Survival Mode also features a currency system which allows players to buy items such as air support, sentry turrets, weapons, and even friendly NPCs to assist the player throughout the match. Revenue is generated by killing enemies and can be multiplied by chaining numerous kill sprees. Six factors at the end of each wave also provide a certain amount, depending on the total time, wave number, enemies slain, accuracy, damage inflicted on the player and headshots. Furthermore, assisting in an enemy kill will reward the assisting player a fraction of the total amount received by the player who performed the kill. Three “Armories” are placed in fixed locations around the map and are activated over the course of the first few rounds; the Weapon Armory provides ammunition for guns, and allows the purchase of bullet-firing weapons and their accessories. The Equipment Armory provides armor, grenades, and special items like sentry turrets and claymores. Finally, the Air Support Armory allows the player to call in assistance from orbiting aircraft, such as Perk drops, Predator missiles and small groups of friendly NPC soldiers delivered by helicopter. Only one Perk can be used at a time, buying and equipping another Perk will simply replace the previous one. Likewise, purchasing a new weapon will remove the previous firearm. Typically a map will place two of the Armories close together and one a significant distance away. Read More: http://callofduty.wikia.com/wiki/Survival_Mode
