Engineer Expert Achievement
Online: unlock 3 weapons in the Engineer kit
The engineer class is the second class you can choose to spawn with. It includes a submachine gun, a repair tool and RPGs/Rocket Launchers.
To unlock more weapons in this class, you must get points with this as your spawned class. (Getting kills/destroying vehicles/destroying M-Com positions/etc).
You need 16,000 XP to unlock the third weapon for this achievement.