Under a Buck (Secret)
Shoot the deer head trophy in the Admiral's Quarters.
This achievement takes place during Chapter 4. Proceed through a the level for a few minutes, and you will be asked to meet up with some team mates in the Admiral's Quarters. You will climb a ladder, and then go towards a console and use your analog sticks to start the elevator. Once you go into the elevator, it will go down. Upon leaving the elevator, turn left, and you will see a deer head mounted on the wall, just shoot it to unlock this achievement.