Weapon Mods
Each weapon in Defiance has between zero and four mod slots; additional mod slots can be added to a weapon that has less than 4 slots by using the Salvage Matrix. Weapon in your inventory list will display small green squares next to the name under the level indicator that indicates their mod slots; these squares turn white if a mod is in them. In the example below, the VBI Sniper Rifle has two empty mod slots, the selected Slickshot sniper rifle has one filled mod slot, and the other weapons have no mod slots:
Examining the weapon details also shows you which mod slots it has, beneath the weapon’s description:
Weapon mods can be added into available mod slots to improve your weapon. They are a reward from most side missions; they can also be purchased from some vendors around the world. Weapon mods come in a variety of types and power levels, with higher level numbers indicating a more powerful modification.
There are four general categories of weapon mods:
1. Barrel: Generally affect damage and accuracy – Examples include: Increased damage per second, increased critical multipliers, increased aimed damage.
2. Magazine: Generally affect reloading or ammo capacity – Examples include: Improved reload time, increased magazine size.
3. Scope: Generally affect accuracy – Examples include: Increased aim accuracy.
4. Stock: Generally affect stability – Examples include: Increased melee damage, improved recoil.
There are four general categories of weapon mods:
1. Barrel: Generally affect damage and accuracy – Examples include: Increased damage per second, increased critical multipliers, increased aimed damage.
2. Magazine: Generally affect reloading or ammo capacity – Examples include: Improved reload time, increased magazine size.
3. Scope: Generally affect accuracy – Examples include: Increased aim accuracy.
4. Stock: Generally affect stability – Examples include: Increased melee damage, improved recoil.
Weapon mods can only be put into the matching type of mod slot on your weapon; for example, you cannot put a Magazine mod into a Scope slot.
In addition to adding regular bonus effects, some rare mods have a Synergy effect also. When put on a weapon with a matching Synergy effect, they grant an additional bonus.
In addition to adding regular bonus effects, some rare mods have a Synergy effect also. When put on a weapon with a matching Synergy effect, they grant an additional bonus.
taken directly from http://community.defiance.com/en/2013/05/14/weapon-mods/