Here is the full list of all 21 HeadCount achievements.
Kill 1000 Enemies
Complete 5 Levels
Complete 10 Levels
Purchase 5 Guns
Purchase 10 Guns
Purchase All Sidearms
Purchase All Primary Firearms
Purchase A Machinegun
Unlock 5 guns
Unlock 10 Guns
Unlock All Sidearms
Unlock All Primary Firearms
Complete Any Level Without Dying
Find All Hidden Guns In Minigames 1
Find All Hidden Guns In Minigames 2
Blow Up 100 Barrels
Complete All Levels On Any Difficulty
Complete All Levels On Hard Difficulty and Max Intensity
Complete All Levels Without Dying, Any Difficulty or Intensity
Complete All Levels Without Dying On Hard Difficulty and Max Intensity
Find Each Trophy In Every Campaign Level