It is granted for constructing one of each custom weapon. Those are:
- Oddly, none of the weapons in the picture (a missile launcher, a sawed-off shotgun, a pistol, and a BB gun) are custom weapons.
- Locations that provide most, if not all, of the components required for custom weapons include Lucky's, Pinkerton's lab in the Rivet City bow (which require items to be stolen), and Old Olney underground.
- Selecting the level 26 perk, Warmonger, makes all custom weapons available to you without having to find their schematics.
- As said by Moira Brown, all of the items needed along with the schematics for the Rock-It Launcher can be bought from her shop.