Ghost ring

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 telescopic sights or reflector (reflex) 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, 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, so that the sights can be adjusted 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.

In the case of firearms, where the projectile follows a Newtonian trajectory, front and rear sights must be aligned with the line of sight of the shooter to the target, known as the 'Point of Aim' (POA), calibrated to the distance of the target and the trajectory of the bullet, so that the bullet hits the target at the 'Point of Impact' (POI). Iron sights provide horizontal and vertical reference points that allow the shooter to train the weapon.

Rear sights are usually mounted in a dovetail on the barrel or receiver, closer to the eye of the shooter, allowing for easy visual pick-up of the notch. Front sights are mounted to the barrel by dovetailing, soldering, screwing, or staking close to the muzzle, frequently on a ramp. Some front sight assemblies include a detachable hood intended to reduce glare, and if the hood is circular, then this provides a reference where the eye will naturally align one within the other.

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Wikipedia