peep sight

Iron sights are a system of shaped alignment markers (usually metal) used as a sighting device to assist in the aiming of a device such as a firearm, crossbow, or telescope, and exclude the use of optics as in reflector (reflex) sights, holographic sights, and telescopic sights.Iron sights are typically composed of two component sights, formed by metal blades: a rear sight mounted perpendicular to the line of sight and a front sight that is a post, bead, ramp, or ring. Open sights use a notch of some sort as the rear sight, while aperture sights use some form of a circular hole. Civilian, hunting, and police firearms usually feature open sights, while many military battle rifles employ aperture sights.
The earliest and simplest iron sights are fixed and cannot be easily adjusted. Many iron sights are designed to be adjustable for sighting in firearms by adjusting the sights for elevation or windage. On many firearms it is the rear sight that is adjustable.
For precision applications such as hunting or sniping, the iron sights are usually replaced by a telescopic sight. Iron sights may still be fitted alongside other sighting devices (or in the case of some models of optics, incorporated integrally) for back-up usage.

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