Full Ammo strategy
In order to use this strategy, you need to be in the second Bunker, Archive, or Control. In the second Bunker, get into a small room while there's still a healthy stream of guys with guns coming and stay there. Shoot everybody that comes by and take their ammo after doing so. Repeat until full. Similar deal with Archive, except you need to be in the first room for it to really work. Open the door and then stand between the doorway and the file cabinets and shoot all that come, taking their ammo afterwards.
Control is a little different. Go up to the staircase leading to the top floor and wait for guys to come buy, shooting them like normal. There's two things that make this different: One, there is a single point that guys come from that can be easily pinpointed – in fact, you can even find the very spot where guys fade in out of thin air to come and kill you – and also, unlike Archive or Bunker II, there are no circumstances under which guys stop coming. You can kill all the stationary guys in the level – which would stop everything in other levels – and those guys will just keep on coming and getting killed.