Call of Duty

There are 6 total Open Combat missions in the Modern Warfare III campaign! When interacting with one, switch any part of your loadout and press the "Confirm" button on all 6 Open Combat missions to unlock the achievement!

Locations:

Precious Cargo
In the Harbormaster building, if entering from the roof, drop down to the top floor, and at the end of the hall, take a left. The loadout crate will be in that room.

Reactor
After the location of the first helicopter, on the way to the building marked with the "A", there will be a loadout crate on top of one of the smaller trailers. Climb up and interact with it for it to count toward the achievement.

Crash Site
Make your way to the center of the map where a building will stand with a sentry turret outside. Either take out the turret or flank the turret to access the loadout crate on the front porch.

Oligarch
The loadout crate will be in the back left of the map. After dropping in, go to the left and hug the back wall until you come across the first building on the left after passing the fountain. This building also has a grenade launcher in it as well.

Highrise
Very similar to the route used in "Elevator Out of Service", make your way up the rappel line, take a left and take out the enemies in the hallway. From there in the last staircase on the left, use the rappel to go up to the top. At the top, take a left and out the window, use the zipline to the ladder that you would use to meet up with Price. Before going up the ladder, take a left and the loadout crate will be in that room.

Gora Dam
Right when you load in, fly to the building to the right. Drop down to the ground floor (or enter using one of the many doors on the sides) and take out the enemies. The loadout crate will be on the ground floor in the main part of the building with the generators.

There are 6 open combat missions

Find a supply box (they are marked as white squares in the map)

Then hold down x and change your gun