To obtain this achievment you will need to find and collect both of the assault rifles within the game. Their names and locations are as follows:
The M4 is obtained, not through finding it physically, but by performing well on missions. You need to earn the silent assassin rating on 5 missions in a row to unlock this weapon. Additionally, you may be able to obtain it during the mission 'The Motorcade Interception' by killing a UN soldier with the fiber wire, causing him to drop the gun.
The AK-74 can be found during 9 different missions, usually by police guards who will be carrying it. 8 of these missions, contain guards carrying the gun and these missions are, 'St. Petersburg Stakeout', 'Kirov Park Meeting', 'Tubeway Torpedo', 'Murder at the Bazaar', 'The Motorcade Interception', 'Tunnel Rat', 'Terminal Hospitality' and 'St. Petersburg Revisited'. The ninth mission has the weapon inside the storage room and the mission is 'The Death of Hannelore'.