To obtain this achievement you need to find and collect all of the handguns within the game. There are 6 to collect. Their names and locations are as follows:
The Silverballers are your beginning pistols, and you will start with them already in your inventory and on display in the hideout. To obtain the silenced version, you must achieve a silent assassin rating on any mission.
You can find the Desert Eagle during 4 different missions, which are; 'Anathema', in the study, 'Tunnel Rat', carried by Yussef Hussein, a target, 'Temple City Ambush' carried by Agent Smith and during 'St. Petersburg Revisited' carried by FSB Agents.
Beretta 9mm Pistol
The Beretta 9mm Pistol can be found during 10 different levels, which are; 'Anathema', 'St. Petersburg Stakeout', 'Kirov Park Meeting', 'Invitation To A Party', 'Tracking Hayamoto', 'Basement Killing', 'The Graveyard Shift', 'The Jacuzzi Job', 'The Death of Hannelore' and 'Redemption at Gontranno. You will find them being carried by bodyguards, embassy guards and security guards throughout these missions. Simply kill one of them and take the weapon.
The .22 Suppressed can only be found during 1 mission, 'Shogun Showdown', inside the museum.
Snub Nosed Revolver (Revolver)
The Snub Nosed Revolver, known simply as 'Revolver' can be found during 3 different missions, 'Anathema', carried by Giuseppe Guillani, 'The Jacuzzi Job', carried by Charlie Sidjan's female bodyguards and 'Murder at the Bazaar' being carried by Mohammad Amin.
The .54 can only be found during 2 different missions and in only 1 location on each. During 'St. Petersburg Stakeout', it is being carried by a soldier at one of the roadblocks and during 'Invitation To A Party' it can be found by the spetsnaz agent.