To obtain this achievement you will need to find and collect all of the sniper rifles within the game. Their names and locations are:
You can find this gun during 2 different missions, 'Invitation To A Party' and 'St. Petersburg Revisited'. During 'Invitation To A Party' you can find it on an outside balcony and during 'St. Petersburg Revisited', You can find it on Agent 17, upon killing him.
W2000 Custom Rifle
This gun is the silenced version of the W2000 and has its own space in your hideout, so it needs to be collected as well. You can acquire this gun during the final mission 'Redemption at Gontranno', being carried by some of the guards.
The Dragunov SVD can be found during 'St. Petersburg Stakeout', 'Kirov Park Meeting' and 'St. Petersburg Revisited' at the agency pickup on each of these missions. During 'Murder at the Bazaar', you can find the gun inside a house that is marked by a point of interest and during 'Redemption at Gontranno', you will find it being carried by several enemies.
The R93 Sniper can be found during 3 different missions, 'Anathema', inside the garage, 'Temple City Ambush' being used by the sniper and 'Terminal Hospitality', in the temple.
There is only 1 location in which to find this weapon. During the mission 'The Motorcade Interception', you will find it at the agency pickup.
There is only 1 location in which you can find this weapon. During the mission 'Hidden Valley', you will find it in the agency cache.