigh ...

to illuminate the distant point of aim, rather than using visual inspection. This method is preferable because it has less

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

, allows more movement in the gun as the projected

laser beam A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of rad ...

will stay true to the bore axis, and does not require removing the bolt. As laser technology has become more affordable and portable, laser boresighters have become popular for sighting in riflescopes. 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 iron sights or scope

reticle A reticle or reticule, also known as a graticule or crosshair, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the electronic v ...

to align the point of aim with the projected laser dot. Another more commonly used type of laser boresighter is attached to the muzzle of the barrel, either inserted straight into the bore ("arbor" type) or held in alignment with the barrel via a

magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

, and projects a laser beam onto the target. Again, the user aligns the sights to the laser dot on the target. No matter which method of boresighting is used, the result is to align the sights to the spot where the barrel is pointing at a particular distance. Because of variations in 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 ...

trajectory and other factors, the boresighted rifle will always shoot below the boresighted spot and is only meant to approximate the actual points of impact, namely getting the shots "on

paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, Textile, rags, poaceae, grasses, Feces#Other uses, herbivore dung, or other vegetable sources in water. Once the water is dra ...

" (landing in proximity to the point of aim). Live ammunition will need to be fired to further 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.

References

{{Firearms Firearm components

Optical boresighting

Laser boresighting

See also

References