Weapon mods can be built from raw materials and attached to weapons at weapons workbenches. There are over 700 mods, generally including sights, barrels, stocks, receivers, magazines and more. The Gun Nut perk is required to create better modifications for guns. The Science! perk is used to create better modifications for energy weapons and create advanced modifications for bullet based firearms. The Blacksmith perk is used to create better modifications for melee weapons.