Different from the other weapon achievements, you need to get 100 head shots with the assault rifle. While it will make it much easier if you wait for your crosshair's to turn red, similar to the pistol or shotgun, it's not required. As long as you get a head shot, it will count towards this achievement, even if you don't wait for a critical hit. Make sure you focus on accuracy and accuracy increasing mods, so as to make this much easier to obtain.
If you have invested into the assault rifle tree, using the Focus Aim skill will make this achievement much easier. It will automatically lock-on and track the closest enemy, so you need only to alter your aim to get a head shot.
You will unlock the perk "Assault Rifle Precision" after scoring fifty headshot's.