ACR-2 Sniper Rifle - At the main menu, select "Single Player", "Start New Game", then press Up(2), Down, Left, Right(2), Down at the difficulty selection menu.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Snake In The Grass (10 points): Kill 10 enemies while crouching in any multiplayer mode.
Every Bullet Counts (20 points): Finish a multiplayer match with 100% accuracy (Minimum of 10 kills).
More Ammo For Me! (10 points): Perform 300 reloads in any multiplayer mode.
Nades For Everyone (20 points): Get a triple kill with an explosive during a single Ranked Match - Team Deathmatch.
Sneaky, Sneaky… (10 points): Accumulate 30 kills with a silencer over Xbox LIVE.
I Got Shotgun… (20 points): Get 20 kills with a shotgun in a single Ranked Match - Capture The Flag.
There Is No Blue Wire! (20 points): Disarm 15 bombs in Demolition mode.
Aim In Vein (20 points): Achieve a 25% vital zone hit ratio in a multiplayer match.
The Impossible Shot (20 points): Achieve 10 kills from a distance greater than 100 feet without a scope in any multiplayer mode.
Assassination King (20 points): Achieve 15 kills with a sniper rifle in a single Ranked Match - Deathmatch.
One Shot Is All I Need! (20 points): Achieve 5 headshots in a row without dying using a sniper rifle in a multiplayer match.
This Is My Rifle… (20 points): Kill the bomb carrier with an assault rifle 5 times in a single Ranked Match - Demolition.
Spray And Pray (20 points): Achieve 5 kills in a row without dying with an SMG in a single Ranked Match - Elimination.
Bullets Everywhere… (20 points): Kill 10 opponents with a machine gun during a single Ranked Match - Team Elimination.
It's Not The Size… (20 points): Achieve 50 kills using pistols in any multiplayer mode.
The Black Knight (20 points): Shoot off all 4 limbs on any combination of players over the course of a multiplayer match.
Do It From Behind (20 points): Kill 50 opponents in Ranked Matches with a knife attack from behind.
Daddy Longlegs (50 points): Accumulate 250 dismemberments in any multiplayer mode.
Head Of The Class (20 points): Accumulate 100 headshots over Xbox LIVE in any mode.
Flag Frag (20 points): Kill 30 enemy flag carriers in Capture The Flag.
Cash And Prizes (20 points): Kill 75 enemies by shooting them in the groin in any multiplayer mode.
Hostest With The Mostest (10 points): Host and complete 100 multiplayer matches.
How Embarrassing (20 points): Kill 4 members of the opposing team while carrying their flag.
Gung-ho (0 points): Have a 1 to 5 or worse Kill to Death ratio in a multiplayer match.
Where's The Competition? (20 points): Have a 5 to 1 or better Kill to Death ratio in a multiplayer match (minimum of 20 kills).
Stop Hittin' Yourself! (0 points): Accumulate 50 suicides over Xbox LIVE in any mode.
You're The Best... Around (10 points): Be the most skillful player in a Ranked Match.
Complete Al Qa'im (20 points): Complete the Al Qa'im level on the hardest difficulty setting.
Complete Mogaung (20 points): Complete the Mogaung levels on the hardest difficulty setting.
Complete Eshkashem (20 points): Complete the Eshkashem levels on the hardest difficulty setting.
Complete Llebo (20 points): Complete the Llebo levels on the hardest difficulty setting.
Complete Donetsk (20 points): Complete the Donetsk levels on the hardest difficulty setting.
The Real Deal (110 points): Complete the game without dying on Normal or Hard difficulty.
Mission Accomplished (50 points): Complete the game on any difficulty setting.
Soldier Of Fortune (100 points): Complete the game on the hardest difficulty setting.
No Mercy (30 points): Kill 1000 enemies throughout the campaign.
Not Enough Milk (30 points): Perform 500 dismemberments throughout the campaign.
Speed Kills (20 points): Kill 5 enemies in 5 seconds in any game mode.
Quicker Than Diet And Exercise (20 points): Perform a dismemberment at every point on the enemies' bodies.
Butcher (20 points): Accumulate 100 kills with the knife over Xbox LIVE in any mode.

Additionally, there are two secret achievements:

Pacifist (20 points): Survive an entire Deathmatch without hurting anyone (minimum of 4 active players).
Going Out With A Bang (20 points): Kill yourself and an opponent with an explosive 10 times in any multiplayer mode.