Achievements
You can get achievements for doing certain tasks in single play and multiplay modes. These achievements count to your gamerscore for your gamercard.

Yes you can get achievements on LIVE.

There are 50 in all and will give you 1000 gamerscore points in all.

Unlockable	How to Unlock
1 Co-op Mission On Dark Agent	Complete 1 Co-Op misson on Dark Agent Difficulty
1 Solo Mission On Dark Agent	Complete 1 Solo misson on Dark Agent Difficulty
10 Explosive Kills	Get 10 kills using explosive in any Multiplayer modes
10 Flags Captured	Capture 10 Flags in Capture The Flag mode
10 Headshot Kills	Get 10 Headshots kills in any Multiplayer mode
10 Kills in Killcount	Get 10 kills in Killcount mode
10 Kills in Team Killcount	Get 10 kills in Team Killcount mode
10 Melee Kills	Get 10 Melee kills in an Multiplayer mode
10 Minutes Survived	Survive 10 minutes in Onlsaught mode
10 Player Infected	Infect 10 Players in Infection mode
10 Rounds Survived	Survive 10 Rounds of Eradication mode
10 Sniper Kills	Get 10 Sniper kills in any Multiplayer modes
10 Territories Held	Score 10 points in Territory Gain mode
10,000 CR Property Destroyed	Destroye 10K CR of property in Sabotage
100 Explosive Kills	Get 100 kills using explosive in any Multiplayer modes
100 Flags Captured	Capture 100 Flags in Capture The Flag mode
100 Headshot Kills	Get 100 Headshots kills in any Multiplayer mode
100 Kills in Killcount	Get 100 kills in Killcount mode
100 Kills in Team Killcount	Get 100 kills in Team Killcount mode
100 Melee Kills	Get 100 Melee kills in an Multiplayer mode
100 Minutes Survived	Survive for 100 minutes Onslaught.
100 Player Infected	Infect 100 Players in Infection mode
100 Rounds Survived	Survive 100 Rounds of Eradication mode
100 Sniper Kills	Get 100 Sniper kills in any Multiplayer modes
100 Territories Held	Score 100 Points in Territorial Gains mode
100,000 CR Property Destroyed	Destroye 100K CR of property in Sabotage
1000 Explosive Kills	Get 1000 Explosive kills in any Multiplayer Mode
1000 Headshot Kills	Get 1000 Headshot kills in any Multiplayer Mode
1000 Kills in Killcount	Get 1000 Kills in Killcount Mode
1000 Kills in Team Killcount	Get 1000 Kills in Team Killcount Mode
1000 Melee Kills	Get 1000 Melee kills in any Multiplayer Mode
1000 Player Infected	Infect 1000 Players in Infection mode
1000 Rounds Survived	Survive 1000 Rounds of Eradication mode
1000 Sniper Kills	Get 1000 Sniper kills in any Multiplayer Mode
5 Co-op Missions On Dark Agent	5 Co-Op missons on Dark Agent Difficulty
5 Solo Mission On Dark Agent	Complete 5 Solo missons on Dark Agent Difficulty
Complete Co-Op Agent	Complete Co-Op on Agent Difficulty
Complete Co-Op Secret Agent	Complete Co-Op on Secret Agent Difficulty
Complete Single Agent	Complete Single Player on Agent Difficulty
Complete Single Dark Agent	Complete Single Player on Dark Agent Difficulty
Complete Single Perfect Agent	Complete Single Player on Perfect Agent Difficulty
Complete Single Secret Agent	Complete Single Player on Secret Agent Difficulty
Completed Co-Op Dark Agent	Complete Co-Op on Dark Agent Difficulty
Completed Co-Op Perfect Agent	Complete Co-Op on Perfect Agent Difficulty
Played 10 Dark-Ops Games	Play 10 Dark-Ops games
Played 10 Deathmatch games	Play 10 Deathmatch games
Played 100 Dark-Ops Games	Play 100 Dark-Ops games
Played 100 Deathmatch Games	Play 100 Deathmatch games
Played 1000 Dark-Ops Games	Play 1000 Dark-Ops games
Played 1000 Deathmatch Games	Play 1000 Deathmatch games

  

Unlock Dark Agent
Beat the game on Perfect Agent mode.
 

Unlock weapons to bring into 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 an exception; you don't get to keep the weapons, because they are not real

The following list is most unlockable weapons, the type of weapon, and the amount of carrying capacity that it takes up. In the second column is the mission that the weapon first appears in.

The viblade is in the room with all the laser tripwires, on a stand in the center
The M60 can be taken from a guard when you fail to get let in by the front gate guard

Oh, and sniper take up 3 inventory spaces, making them unwieldy, but the jackal and shockwave can be found in rooftops escape, and bridge assault respectively.
 

Unlockable	How to Unlock
CMP-150 (SMG 2)	Sewer Retrieval
Combat Sheild (Sheild 1)	Trinity Infiltraiton
DEF-12 Shotgun (Close Combat 2)	Nightclub Stakeout
DW-P5 (SMG 2)	Nightclub Stakeout
FAC-16 (Assault 2)	Trinity Escape (off dead ally)
Falcon (Pistol 1)	Nightclub Stakeout
Frag Grenade (Thrown 1*5)	Nightclub Stakeout
Hawk Boomerang (Thrown 1)	Rooftops Escape
KSI-74 (Assault 2)	Lab Rescue
Laptop (Assault 2)	Trinity Infiltration
M60 (Heavy 3)	Mansion Infiltration
Magnum (Pistol 1)	Rooftops Escape
Magsec 4 (Pistol 1)	Mansion Infiltration
Multi Mine (Thrown 1*5)	Temple Surveillance
Phychosis Gun (Pistol 1)	Trinity Infiltration
Plasma Rifle (Heavy 3)	Temple Surveillance
RCP-90 (SMG 2)	Jungle Storm
Rocket Launcher (Heavy 3)	Bridge Assult
SuperDragon (Assault 2)	Outpost Rescue
UGL Liberator (SMG 2)	Nightclub Stakeout
Viblade (Close Combat 1)	Mansion Infiltration (red triplaser room)