The description says Kill 500 enemies with the Heavy Machine Gun
Note that only enemies in the Mission Mode count as enemies.
Enemies in Multiplayer do not work for this Achievement.
The reward for this achievement is 25 Score Points (when achieved during a match).
Tips & Tricks
The heavy machine gun is especially effective against unprepared enemies when used with skill,demolishing unarmored player with less than half a clip and quickly wearing down heavily armored ones