During game play press the Back button (next to the X-Guide button) and input the following codes to unlock the corresponding:

Restock Ammo:
Red, Green, Blue, Yellow, Left, Right, Left

Restore Life:
Red, Green, Red, Green, Up, Up, Down, Blue

Load Up:
Up, Up, Down, Down, Left, Right, Left, Right, Red, Green

Invulnerability:
Down, Up, Down, Down, Up, Right, Left, Right, Yellow, Blue, Blue, Green

Unlockable Single-Player Achievements
To unlock these achievements for Gamerscore points play a Single-Player game without any in-game codes and accomplish the required task.

Untouchable - 10 Gamerscore Points:
Complete level with no damage taken

Ballistics Expert - 15 Gamerscore Points:
Completed a level using only the Rocket Launcher

Blaster Master - 15 Gamerscore Points:
Completed a level using only the Blaster

Blazing Buckshot -15 Gamerscore Points:
Completed a level using only the Shotgun

Enlightening - 15 Gamerscore Points:
Completed a level using only the Lightning Gun

Hyperactive - 15 Gamerscore Points:
Completed a level using only the Hyper-Blaster

Pineapple King - 15 Gamerscore Points:
Completed a level using only the Grenade Launcher

The Dark Lord - 15 Gamerscore Points:
Completed a level using only the Dark Matter Gun

The Machine - 15 Gamerscore Points:
Completed a level using only the machine Gun

The Sniper - 15 Gamerscore Points:
Completed a level using only the Rail Gun

Tooth and Nail - 15 Gamerscore Points:
Completed a level using only the Nail Gun

Private-Act 1 - 15 Gamerscore Points:
Complete Act 1 with Rank of Private

Private-Act 2 - 20 Gamerscore Points:
Complete Acts 1 and 2 with Rank of Private

Full Arsenal - 20 Gamerscore Points:
Landed a shot from every weapon

Sky Marshall's Medal of Valor - 20 Gamerscore Points:
Rescue PVT Lanier of Eagle Squad

Solar Star of Courage - 20 Gamerscore Points:
Finish the level with helping the Viper Squad

Galactic Order of Heroism - 20 Gamerscore Points:
Finish the level with helping the Raven Squad

General-Act 1 - 25 Gamerscore Points:
Complete Act 1 with Rank of General

General-Act 2 - 25 Gamerscore Points:
Complete Act 2 with Rank of General

Corporal-Act 1 - 25 Gamerscore Points:
Complete Act 1 with Rank of Corporal

Corporal-Act 1 - 25 Gamerscore Points:
Complete Acts 1 and 2 with rank of Corporal

Lieutenant-Act 1 - 25 Gamerscore Points:
Complete Act 1 with Rank of Lieutenant

Private-Defeated the Strogg - 25 Gamerscore Points:
Complete the game with Rank of Private

Lieutenant-Act 2 - 25 Gamerscore Points:
Complete Acts 1 and 2 with Rank of Lieutenant

Lieutenant-Defeated the Strogg - 30 Gamerscore Points:
Complete the game with Rank of Lieutenant

Corporal-Defeated the Strogg - 30 Gamerscore Points:
Complete the game with Rank of Corporal

General-Defeated the Strogg - 30 Gamerscore Points:
Complete the game with Rank of General


Unlockable Multiplayer Achievements
To unlock the following achievements for Gamerscore Points play a Ranked Multiplayer game and accomplish a certain task.

Private-Multiplayer - 5 Gamerscore Points:
Over 100 Win Points

Perfect Win - 5 Gamerscore Points:
Won a Match without dying

Basic Training - 5 Gamerscore Points:
Play over 100 Matches

Bronze Flag - 5 Gamerscore Points:
Over 100 captures

Best Impression - 10 Gamerscore Points:
Over 250 Impressives

Corporal-Multiplayer - 10 Gamerscore Points:
Over 500 Win Points

Silver Flag - 10 Gamerscore Points:
Over 500 Captures

You Excellency - 10 Gamerscore Points:
Over 500 Excellents

Defender - 10 Gamerscore Points:
Over 250 Defense Awards

Guts but no Glory - 10 Gamerscore Points:
Over 100 Assists

First tour of duty - 10 Gamerscore Points:
Play over 500 Matches

Viper Squad - 5 Gamerscore Points:
Over 100 Frags

Warthog Squad - 10 Gamerscore Points:
Over 500 Frags

Golden Gauntlet - 20 points:
Humiliate over 250 players

Number 1 - 20 Gamerscore Points:
Reach number one on the ALL Game types ranked leader board

Gold Flag - 25 Gamerscore Points:
Over 1,000 captures

Combat Veteran - 25 Gamerscore Points:
Play over 1,000 matches

Lieutenant-Multiplayer - 25 Gamerscore Points:
Over 1,000 Win points

General-Multiplayer - 50 Gamerscore Points:
Over 5,000 Win Points

Raven Squad - 50 Gamerscore Points:
Over 5,000 Frags

Seasoned Warrior - 50 Gamerscore Points:
Play over 5,000 Matches

Rhino Squad - 75 Gamerscore Points:
Over 10,000 Frags