When the Chimera is added into Modern Warfare 3 with Season 1 Reloaded, you will be able to unlock it via a weapon challenge or a store bundle. The challenge for the weapon is to do one of the following:
Overall, this is pretty easily, especially if you are playing Modern Warfare 3. DMZ Building 21 isn’t on the Warzone 2 Al Mazrah map so finding it is a bit more of a challenge.
Now you know how to unlock the Chimera in Modern Warfare 3 once it arrives, you can check out some of the other Season 1 additions, including the new Modern Warfare 3 prestige system and the addition of the Shoot House map.