Weapon Mods in The Division can be added to Weapons or changed to improve or add new effects. They have different qualities: Worn, Standard, Specialized, Superior and High-End. These qualities not only increase the effect of the Attributes found on the Weapon Mods, but increase the number of Attributes found. The percentages of improvement on these mods seem to be somewhat RNG based, falling inside a number range for the level of mod and quality of it. Weapon Mods can also be crafted. See the Weapon Mod Blueprints page to see exactly which ones can be.
There are 5 Mod types for Weapons. These Mod types are:
Skins change the appearance of the Weapon, but have no effect otherwise. You can acquire Skins from helping NPCs and ?
Optics affects certain Attributes of the player as well as affecting the possible zoom. There are Small and Large Optics, and different Weapons will require you to have one or the other (some let you use both types). Each Optic Mod will always have the bonus to the Attribute listed. If the Mod has more than one Attribute bonus, one of them will always be the one listed below. Higher quality Optics Mods will come with more bonuses. Please see the Optics page for a list of them.
Magazines can be changed in favor of those that carry more Ammo, reload faster or increase your Rate of Fire. So far there are only 3 known Mag types and they work with any Weapon that has a Magazine Mod slot. Higher quality Magazines will also come with other bonuses. Please see the Magazines page for a list of those bonuses.
Underbarrels add Stability and Accuracy to your Weapon. There are two types of Underbarrels: Small and Large. Different Weapons use different types, and you must match them accordingly (some Weapons allow for either type). Each Underbarrel below will always come with the listed Attribute bonus. If there are more than one bonus, then at least one of them will be the one listed below. Higher quality Underbarrels will also come with other bonuses. Please see the Underbarrels page for a list of those bonuses.
Muzzles effect the Stability and Threat Attributes of the Player. There are two types of Muzzles: Small Caliber and Large Caliber. You must match your Muzzle Mod to the size of the slot on the Weapon you wish to modify (again some use both types). Each Muzzle below will always come with the listed Attribute bonus. If there are more than one bonus, then at least one of them will be the one listed below. Higher quality Muzzles will also come with other bonuses. Please see the Muzzles page for a list of those bonuses.