Inside the bunker is the experimental MIRV (and a plasma grenade hidden in a cooking pot, on the first set of shelves left-hand side). The National Guard bunker, accessed through the National Guard armory, can be opened by completing the unmarked quest Keller Family Refuge or via a relatively easy glitch (see relevant section).
Activating the second switch inside the utility room will open a trap door that leads into the armory itself. There is a sentry bot inside the room just behind the door. The utility door cannot be lockpicked, even though it is shown to have a Very Hard lock. This will unlock the utility door two stories below, which in turn allows one entry into the armory. Exiting back into the depot, there is an electrical switch about 15 feet to the left.
The armory is only accessible after going through the National Guard training wing, the National Guard depot offices, and then exiting from the offices back to the first area of the National Guard depot. Once back in the depot, there is a room with a toppled vending machine and the switch to the utility door, which is in the southwest corner of the room. The door at the top of the rubble has a sentry bot behind it and leads to the depot's upper floors. On the counter is a Duck and Cover! In the far northwest corner of the room, there is some rubble that can be climbed to reach the next area. Go up either of the two staircases flanking the turret control room and enter the room to the south. At the west end of the cubicle-filled area down the first staircase is a room with a turret control terminal.