To unlock this achievement you must find and collect all of the hand guns within the game. There are 12 handguns that need to be obtained, but you will start with 1, 'Silverballers', so you only need to collect a further 11. Their names and locations are as follows:
Silverballer - Single, Suppressed
You can find this gun during the mission, 'The Meat King's Party'. It is being carried by the lawyer, who is one of your targets.
Silverballers - Dual-Wielded, Suppressed
To obtain this weapon, you need to beat the mission 'The Bkarhov Bomb', with a silent assassin rating.
The GK17 can be found during 7 different missions. During 'Beldingford Manor' and 'Traditions of the Trade' they are carried by the guards. During 'Deadly Cargo', 'The Seafood Massacre' and 'Hunter and Hunted' they are carried by policemen. Some members of the Blue Lotus Triad are carrying 1 during 'Slaying a Dragon' and 1 of the patrolling guards will be carrying it during the training mission.
GK17 - Dual-Wielded
To obtain the dual-wielded version of the GK17, you must beat the mission 'The Seafood Massacre', with a silent assassin rating.
The CZ2000 can be found during 4 different missions. During 'The Meat King's Party' and 'Rendezvous in Rotterdam', the various enemies and guards will be carrying it. During 'The Bjarkhov Bomb', Fabian Fuchs will be carrying 1 and you can also find it during 'Asylum Aftermath' on a table near the elevator and on the dead bodies of the agent 48's.
CZ2000 - Dual-Wielded
You can obtain the dual-wielded version of this weapon by beating the mission 'Asylum Aftermath', with a silent assassin rating.
The SG220.S can be found during 4 different missions. You can find it on a desk in Sergei Bjarkhov's office during 'The Bjarkhov Bomb' and on a desk near the bathroom that Franz Fuchs is showering in during 'Traditions of the Trade'. You can also find this gun in the room with the dead civilian during 'Hunter and Hunted'. This weapon is also carried by the Red Dragon negotiator during the mission 'The Seafood Massacre'.
SG220.S - Dual-Wielded
To obtain the dual-wielded version of this weapon, you need to beat the mission 'Traditions of the Trade', with a silent assassin rating.
Gold Desert Eagle
There is only 3 specific people carrying this gun over 3 different missions. During 'Beldingford Manor', Alistair Beldingfords companion will be carrying it and during 'Rendezvous in Rotterdam', Rutgert Van Leuven will be carrying it. This gun is also carried by Zun during 'The Lee Hong Assassination'.
Gold Desert Eagle - Dual-Wielded
See the description for 'Double Eagle', below.
You can acquire the Magnum 500 from some of the bikers during 'Rendezvous in Rotterdam' and from the bodyguards during 'Traditions of the Trade'. During 'The Seafood Massacre, the police chief will be carrying 1 as will Boris Ivanovich Duruska, during 'Deadly Cargo'.
Magnum 500 - Dual-Wielded
You can acquire the dual magnum 500's by attaining a silent assassin rating on the mission 'Beldingford Manor'.