From Halopedia, the Halo wiki
Swap Meet is an achievement in Halo: Reach. To unlock this achievement, a player must trade weapons with an AI ally in campaign or Firefight. The in-game description states that the achievement can only be earned in campaign because at launch none of the Firefight maps featured AI allies. Installation 04, part of the Anniversary Map Pack released in November 2012, features ODST AI allies with whom the player can swap weapons and unlock the achievement.