You can obtain achievements for doing certain tasks in single play and multiplay modes. These achievements count to your gamerscore for your gamercard. And you can get achievements on Xbox Live.
The following is a list of how to unlock each of the achievements.
Complete 1 Co-Op misson on Dark Agent Difficulty
Complete 1 Solo misson on Dark Agent Difficulty
Get 10 kills using explosive in any Multiplayer modes
Capture 10 Flags in Capture The Flag mode
Get 10 Headshots kills in any Multiplayer mode
Get 10 kills in Killcount mode
Get 10 kills in Team Killcount mode
Get 10 Melee kills in an Multiplayer mode
Survive 10 minutes in Onlsaught mode
Infect 10 Players in Infection mode
Survive 10 Rounds of Eradication mode
Get 10 Sniper kills in any Multiplayer modes
Score 10 points in Territory Gain mode
Destroy 10K CR of property in Sabotage
Get 100 kills using explosive in any Multiplayer modes
Capture 100 Flags in Capture The Flag mode
Get 100 Headshots kills in any Multiplayer mode
Get 100 kills in Killcount mode
Get 100 kills in Team Killcount mode
Get 100 Melee kills in an Multiplayer mode
Survive for 100 minutes Onslaught.
Infect 100 Players in Infection mode
Survive 100 Rounds of Eradication mode
Get 100 Sniper kills in any Multiplayer modes
Score 100 Points in Territorial Gains mode
Destroyed Destroye 100K CR of property in Sabotage
Get 1000 Explosive kills in any Multiplayer Mode
Get 1000 Headshot kills in any Multiplayer Mode
Get 1000 Kills in Killcount Mode
Get 1000 Kills in Team Killcount Mode
Get 1000 Melee kills in any Multiplayer Mode
Infect 1000 Players in Infection mode
Survive 1000 Rounds of Eradication mode
Get 1000 Sniper kills in any Multiplayer Mode
Complete 5 Co-Op missons on Dark Agent Difficulty
Complete 5 Solo missons on Dark Agent Difficulty
Complete Co-Op on Agent Difficulty
Complete Co-Op on Secret Agent Difficulty
Complete Single Player on Agent Difficulty
Complete Single Player on Dark Agent Difficulty
Complete Single Player on Perfect Agent Difficulty
Complete Single Player on Secret Agent Difficulty
Complete Co-Op on Dark Agent Difficulty
Complete Co-Op on Perfect Agent Difficulty
Play 10 Dark-Ops games
Play 10 Deathmatch games
Play 100 Dark-Ops games
Play 100 Deathmatch games
Play 1000 Dark-Ops games
Play 1000 Deathmatch games
Unlock Dark Agent
Defeat the game on Perfect Agent mode.
Unlock weapons and use them in other missions
In order to unlock a weapon, you need to start a level that has that weapon in it, find that weapon, and finish the level with it. Simulations such as the level 0 are exceptions; you don't get to keep the weapons, because they are only simulations.
The following is a lits of unlockable weapons. Listed is the type of weapon, and the amount of carrying capacity that it takes up, then the mission where the weapon first appears.
