To earn this achievement you must collect all AR's within the game. To "collect" them, you must pick them up in a mission, and then finish the mission with it in your possesion.
If you start a mission with a collected gun, but drop it and dont finish with that gun, it will be lost and have to be recollected.
**AK74** (Carried by guards all over the game).
The Bjarkhov Bomb
Rendevouz in Rotterdam
Deadly Cargo
Slaying a Dragon
The Wang Fou Incident
The Seafood Massacre
Lee Hong Assassination
**AK74-S**
Complete "The Wang Fou Incident" with Silent Assasin ranking
**M4 Carbine**
*Deadly Cargo - (Hint)- People say this is hard to get. Its not. Dont wait for SWAT to storm the boat. Complete the mission as normal, but on your exit enter the northern most warehouse (the building just south of where the worker outfit is). In here is a SWAT officer. Kill him, take his M4, and then just leave through the Police gate and the exit.
-M4 Carbine .S
Complete "Deadly Cargo" with Silent Assasin ranking
Any questions, just ask.