You have to have every weapon in your inventory. Unlike the treasure achievement, where you simply have to find the items.

Just be sure that you have a rocket launcher, a land mine, and the three types of grenades. (Explosive, Flame, and Flash)

You should unlock the achievement the first time you return to the inventory screen during a game. The pre-game inventory screen didn't count for me, but when I completed the chapter, I got the achievement.

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