Mark the section below to make the text readable. (SPOILER)

To sucessfully add this mod to your weapon, you need the Original Nuka-Launcher Mod Plan.

For this, it's required to solve the Sidequest found in Nuka-Town called "Cappy in a Haystack"

In the end when asked to kill John-Caleb Bradberton, DO IT.

This will grant Access to his Safe, give you the reciept for the original Nuka-Cola and also the plan for the nuka launcher mod.

After this you can give the reciept to Sierra Petrovita, which finishes the quest.

END.