A firearm sighting optic that can have the lenses easily and quickly replaced by the user is disclosed. This sighting optics allows the lens to be inserted from the bottom of a red dot sighting unit and then be secured by set screws. The firearm sighting optic includes a base and a hood. The lens is removably retained with the firearm sighting optic by user removable fasteners securing the hood to the base. The lens is housed in a hood that makes sliding the replacement lens into the unit much easier. The hood and base include an interconnecting protrusion and recess for reliable alignment of the lens within the hood and the base.