Weapon Mods (Fallout Supplement)
From D&D Wiki
Weapons in Fallout tend to be personal, sometimes kit-bashed affairs, with typically weaker weapons being modified using spare parts to produce a weapon in some ways superior to most stock weapons. Weapon modifications, aka mods, can be created and attached at a Weapon Crafting Table using the Craft and Repair skills respectively; moreover, while weapon mods can be custom-built, they can sometimes be purchased at weapon shops or found as loot.
How to Use Weapon Mods
The easiest way to obtain weapon mods is to purchase them at a weapon shop or loot them off dead enemies, but the most common way is to craft them at a Weapon Crafting Table. Each mod has a purchase price listed in Trade Units (TUs) and Bottle Caps (Caps) that can be used if your GM decides that you're at a place you can purchase the mod, along with a scarcity rating to determine how rarely the mod can be found out in the world. Then comes the Repair skill check DC to install the mod and the time it takes to do so. Note that installing a mod requires no parts, as you must first either buy, find or craft the mod. You can only attempt to install a particular mod once per level: if you fail, you lack the experience to install it yet. Mods cannot be uninstalled once attached to a weapon. Crafting a mod works the same as the core d20 Modern and Apocalypse and the description lists the Parts DC and Parts Used, along with the skill check DC for Craft (mechanical) or Craft (electrical) and the time it takes to craft.
Weapons are split into distinct categories for different types of weapon, which determines not which mods they can use, but which mods use up which slots. A weapon can only have one mod per slot (except for miscellaneous mods), so an AER9 laser rifle can have either night sight or the scope, rifle, but not both; however, this does not limit the mods available for the capacitor, magazine or barrel. Note: The mods available for each weapon is listed in the description for that weapon.
Personal Firearms/Energy Weapons