Use the Assault Rifle to kill 5 enemies in a row without reloading.

This is very likely to be your first achievement and can be unlocked during the very first mission when you are given the Assault Rifle. If you're playing on Easy difficulty, enemies will die quickly, so you may want to lower it to Easy for the sake of this achievement if you're on a higher difficulty. Simply find a group of 5 or more enemies and take your time lining head shots up until you have killed 5 without reloading.