From Halopedia, the Halo wiki
The KFA-2 x2 is a 2× magnification, smart-linked scope that relays visual data directly into the user's HUD. This stands in contrast to normal optical scopes such as the one featured on the BR55 Service Rifle, which requires the user to look into the scope, though the latter can also relay the visual data directly to the user's HUD. Infantry and other military personnel who do not wear a helmet with a visor or an eyepiece have this HUD interface overlaid over their normal sight via their Standard or Command Neural Interface.
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