So you can do the Lock and Load Achievement fast and easy by doing the following. Start Russian Consulate on Challenge and Brutal Onslaught. Get to the part where you are at the library. When you have 2 NPC's left everyone die and re spawn. Next one player goes and kills ONE NPC. That will unlock the Special Ammo each time instead of 2 players needing to kill 2 NPC's for everyone in the group to get it.