Weapon Mods
Weapon Mods can be obtained through Weapon Mod Doors for the respective class's Primary Weapon, and Heavy Weapon Mod Doors for Secondary Weapons from all unlocked classes, which means you can have secondary weapons & their respective mods from any other class you have unlocked.
Primary Weapons have 3 elemental weapon mods that just add elemental damage, as well as Stat Boost weapon mods.
Secondary Weapons only have 3 elemental weapon mods, but the weapon mods Completely Alter the gun.
Soldier
Increases Fire Rate from 66 to 92.4 rpm.
Boosts Reload Speed.
Reload Speed 2.5 -> 1.5 seconds
Short Reload Speed 1.2 -> 0.72 seconds
Hunter
Increases Max Ammo capacity from 7 to 9.
Increases Reload Speed.
Reload Speed 1.75 -> 1.125 seconds
Short Reload Speed 0.8125 -> 0.6125 seconds
Increases Weakpoint damage from 150% to 570%, but Capped Fire Rate.
Engineer
Increases Fire Rate from 300 to 420 rpm.
When combined with Burst Fire, Fire Rate increases to 739.2 rpm.
Increases Reload Speed.
Reload Speed 1.8 -> 1.08 seconds
Short Reload Speed 1.2 -> 0.72 seconds
Changes fire mode to a Three-Round Burst, increasing Fire Rate to 528 rpm.
If All Shots hit a target, the Last Round will deal Triple Damage (can be different targets).
When combined with the Fire Rate mod, Fire Rate increases to 739.2 rpm
Projectiles Penetrate enemies, dealing 80% Damage to targets Behind Them.
Juggernaut
Increases Reload Speed.
Reload Speed 1.5 -> 0.9 seconds
Short Reload Speed 1 -> 0.6 seconds
Sawed Off Shotgun will Fire Both Barrels on trigger pull, Doubling Damage and increasing recoil by 35%, but effectively doubling ammo cost.
The projectiles will Ricochet Off Surfaces, dealing 75% more damage after the initial bounce.