The quality of a Mod is directly tied to its Rank. Rank I is Common and Rank IV is Epic. Rank V should be Legendary but we have yet to see one in-game.
We are not going to do individual item pages for Mods. This table is the only needed reference.
Mod Vendors can be found in the following locations:
Iron Demon Ranch in Madera, all Rank I mods, 500 each.
Cronkhite Post in Sausalito, all Rank I mods, 500 each.
Headlands Transit Depot in Sausalito, all Rank I mods, 500 each.
For mods above Rank I you will need to do Side Missions to get Mod Caches, or buy Synergy Mod Caches from Faction Vendors.
Synergy is the interaction of multiple elements in a system to produce an effect different from or greater than the sum of their individual effects.
Equipping two weapons with the same Weapon Synergy, or adding one to four mods with the same Mod Synergy to a weapon unlocks bonuses when using that weapon. Synergies that are listed above in BOLD are Weapon Synergies.
Missions that reward a Mod Cache will appear below in the Articles list. All Side Missions and Rampage Missions reward a Mod Cache which will contain a random mod of any type and Rank II or III.
Hotshot Missions reward a Mod Cache too, but for a particular weapon type. It will only yield mods of that type. So a Shotgun Mod Cache will always yield a Shotgun mod. You still cannot predict which mod slot it will be for or whether it will be Rank II or III.
it is not unlikely that Mod Cache mods will also have Synergy.
Most of the data on this page was gathered by a scan of the Rank I mods available at Iron Demon Ranch.
each.