Equipped a class mod for your character

Earned level 20

Sold 50 guns to a shop

Finish the game in Normal Mode

Knife an enemy without him ever knowing you were there in Single Player or Special Ops.

Release Melissa from her Trap.

First kill with a Gun.

Release Jennings from his Trap.

Unlock 5 locked doors.

First kill with a Bat.

First kill with a Table Leg.

Killed a Collared Minion with a collar detonation.