Telescopic sight
From Halopedia, the Halo wiki
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A telescopic sight, commonly called a scope, is a device used to give additional accuracy using a point of aim for weapons such as firearms, airguns and crossbows. Other sighting systems are iron sights, red dot sights, and laser sights.
In the Halo universe, scopes were used on a wide variety of firearms, e.g. the Battle Rifle, Sniper Rifle, Beam Rifle, and Carbine.
Each scope varies upon its weapon. For example, in a Sniper Rifle, the scope is electronic, contains a night vision function, and can zoom in from 2x to 10x magnification.