This unlocks once you have found all 8 weapons in the game. This does not include weapon modifications/upgrades or harpoon tips.
The hardest ones to find are the ice gun and sticky bomb gun. Both of them can be found in the glacier. Just keep opening weapon crates and collect blueprints. You can check your progress when you open the yellow chest on the boat.
Once you have all blueprints Duff will call you and your achievement unlocks.