The lights are on
When you finally get to the Secret Armory, you get 2 1/2 minutes, right? That's not nearly enough time to check all those weapons cases, so what do you do?
First, do not activate the timer, in fact, stay out of the control
room, especially if you are doing this single player. If you complete the quest objective 'get loot' accidentally by entering the control room while getting all the loot, you will not be able to finish the quest. If this happens, quit without saving. Go back to the
point where the first big blast door dropped into the floor after you
put in the color code. look straight down at the door and crouch. walk
back and forth a bit and you should fall through the floor. Head
towards the control room and you should be able to drop into the area
with all the weapons cases. Plunder at your leisure.
Already completed the mission? well keep playing, Marcus will give you the Mop Up mission
requiring you to kill a number of each kind of lance trooper. The easy
way to do this is the Circle of Duty side missions. Once you turn in
the mission you will have another crack at the Armory.