Rock, Paper, Genocide is a series of four, very short missions that revolve around learning about the elemental weapons in Borderlands 2. They will unlock after completing The Road to Sanctuary and all four can be completed in a row. For each mission, a specific weapon will be placed in your inventory, which will be the element needed for that particular mission.

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If you don’t have the respective elemental weapons, Marcus will loan you some to use for this mission.

As you will notice, this weapon is listed under a category called Mission Weapons, meaning that it will only be available while you’re doing that mission. Once you complete it, the weapon will be removed from your inventory (it doesn’t take up an inventory slot). After starting the first mission, Fire Weapons!, follow Marcus into the shooting range and shoot the dummy with each weapon to trigger the elemental effect. The only difference in any of them is that you need to shoot with a different gun for the Slag Weapons! mission.