Full list of all 69 Halo: Reach achievements worth 1,700 gamerscore. It takes around 40-50 hours to unlock all of the achievements in the base game on Xbox 360.
The base game contains 49 achievements worth 1,000 Gamerscore, and there are 5 DLC packs containing 20 achievements worth 700 Gamerscore.
Completed a Weekly Challenge.
Advanced a Commendation to a Silver state.
Won a game of Invasion in the 1st phase.
As an Elite, killed 5 Spartan players in Matchmaking.
Let a teammate spawn on you 5 times in an Invasion Matchmaking game.
Scored 1,000,000 points in a Firefight game.
Completed a Firefight set on Legendary without dying.
Earned a Killionaire medal in Firefight.
Scored 200,000 points in a Firefight game.
Killed 10 enemies in Firefight or Campaign with a supercombine explosion.
Completed all of the Daily Challenges in a given day.
Killed 10 enemies in a Firefight or Campaign session with the DMR.
Scored 50,000 points in a Firefight game.
Scored 15,000 points in Score Attack Firefight Matchmaking.
Used the File Browser to upload a file to your File Share.
Recommended a file to someone.
Earned a Triple Kill while Jetpacking in Campaign, Firefight or Matchmaking.
Killed 10 enemies in a Firefight or Campaign session with the M6G pistol.
Used a Health Pack to replenish life after taking body damage.
Purchased an item from the Armory.
Performed an Assassination on an enemy.
Reached the rank of Corporal in the UNSC.
Earned a Killing Spree in multiplayer Matchmaking.
Earned a First Strike Medal in a Matchmaking game.
Complete each Campaign Mission on Legendary - alone.
Complete the Campaign on Legendary difficulty.
Clear the 2nd mission without setting foot in a drivable vehicle.
Kill 7 Moa during the 2nd mission of the Campaign.
Complete the 2nd mission on Normal or harder.
Kill 2 vehicles at once with the Target Locator during the 3rd mission.
Trade weapons with an AI ally in Campaign.
Complete the 3rd mission on Normal or harder.
Complete the 4th mission on Normal or harder.
Kill the Elite Zealot before he can escape during the 5th mission.
Complete the 5th mission on Normal or harder.
Destroy the Corvette's engines & escort in under 3 minutes in the 6th mission on Heroic or harder.
Complete the 6th mission on Normal or harder.
Complete the 7th mission on Normal or harder.
Kill 3 of the anti-aircraft batteries during the 8th mission.
Complete the 8th mission on Normal or harder.
Finish the 9th mission on Legendary with the Scorpion intact.
Hijack a Banshee during the Reach Campaign.
Complete the 9th mission on Normal or harder.
Perform an Assassination against an Elite to survive a fall that would've been fatal.
Complete the 10th mission on Normal or harder.
Complete the Campaign on Heroic difficulty.
Complete the Campaign on Normal difficulty.
Reach the rank of Captain in the UNSC.
Purchase an item from the Armory that requires the rank of Lt. Colonel.
Noble 1: Destroyed a vehicle using Armor Lock in a matchmade game.
Noble 1: Killed a player at long range with the DMR in a matchmade Slayer game.
Noble 1: Your team collected all of the flags in a matchmade Stockpile game.
Noble 1: Earned a Double Kill with the shotgun in multiplayer Matchmaking.
Noble 1: Earned a kill in a matchmade Rocket Race game.
Noble 1: Earned a Double Kill from the Grave in multiplayer Matchmaking.
Noble 1: Your team blew up the research facility in a matchmade Invasion game on Breakpoint.
Defiant: Killed 3 players with the Oddball in a matchmade Oddball game.
Defiant: Captured two flags from an enemy capture zone in a matchmade Stockpile game.
Defiant: Returned two flags in a matchmade CTF game.
Defiant: Assassinated a player that just exited Armor-Lock in a matchmade game.
Defiant: Prevented the attacking team from touching the flag in a matchmade 1-Flag CTF game.
Defiant: Was the last surviving uninfected player in a matchmade Infection game.
Anniversary: Captured 10 skulls at once in a matchmade Headhunter game.
Anniversary: Killed the flag carrier with a Plasma Grenade in a matchmade game.
Anniversary: Performed a Beat Down on an enemy while using the Jetpack during a matchmade game.
Anniversary: Infected 3 players in a single round in a matchmade Infection game.
Anniversary: Killed an enemy with their own grenade that was stuck to you in a matchmade game.
Anniversary: Earned 3 headshots in a row in a matchmade game.
Anniversary: Ran over 5 people with a vehicle in a matchmade game.