A type of iron sight. The rear sight consists of a small disk with a center hole through which the shooter views the front sight and the target. The front sight may be a post, bead, or another aperture. Light entering the center hole of the sight frames the front sight, which enables the eye to focus more easily than in could when sighting through open sights, and thus facilitates the centering of the front sight on a target. Also known as orthoptic sight, peep sight, aperture back sight, or receiver sight (if mounted to the receiver). May be categorized according to position on a firearm as follows:
Action mounting – A conventional aperture sight which mounts on the action of a firearm and may vary in design with firearms of different purposes.
Bolt mounting – An aperture sight which mounts to the bolt sleeve or cocking piece of bolt action firearms.
Tang mounting – An aperture sight which mounts to the tang of a firearm.
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