Gung Ho Achievement
Snipe 100 moving targetsNote: Only scoped kills count towards this achievement.
This is a cumulative achievement that should unlock naturally over the course of all game modes. Every time you kill a moving target it will count toward this. A moving target is an enemy that is running in any direction, usually looking for cover. Note that they have to be running and not walking a patrol or moving while shooting at you.
To see how many you have done, from the main menu, navigate to Extras then Career Stats. Press to highlight 'Single Player' then look for 'Scoped Moving Target Kills'.