To unlock this achievement, you must find and collect all of the sniper rifles within the game. There are 6 sniper rifles and their locations are as follows:
Lee Enfield Enforcer
There are only 2 missions in which this gun can be found. You can find 2 during 'Beldingford Manor', 1 of which is in the barn loft, being carried by a guard and the other is in a gun cabinet inside the manor. You can also find it during the mission 'Asylum Aftermath' where it can be found in a gun cabinet on the second floor, or on the roof of the building.
The R93 can only be found during 2 missions, 'Deadly Cargo' and 'The Seafood Massacre.' It is being wielded by a sniper in both instances. The sniper is on the warehouse roof during 'Deadly Cargo' and on the restaurant roof during 'The Seafood Massacre'.
PGM Sniper Rifle
This gun can only be found during the mission 'Hunter and Hunted' and it is carried by a sniper on the roof.
PGM Sniper Rifle - Supressed
You will receive this weapon by earning a silent assassin rating on the final mission, 'Hunter and Hunted'.
The dragunov can be found during 4 different missions. During 'The Bjarkhov Bomb' and 'Slaying a Dragon' it is carried by the sniper guards. During the mission 'Asylum Aftermath' some of the SWAT team members are carrying it. During 'The Meat King's Party' there is 1 on a box in the main butchery area.
The W2000 is available on 6 different missions. 3 of these missions, you will start with the gun already in your possession, these are; 'Traditions of the Trade', 'Slaying a Dragon' and 'The Seafood Massacre'. You can find this gun in the guards quarters during 'The Wang Fou Incident' and during the mission 'The Lee Hong Assassination' you can find it in the weapons depot. The final place where you will find this gun is during 'Hunter and Hunted' where it will be in the cabinet in 47's room.