I believe that you can equip the weapon then go to your inventory and hit 'X' (While the weapon is highlighted), it will give you the option to install a mod. The best option is install The Weapon Mod Menu at http://newvegas.nexusmods.com/mods/44515 The Weapon Mod Menu replaces the existing menu when choosing to "Mod Weapon" in the PipBoy. You use it by pressing "X" as you normally would to open the mod menu. There are four main features added by The Weapon Mod Menu: Removal of attached weapon mods Specific information on the amount of improvement each mod gives Information about all of the weapon's mods, even if you don't own any of them The number of available mods in your inventory See The Weapon Mod Menu in action with Gopher's Fallout New Vegas Mod Clinic - Part 11 and TheTuninator's Fallout: New Vegas-Mods of the Week-Episode 31. A very special thanks goes to schlangster, who created the new NVSE functions that make this mod possible. (And for letting me have a sneak peek at them so I could have the mod ready when NVSE was updated.)