To unlock this achievement you will need to find and collect all of the assault rifles within the game. There are 4 assault rifle. Their locations are as follows:
You can only find the M4 Carbine during 1 mission in the game, which is 'Deadly Cargo' and it is being carried by the SWAT team.
M4 Carbine - Suppressed
To unlock the suppressed variant, you need to beat 'Deadly Cargo' with a silent assassin rating.
You can find the AK-74 during 7 different missions. During 'The Bjarkhov Bomb', 'Rendezvous in Rotterdam' and 'Deadly Cargo', it is carried by the various guards and enemies. During 'Slaying a Dragon' it is carried by some members of the blue lotus triad, guarding the park and during 'The Wang Fou Incident', it is carried by some members of the red dragon triad in the restaurant. Some of the police during the mission 'The Seafood Massacre' will be carrying 1 and you can also find it in a weapons cache on the second floor, during 'The Lee Hong Assassination'.
AK-74 - Suppressed
This weapon is obtained after beating the mission 'The Wang Fou Incident' with a silent assassin rating.