When you begin this section, you start in a room with a large door both in back and in front of you. Also, in front of you is a supply of ammunition. Normally when you leave this room, the door will close behind you and will not re-open. Use the following trick to keep it open. After sniping everybody you can from this room, turn around and face the back of the room. Then, look all the way up. As you are looking up, walk backwards so you can see the front (open) door. Continue to walk just out the door until you see the door begin to close. Wait until it is half way down and go back into the room. The door will re-open. It should now be stuck open. If not, then repeat the process until it sticks. You now have the remaining ammunition at your disposal.