Boresight (firearm)

Source: Wikipedia, the free encyclopedia.
This article is about the use of the term in firearms. For other uses, see Boresight (disambiguation).
Rifle bore sighter with arbor and leather case
A German Luftwaffe technician uses a collimator to boresight the gun firing through the propeller hub of a Messerschmitt Bf 109 (1941)
Boresighting the gun on an M1 Abrams tank (2005)

Boresighting is a method of visually pre-aligning a

iron sights). The process is usually performed on a rifle, and can be accomplished either with the naked eye, or with a specialized device called a boresighter.[1]

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

arbor and projects a grid pattern onto the sight picture.[2]
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 beam
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

muzzle of the barrel, either inserted straight into the bore ("arbor" type) or held in alignment with the barrel via a magnet
, 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 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" (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.

See also

References

  1. ^ Field & Stream. September 1970. p. 103.
  2. ^ Bonnier Corporation (June 1969). Popular Science. Bonnier Corporation. p. 160.
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