Unique weapons cannot be equipped with weapon mods, with the exceptions of the weathered 10mm pistolMercenary's grenade rifleholorifleand Elijah's Advanced LAER. While in Fallout 3 this was limited to the creation of custom weapons, New Vegas takes this a step further by allowing the creation of weapon variants based on basic weapons by adding weapon mods.
One of the features added to New Vegas is the ability to custom modify certain base weapons.
