Instructions
Progress through "Multiplayer" mode from levels 1 to 70. For every level you complete, new items will be unlocked. There are over 100 unlockable items in "Multiplayer" mode, so if you want to get everything, your only option is to keep pushing forward. Certain unlockable weapons are only available if you are playing the "Multiplayer" campaign.
Complete the "Ghillie In The Mist" challenges to unlock additional camo gear. There are three levels to this challenge, each of which unlocks a different camo set. Each level requires a certain number of one-shot kills with a sniper rifle. The first level requires 50, the second level 100, and the third level 200.
Upgrade your existing perks by completing challenges. Each perk upgrade requires you to perform a certain feat while using the perk you are upgrading. For example, to upgrade the "Marathon" perk, you must run 26 miles with the "Marathon" perk enabled. Each of the 14 perks has a different upgrade requirement. It is best to research specific perk upgrades beforehand, rather than stumbling across the upgrades by accident.
Rack up killstreaks in "Multiplayer" mode to unlock beneficial rewards. The more kills you get in a row, the better your rewards will be. On the low end of the spectrum, a killstreak of three will reveal enemies on your minimap. The greatest feat is a killstreak of 25, which is exceptionally difficult to achieve. A killstreak of 25 is rewarded with a devastating nuclear strike.