This is a generic mod used by multiple weapons, and adds a zero-magnification red-dot sight to aid in rapid target-acquisition.
There are several different variations of this mod to be found on the various weapons in the Mega-Pack, but they are all made using parts from either the .44 Revolver Scope, the Marksman Carbine, or the 9mm Pistol Scope. The lenses are all retextured using the window glass from the Vault diner window mesh, and the red dot is a repurposed glow sight from the 9mm Pistol, retextured using the glowing red status light from the C4 Detonator.
The following Mega-Pack weapons can equip this mod: